TypeScript Release Notes

Today we are excited to announce the release of TypeScript 5.9!

If you’re not familiar with TypeScript, it’s a language that builds on JavaScript by adding syntax for types. With types, TypeScript makes it possible to check your code to avoid bugs ahead of time. The TypeScript type-checker does all this, and is also the foundation of great tooling in your editor and elsewhere, making coding even easier. If you’ve written JavaScript in editors like Visual Studio and VS Code, TypeScript even powers features you might already be using like completions, go-to-definition, and more. You can learn more about TypeScript at our website.

But if you’re already familiar, you can start using TypeScript 5.9 today!
npm install -D typescript

Let’s take a look at what’s new in TypeScript 5.9!

• Minimal and Updated tsc --init
• Support for import defer
• Support for --module node20
• Summary Descriptions in DOM APIs
• Expandable Hovers (Preview)
• Configurable Maximum Hover Length
• Optimizations
• Notable Behavioral Changes

What’s New Since the Beta and RC?

There have been no changes to TypeScript 5.9 since the release candidate.
A few fixes for reported issues have been made since the 5.9 beta, including the restoration of AbortSignal.abort() to the DOM library. Additionally, we have added a section about Notable Behavioral Changes.

Minimal and Updated tsc --init

For a while, the TypeScript compiler has supported an --init flag that can create a tsconfig.json within the current directory. In the last few years, running tsc --init created a very “full” tsconfig.json, filled with commented-out settings and their descriptions. We designed this with the intent of making options discoverable and easy to toggle.
However, given external feedback (and our own experience), we found it’s common to immediately delete most of the contents of these new tsconfig.json files. When users want to discover new options, we find they rely on auto-complete from their editor, or navigate to the tsconfig reference on our website (which the generated tsconfig.json links to!). What each setting does is also documented on that same page, and can be seen via editor hovers/tooltips/quick info. While surfacing some commented-out settings might be helpful, the generated tsconfig.json was often considered overkill.
We also felt that it was time that tsc --init initialized with a few more prescriptive settings than we already enable. We looked at some common pain points and papercuts users have when they create a new TypeScript project. For example, most users write in modules (not global scripts), and --moduleDetection can force TypeScript to treat every implementation file as a module. Developers also often want to use the latest ECMAScript features directly in their runtime, so --target can typically be set to esnext. JSX users often find that going back to set --jsx is needless friction, and its options are slightly confusing. And often, projects end up loading more declaration files from node_modules/@types than TypeScript actually needs; but specifying an empty types array can help limit this.
In TypeScript 5.9, a plain tsc --init with no other flags will generate the following tsconfig.json :

{
 // Visit https://aka.ms/tsconfig to read more about this file
 "compilerOptions" : {
  // File Layout
  // "rootDir": "./src",
  // "outDir": "./dist",
  // Environment Settings
  // See also https://aka.ms/tsconfig_modules
  "module" : "nodenext",
  "target" : "esnext",
  "types" : [
  ],
  // For nodejs:
  // "lib": ["esnext"],
  // "types": ["node"],
  // and npm install -D @types/node
  // Other Outputs
  "sourceMap" : true,
  "declaration" : true,
  "declarationMap" : true,
  // Stricter Typechecking Options
  "noUncheckedIndexedAccess" : true,
  "exactOptionalPropertyTypes" : true,
  // Style Options
  // "noImplicitReturns": true,
  // "noImplicitOverride": true,
  // "noUnusedLocals": true,
  // "noUnusedParameters": true,
  // "noFallthroughCasesInSwitch": true,
  // "noPropertyAccessFromIndexSignature": true,
  // Recommended Options
  "strict" : true,
  "jsx" : "react-jsx",
  "verbatimModuleSyntax" : true,
  "isolatedModules" : true,
  "noUncheckedSideEffectImports" : true,
  "moduleDetection" : "force",
  "skipLibCheck" : true
 }
}

For more details, see the implementing pull request and discussion issue.

Support for import defer

TypeScript 5.9 introduces support for ECMAScript’s deferred module evaluation proposal using the new import defer syntax. This feature allows you to import a module without immediately executing the module and its dependencies, providing better control over when work and side-effects occur.
The syntax only permits namespace imports:

import defer * as feature from "./some-feature.js";

The key benefit of import defer is that the module is only evaluated when one of its exports is first accessed. Consider this example:

// ./some-feature.ts
initializationWithSideEffects();
function initializationWithSideEffects() {
 // ...
 specialConstant = 42;
 console.log("Side effects have occurred!");
}
export let specialConstant: number;

When using import defer, the initializationWithSideEffects() function will not be called until you actually access a property of the imported namespace:

import defer * as feature from "./some-feature.js";
// No side effects have occurred yet
// ...
// As soon as `specialConstant` is accessed, the contents of the `feature` 
// module are run and side effects have taken place.
console.log(feature.specialConstant); // 42

Because evaluation of the module is deferred until you access a member off of the module, you cannot use named imports or default imports with import defer :

// ❌ Not allowed
import defer { doSomething } from "some-module";
// ❌ Not allowed
import defer defaultExport from "some-module";
// ✅ Only this syntax is supported
import defer * as feature from "some-module";

Note that when you write import defer, the module and its dependencies are fully loaded and ready for execution. That means that the module will need to exist, and will be loaded from the file system or a network resource. The key difference between a regular import and import defer is that the execution of statements and declarations is deferred until you access a property of the imported namespace.
This feature is particularly useful for conditionally loading modules with expensive or platform-specific initialization. It can also improve startup performance by deferring module evaluation for app features until they are actually needed.
Note that import defer is not transformed or “downleveled” at all by TypeScript. It is intended to be used in runtimes that support the feature natively, or by tools such as bundlers that can apply the appropriate transformation. That means that import defer will only work under the --module modes preserve and esnext.
We’d like to extend our thanks to Nicolò Ribaudo who championed the proposal in TC39 and also provided the implementation for this feature.

Support for --module node20

TypeScript provides several node* options for the --module and --moduleResolution settings. Most recently, --module nodenext has supported the ability to require() ECMAScript modules from CommonJS modules, and correctly rejects import assertions (in favor of the standards-bound import attributes).
TypeScript 5.9 brings a stable option for these settings called node20, intended to model the behavior of Node.js v20. This option is unlikely to have new behaviors in the future, unlike --module nodenext or --moduleResolution nodenext. Also unlike nodenext, specifying --module node20 will imply --target es2023 unless otherwise configured. --module nodenext, on the other hand, implies the floating --target esnext.
For more information, take a look at the implementation here.

Summary Descriptions in DOM APIs

Previously, many of the DOM APIs in TypeScript only linked to the MDN documentation for the API. These links were useful, but they didn’t provide a quick summary of what the API does. Thanks to a few changes from Adam Naji, TypeScript now includes summary descriptions for many DOM APIs based on the MDN documentation. You can see more of these changes here and here.

Expandable Hovers (Preview)

Quick Info (also called “editor tooltips” and “hovers”) can be very useful for peeking at variables to see their types, or at type aliases to see what they actually refer to. Still, it’s common for people to want to go deeper and get details from whatever’s displayed within the quick info tooltip. For example, if we hover our mouse over the parameter options in the following example:

export function drawButton(options: Options): void

We’re left with (parameter) options: Options.
Do we really need to jump to the definition of the type Options just to see what members this value has?
Previously, that was actually the case. To help here, TypeScript 5.9 is now previewing a feature called expandable hovers, or “quick info verbosity”. If you use an editor like VS Code, you’ll now see a + and - button on the left of these hover tooltips. Clicking on the + button will expand out types more deeply, while clicking on the - button will collapse to the last view.
This feature is currently in preview, and we are seeking feedback for both TypeScript and our partners on Visual Studio Code. For more details, see the PR for this feature here.

Configurable Maximum Hover Length

Occasionally, quick info tooltips can become so long that TypeScript will truncate them to make them more readable. The downside here is that often the most important information will be omitted from the hover tooltip, which can be frustrating. To help with this, TypeScript 5.9’s language server supports a configurable hover length, which can be configured in VS Code via the js/ts.hover.maximumLength setting.
Additionally, the new default hover length is substantially larger than the previous default. This means that in TypeScript 5.9, you should see more information in your hover tooltips by default. For more details, see the PR for this feature here and the corresponding change to Visual Studio Code here.

Optimizations

Cache Instantiations on Mappers
When TypeScript replaces type parameters with specific type arguments, it can end up instantiating many of the same intermediate types over and over again. In complex libraries like Zod and tRPC, this could lead to both performance issues and errors reported around excessive type instantiation depth. Thanks to a change from Mateusz Burzyński, TypeScript 5.9 is able to cache many intermediate instantiations when work has already begun on a specific type instantiation. This in turn avoids lots of unnecessary work and allocations.
Avoiding Closure Creation in fileOrDirectoryExistsUsingSource
In JavaScript, a function expression will typically allocate a new function object, even if the wrapper function is just passing through arguments to another function with no captured variables. In code paths around file existence checks, Vincent Bailly found examples of these pass-through function calls, even though the underlying functions only took single arguments. Given the number of existence checks that could take place in larger projects, he cited a speed-up of around 11%. See more on this change here.

Notable Behavioral Changes

lib.d.ts Changes
Types generated for the DOM may have an impact on type-checking your codebase.
Additionally, one notable change is that ArrayBuffer has been changed in such a way that it is no longer a supertype of several different TypedArray types. This also includes subtypes of UInt8Array, such as Buffer from Node.js. As a result, you’ll see new error messages such as:
error TS2345: Argument of type 'ArrayBufferLike' is not assignable to parameter of type 'BufferSource'.
error TS2322: Type 'ArrayBufferLike' is not assignable to type 'ArrayBuffer'.
error TS2322: Type 'Buffer' is not assignable to type 'Uint8Array'.
error TS2322: Type 'Buffer' is not assignable to type 'ArrayBuffer'.
error TS2345: Argument of type 'Buffer' is not assignable to parameter of type 'string | Uint8Array'.
If you encounter issues with Buffer, you may first want to check that you are using the latest version of the @types/node package. This might include running
npm update @types/node --save-dev
Much of the time, the solution is to specify a more specific underlying buffer type instead of using the default ArrayBufferLike (i.e. explicitly writing out Uint8Array rather than a plain Uint8Array). In instances where some TypedArray (like Uint8Array) is passed to a function expecting an ArrayBuffer or SharedArrayBuffer, you can also try accessing the buffer property of that TypedArray like in the following example:

let data = new Uint8Array([0, 1, 2, 3, 4]);
- someFunc(data)
+ someFunc(data.buffer)

Type Argument Inference Changes

In an effort to fix “leaks” of type variables during inference, TypeScript 5.9 may introduce changes in types and possibly new errors in some codebases. These are hard to predict, but can often be fixed by adding type arguments to generic functions calls. See more details here.

What’s Next?

Now that TypeScript 5.9 is out, you might be wondering what’s in store for the next version: TypeScript 6.0.
As you might have heard, much of our recent focus has been on the native port of TypeScript which will eventually be available as TypeScript 7.0. So what does that mean for TypeScript 6.0?
Our vision for TypeScript 6.0 is to act as a transition point for developers to adjust their codebases for TypeScript 7.0. While TypeScript 6.0 may still ship updates and features, most users should think of it as a readiness check for adopting TypeScript 7.0. This new version is meant to align with TypeScript 7.0, introducing deprecations around certain settings and possibly updating type-checking behavior in small ways. Luckily, we don’t predict most projects will have too much trouble upgrading to TypeScript 6.0, and it will likely be entirely API compatible with TypeScript 5.9.
We’ll have more details coming soon. That includes details on TypeScript 7.0 as well, where you can try it out in Visual Studio Code today and install it right in your project.
Otherwise, we hope that TypeScript 5.9 treats you well, and makes your day-to-day coding a joy.
Happy Hacking!
– Daniel Rosenwasser and the TypeScript Team

Original source Report a problem

Release Candidate (RC) of TypeScript 5.9

Today we are excited to announce the Release Candidate (RC) of TypeScript 5.9!
To get started using the Release Candidate, you can get it through npm with the following command:
npm install -D typescript@rc
Let’s take a look at what’s new in TypeScript 5.9!

What’s New in TypeScript 5.9

• Minimal and Updated tsc --init
• Support for import defer
• Support for --module node20
• Summary Descriptions in DOM APIs
• Expandable Hovers (Preview)
• Configurable Maximum Hover Length
• Optimizations
• Notable Behavioral Changes

What’s New Since the Beta?

A few reported fixes have been made since the 5.9 beta, including the restoration of AbortSignal.abort() to the DOM library. Additionally, we have added a section about Notable Behavioral Changes.

Minimal and Updated tsc --init

For a while, the TypeScript compiler has supported an --init flag that can create a tsconfig.json within the current directory. In the last few years, running tsc --init created a very “full” tsconfig.json, filled with commented-out settings and their descriptions. We designed this with the intent of making options discoverable and easy to toggle.
However, given external feedback (and our own experience), we found it’s common to immediately delete most of the contents of these new tsconfig.json files. When users want to discover new options, we find they rely on auto-complete from their editor, or navigate to the tsconfig reference on our website (which the generated tsconfig.json links to!). What each setting does is also documented on that same page, and can be seen via editor hovers/tooltips/quick info. While surfacing some commented-out settings might be helpful, the generated tsconfig.json was often considered overkill.
We also felt that it was time that tsc --init initialized with a few more prescriptive settings than we already enable. We looked at some common pain points and papercuts users have when they create a new TypeScript project. For example, most users write in modules (not global scripts), and --moduleDetection can force TypeScript to treat every implementation file as a module. Developers also often want to use the latest ECMAScript features directly in their runtime, so --target can typically be set to esnext. JSX users often find that going back to set --jsx is a needless friction, and its options are slightly confusing. And often, projects end up loading more declaration files from node_modules/@types than TypeScript actually needs – but specifying an empty types array can help limit this.
In TypeScript 5.9, a plain tsc --init with no other flags will generate the following tsconfig.json :

// Visit https://aka.ms/tsconfig to read more about this file
"compilerOptions": {
  // File Layout
  // "rootDir": "./src",
  // "outDir": "./dist",
  // Environment Settings
  // See also https://aka.ms/tsconfig_modules
  "module": "nodenext",
  "target": "esnext",
  "types": [],
  // For nodejs:
  // "lib": ["esnext"],
  // "types": ["node"],
  // and npm install -D @types/node
  // Other Outputs
  "sourceMap": true,
  "declaration": true,
  "declarationMap": true,
  // Stricter Typechecking Options
  "noUncheckedIndexedAccess": true,
  "exactOptionalPropertyTypes": true,
  // Style Options
  // "noImplicitReturns": true,
  // "noImplicitOverride": true,
  // "noUnusedLocals": true,
  // "noUnusedParameters": true,
  // "noFallthroughCasesInSwitch": true,
  // "noPropertyAccessFromIndexSignature": true,
  // Recommended Options
  "strict": true,
  "jsx": "react-jsx",
  "verbatimModuleSyntax": true,
  "isolatedModules": true,
  "noUncheckedSideEffectImports": true,
  "moduleDetection": "force",
  "skipLibCheck": true
}
}

For more details, see the implementing pull request and discussion issue.

Support for import defer

TypeScript 5.9 introduces support for ECMAScript’s deferred module evaluation proposal using the new import defer syntax. This feature allows you to import a module without immediately executing the module and its dependencies, providing better control over when work and side-effects occur.
The syntax only permits namespace imports:
import defer * as feature from "./some-feature.js";
The key benefit of import defer is that the module is only evaluated on the first use. Consider this example:

// ./some-feature.ts
initializationWithSideEffects();
function initializationWithSideEffects() {
  // ...
  specialConstant = 42;
  console.log("Side effects have occurred!");
}
export let specialConstant: number;

When using import defer, the initializationWithSideEffects() function will not be called until you actually access a property of the imported namespace:

// No side effects have occurred yet
import defer * as feature from "./some-feature.js";

// As soon as `specialConstant` is accessed, the contents of the `feature` module are run and side effects have taken place.
console.log(feature.specialConstant);
// 42

Because evaluation of the module is deferred until you access a member off of the module, you cannot use named imports or default imports with import defer :

// ❌ Not allowed
import defer { doSomething } from "some-module";
// ❌ Not allowed
import defer defaultExport from "some-module";
// ✅ Only this syntax is supported
import defer * as feature from "some-module";

Note that when you write import defer , the module and its dependencies are fully loaded and ready for execution. That means that the module will need to exist, and will be loaded from the file system or a network resource. The key difference between a regular import and import defer is that the execution of statements and declarations is deferred until you access a property of the imported namespace.
This feature is particularly useful for conditionally loading modules with expensive or platform-specific initialization. It can also improve startup performance by deferring module evaluation for app features until they are actually needed.
Note that import defer is not transformed or “downleveled” at all by TypeScript. It is intended to be used in runtimes that support the feature natively, or by tools such as bundlers that can apply the appropriate transformation. That means that import defer will only work under the --module modes preserve and esnext .
We’d like to extend our thanks to Nicolò Ribaudo who championed the proposal in TC39 and also provided the implementation for this feature.

Support for --module node20

TypeScript provides several node* options for the --module and --moduleResolution settings. Most recently, --module nodenext has supported the ability to require() ECMAScript modules from CommonJS modules, and correctly rejects import assertions (in favor of the standards-bound import attributes).
TypeScript 5.9 brings a stable option for these settings called node20 , intended to model the behavior of Node.js v20. This option is unlikely to have new behaviors in the future, unlike --module nodenext or --moduleResolution nodenext . Also unlike nodenext , specifying --module node20 will imply --target es2023 unless otherwise configured. --module nodenext , on the other hand, implies the floating --target esnext .
For more information, take a look at the implementation here.

Summary Descriptions in DOM APIs

Previously, many of the DOM APIs in TypeScript only linked to the MDN documentation for the API. These links were useful, but they didn’t provide a quick summary of what the API does. Thanks to a few changes from Adam Naji , TypeScript now includes summary descriptions for many DOM APIs based on the MDN documentation. You can see more of these changes here and here .

Expandable Hovers (Preview)

Quick Info (also called “editor tooltips” and “hovers”) can be very useful for peeking at variables to see their types, or at type aliases to see what they actually refer to. Still, it’s common for people to want to go deeper and get details from whatever’s displayed within the quick info tooltip. For example, if we hover our mouse over the parameter options in the following example:

export function drawButton ( options : Options ): void

We’re left with (parameter) options: Options .
Do we really need to jump to the definition of the type Options just to see what members this value has?
To help here, TypeScript 5.9 is now previewing a feature called expandable hovers , or “quick info verbosity”. If you use an editor like VS Code, you’ll now see a + and - button on the left of these hover tooltips. Clicking on the + button will expand out types more deeply, while clicking on the - will go back to the last view.
This feature is currently in preview, and we are seeking feedback for both TypeScript and our partners on Visual Studio Code. For more details, see the PR for this feature here .

#### Configurable Maximum Hover Length
Occasionally, quick info tooltips can become so long that TypeScript will truncate them to make them more readable. The downside here is that often the most important information will be omitted from the hover tooltip, which can be frustrating. To help with this, TypeScript 5.9’s language server supports a configurable hover length, which can be configured in VS Code via the js/ts.hover.maximumLength setting.
Additionally, the new default hover length is substantially larger than the previous default. This means that in TypeScript 5.9, you should see more information in your hover tooltips by default. For more details, see the PR for this feature here and the corresponding change to Visual Studio Code here .

#### Optimizations
- Cache Instantiations on Mappers
  When TypeScript replaces type parameters with specific type arguments, it can end up instantiating many of the same intermediate types over and over again. In complex libraries like Zod and tRPC, this could lead to both performance issues and errors reported around excessive type instantiation depth. Thanks to a change from Mateusz Burzyński , TypeScript 5.9 is able to cache many intermediate instantiations when work has already begun on a specific type instantiation. This in turn avoids lots of unnecessary work and allocations.
- Avoiding Closure Creation in fileOrDirectoryExistsUsingSource
  In JavaScript, a function expression will typically allocate a new function object, even if the wrapper function is just passing through arguments to another function with no captured variables. In code paths around file existence checks, Vincent Bailly found examples of these pass-through function calls, even though the underlying functions only took single arguments. Given the number of existence checks that could take place in larger projects, he cited a speed-up of around 11%. See more on this change here .

#### Notable Behavioral Changes
- lib.d.ts Changes
  Types generated for the DOM may have an impact on type-checking your codebase.
  Additionally, one notable change is that ArrayBuffer has been changed in such a way that it is no longer a supertype of several different TypedArray types. This also includes subtypes of UInt8Array, such as Buffer from Node.js. As a result, you’ll see new error messages such as:
  error TS2345: Argument of type 'ArrayBufferLike' is not assignable to parameter of type 'BufferSource'.
  error TS2322: Type 'ArrayBufferLike' is not assignable to type 'ArrayBuffer'.
  error TS2322: Type 'Buffer' is not assignable to type 'Uint8Array<ArrayBufferLike>'.
  error TS2322: Type 'Buffer' is not assignable to type 'ArrayBuffer'.
  error TS2345: Argument of type 'Buffer' is not assignable to parameter of type 'string | Uint8Array<ArrayBufferLike>'.
  If you encounter issues with Buffer, you may first want to check that you are using the latest version of the @types/node package. This might include running
  npm update @types/node --save-dev
  Much of the time, the solution is to specify a more specific underyling buffer type instead of using the default ArrayBufferLike (i.e. explicitly writing out Uint8Array<ArrayBuffer> rather than a plain Uint8Array).

- Type Argument Inference Changes
  In an effort to fix “leaks” of type variables during inference, TypeScript 5.9 may introduces changes in types and possibly new errors in some codebases. These are hard to predict, but can often be fixed by adding type arguments to generic functions calls. See more details here .

- What’s Next?
  As you might have heard, much of our recent focus has been on the native port of TypeScript which will eventually be available as TypeScript 7. You can actually try out the native port today by checking out TypeScript native previews, which are released nightly.
  Given that not much has changed since the TypeScript 5.9 beta, we actually plan to release TypeScript 5.9 as a final release candidate within the next week. So we hope you try out the RC today and let us know what you think!
  Happy Hacking!
  – Daniel Rosenwasser and the TypeScript Team

Original source Report a problem

Today we are excited to announce the availability of TypeScript 5.9 Beta.
To get started using the beta, you can get it through npm with the following command:
npm install -D typescript@beta
Let’s take a look at what’s new in TypeScript 5.9!

• Minimal and Updated tsc --init
• Support for import defer
• Support for --module node20
• Summary Descriptions in DOM APIs
• Expandable Hovers (Preview)
• Configurable Maximum Hover Length
• Optimizations

Minimal and Updated tsc --init

For a while, the TypeScript compiler has supported an --init flag that can create a tsconfig.json within the current directory. In the last few years, running tsc --init created a very “full” tsconfig.json, filled with commented-out settings and their descriptions. We designed this with the intent of making options discoverable and easy to toggle.
However, given external feedback (and our own experience), we found it’s common to immediately delete most of the contents of these new tsconfig.json files. When users want to discover new options, we find they rely on auto-complete from their editor, or navigate to the tsconfig reference on our website (which the generated tsconfig.json links to!). What each setting does is also documented on that same page, and can be seen via editor hovers/tooltips/quick info. While surfacing some commented-out settings might be helpful, the generated tsconfig.json was often considered overkill.
We also felt that it was time that tsc --init initialized with a few more prescriptive settings than we already enable. We looked at some common pain points and papercuts users have when they create a new TypeScript project. For example, most users write in modules (not global scripts), and --moduleDetection can force TypeScript to treat every implementation file as a module. Developers also often want to use the latest ECMAScript features directly in their runtime, so --target can typically be set to esnext. JSX users often find that going back to set --jsx is a needless friction, and its options are slightly confusing. And often, projects end up loading more declaration files from node_modules/@types than TypeScript actually needs – but specifying an empty types array can help limit this.
In TypeScript 5.9, a plain tsc --init with no other flags will generate the following tsconfig.json :
{
// Visit https://aka.ms/tsconfig to read more about this file
"compilerOptions" : {
// File Layout
// "rootDir": "./src",
// "outDir": "./dist",
// Environment Settings
// See also https://aka.ms/tsconfig_modules
"module" : "nodenext" ,
"target" : "esnext" ,
"types" : [ ] ,
// For nodejs:
// "lib": ["esnext"],
// "types": ["node"],
// and npm install -D @types/node
// Other Outputs
"sourceMap" : true ,
"declaration" : true ,
"declarationMap" : true ,
// Stricter Typechecking Options
"noUncheckedIndexedAccess" : true ,
"exactOptionalPropertyTypes" : true ,
// Style Options
// "noImplicitReturns": true,
// "noImplicitOverride": true,
// "noUnusedLocals": true,
// "noUnusedParameters": true,
// "noFallthroughCasesInSwitch": true,
// "noPropertyAccessFromIndexSignature": true,
// Recommended Options
"strict" : true ,
"jsx" : "react-jsx" ,
"verbatimModuleSyntax" : true ,
"isolatedModules" : true ,
"noUncheckedSideEffectImports" : true ,
"moduleDetection" : "force" ,
"skipLibCheck" : true ,
}
}
For more details, see the implementing pull request and discussion issue .

Support for import defer

TypeScript 5.9 introduces support for ECMAScript’s deferred module evaluation proposal using the new import defer syntax. This feature allows you to import a module without immediately executing the module and its dependencies, providing better control over when work and side-effects occur.
The syntax only permits namespace imports:
import defer * as feature from "./some-feature.js";
The key benefit of import defer is that the module is only evaluated on the first use. Consider this example:
// ./some-feature.ts
initializationWithSideEffects();
function initializationWithSideEffects() {
// ...
specialConstant = 42;
console.log("Side effects have occurred!");
}
export let specialConstant: number;
When using import defer, the initializationWithSideEffects() function will not be called until you actually access a property of the imported namespace:
import defer * as feature from "./some-feature.js";
// No side effects have occurred yet
// ...
// As soon as specialConstant is accessed, the contents of the feature
// module are run and side effects have taken place.
console.log(feature.specialConstant); // 42
Because evaluation of the module is deferred until you access a member off of the module, you cannot use named imports or default imports with import defer :
// ❌ Not allowed
import defer { doSomething } from "some-module";
// ❌ Not allowed
import defer defaultExport from "some-module";
// ✅ Only this syntax is supported
import defer * as feature from "some-module";
Note that when you write import defer , the module and its dependencies are fully loaded and ready for execution. That means that the module will need to exist, and will be loaded from the file system or a network resource. The key difference between a regular import and import defer is that the execution of statements and declarations is deferred until you access a property of the imported namespace.
This feature is particularly useful for conditionally loading modules with expensive or platform-specific initialization. It can also improve startup performance by deferring module evaluation for app features until they are actually needed.
Note that import defer is not transformed or “downleveled” at all by TypeScript. It is intended to be used in runtimes that support the feature natively, or by tools such as bundlers that can apply the appropriate transformation. That means that import defer will only work under the --module modes preserve and esnext .
We’d like to extend our thanks to Nicolò Ribaudo who championed the proposal in TC39 and also provided the implementation for this feature.

Support for --module node20

TypeScript provides several node* options for the --module and --moduleResolution settings. Most recently, --module nodenext has supported the ability to require() ECMAScript modules from CommonJS modules, and correctly rejects import assertions (in favor of the standards-bound import attributes ).
TypeScript 5.9 brings a stable option for these settings called node20 , intended to model the behavior of Node.js v20. This option is unlikely to have new behaviors in the future, unlike --module nodenext or --moduleResolution nodenext . Also unlike nodenext , specifying --module node20 will imply --target es2023 unless otherwise configured. --module nodenext , on the other hand, implies the floating --target esnext .
For more information, take a look at the implementation here .

Summary Descriptions in DOM APIs

Previously, many of the DOM APIs in TypeScript only linked to the MDN documentation for the API. These links were useful, but they didn’t provide a quick summary of what the API does. Thanks to a few changes from Adam Naji , TypeScript now includes summary descriptions for many DOM APIs based on the MDN documentation. You can see more of these changes here and here .

Expandable Hovers (Preview)

Quick Info (also called “editor tooltips” and “hovers”) can be very useful for peeking at variables to see their types, or at type aliases to see what they actually refer to. Still, it’s common for people to want to go deeper and get details from whatever’s displayed within the quick info tooltip. For example, if we hover our mouse over the parameter options in the following example:
export function drawButton(options: Options): void
We’re left with (parameter) options: Options .
Do we really need to jump to the definition of the type Options just to see what members this value has?
To help here, TypeScript 5.9 is now previewing a feature called expandable hovers , or “quick info verbosity”. If you use an editor like VS Code, you’ll now see a + and - button on the left of these hover tooltips. Clicking on the + button will expand out types more deeply, while clicking on the - will go back to the last view.
This feature is currently in preview, and we are seeking feedback for both TypeScript and our partners on Visual Studio Code. For more details, see the PR for this feature here .

Configurable Maximum Hover Length

Occasionally, quick info tooltips can become so long that TypeScript will truncate them to make them more readable. The downside here is that often the most important information will be omitted from the hover tooltip, which can be frustrating. To help with this, TypeScript 5.9’s language server supports a configurable hover length, which can be configured in VS Code via the js/ts.hover.maximumLength setting.
Additionally, the new default hover length is substantially larger than the previous default. This means that in TypeScript 5.9, you should see more information in your hover tooltips by default. For more details, see the PR for this feature here and the corresponding change to Visual Studio Code here .

Optimizations

• Cache Instantiations on Mappers
  When TypeScript replaces type parameters with specific type arguments, it can end up instantiating many of the same intermediate types over and over again. In complex libraries like Zod and tRPC, this could lead to both performance issues and errors reported around excessive type instantiation depth. Thanks to a change from Mateusz Burzyński , TypeScript 5.9 is able to cache many intermediate instantiations when work has already begun on a specific type instantiation. This in turn avoids lots of unnecessary work and allocations.

• Avoiding Closure Creation in fileOrDirectoryExistsUsingSource
  In JavaScript, a function expression will typically allocate a new function object, even if the wrapper function is just passing through arguments to another function with no captured variables. In code paths around file existence checks, Vincent Bailly found examples of these pass-through function calls, even though the underlying functions only took single arguments. Given the number of existence checks that could take place in larger projects, he cited a speed-up of around 11%. See more on this change here .

What’s Next?

As you might have heard, much of our recent focus has been on the native port of TypeScript which will eventually be available as TypeScript 7. You can actually try out the native port today by checking out TypeScript native previews , which are released nightly.
However, we are still developing TypeScript 5.9 and addressing issues, and encourage you to try it out and give us feedback. If you need a snapshot of TypeScript that’s newer than the beta, TypeScript also has nightly releases , and you can try out the latest editing experience by installing the JavaScript and TypeScript Nightly extension for Visual Studio Code .
So we hope you try out the beta or a nightly release today and let us know what you think!
Happy Hacking!
– Daniel Rosenwasser and the TypeScript Team

Original source Report a problem

TypeScript Native Previews

This past March we unveiled our efforts to port the TypeScript compiler and toolset to native code. This port has achieved a 10x speed-up on most projects not just by using a natively-compiled language (Go), but also through using shared memory parallelism and concurrency where we can benefit. Since then, we have made several strides towards running on large complex real-world projects.

Today, we are excited to announce broad availability of TypeScript Native Previews. As of today, you will be able to use npm to get a preview of the native TypeScript compiler. Additionally, you’ll be able to use a preview version of our editor functionality for VS Code through the Visual Studio Marketplace.

To get the compiler over npm, you can run the following command in your project:

npm install -D @typescript/native-preview

This package provides an executable called tsgo. This executable runs similarly to tsc, the existing executable that the typescript package makes available:

npx tsgo --project ./src/tsconfig.json

Eventually we will rename tsgo to tsc and move it to the typescript package. For now, it lives separately for easier testing. The new executable is still a work in progress, but is suitable to type-check and build many real-world projects.

But we know that a command-line compiler is only half the story. We’ll have heard teams are eager to see what the new editing experience is like too, and so you can now install the new TypeScript (Native Preview) extension in Visual Studio Code. You can easily install it off of the VS Code Extension Marketplace.

Because the extension is still in early stages of development, it defers to the built-in TypeScript extension in VS Code. For that reason, the extension will need to be enabled even after installation. You can do this by opening VS Code’s command palette and running the command “TypeScript Native Preview: Enable (Experimental)”.

Alternatively, you can toggle this in your settings UI by configuring “TypeScript > Experimental: Use Tsgo” or by adding the following line to your JSON settings:

"typescript.experimental.useTsgo": true,

Updates, Release Cadence, and Roadmap

These previews will eventually become TypeScript 7 and will be published nightly so that you can easily try the latest developments on the TypeScript native port effort. If you use the VS Code extension, you should get automatic updates by default. If for whatever reason you find any sort of disruption, we encourage you to file an issue and temporarily disable the new language service with the command “TypeScript Native Preview: Disable” or by configuring any of the settings mentioned above.

Keep in mind, these native previews are missing lots of functionality that stable versions of TypeScript have today. That includes command-line functionality like --build (though individual projects can still be built with tsgo), --declaration emit, and certain downlevel emit targets. Similarly, editor functionality like auto-imports, find-all-references, and rename are still pending implementation. But we encourage developers to check back frequently, as we’ll be hard at work on these features!

What’s New?

Since our initial announcement, we have made some notable strides in type-checking support, testability, editor support, and APIs. We would also love to give a brief update of what we’ve accomplished, plus what’s on the horizon.

Note that while the native preview will eventually be called TypeScript 7, we’ve casually been referring to it as Project Corsa in the meantime. We’ve also been more explicit in referring to the codebase that makes up TypeScript 5.8 as our current JS-based codebase, or Strada. So in our updates, you’ll see us differentiate the native and stable versions of TypeScript as Corsa (TS7) and Strada (TS 5.8).

Fuller Type-Checking Support

The majority of the type-checker has been ported at this time. That is to say, most projects should see the same errors apart from those affected by some intentional changes (e.g. this change to the ordering of types) and some stale definitions in lib.d.ts. If you see any divergences and differences, we encourage you to file an issue to let us know.

It is worth calling out support for two major type-checking features that have been added since our initial announcement: JSX and JavaScript+JSDoc.

JSX Checking Support

When developers first got access to the TypeScript native port, we had to temper expectations. While type-checking was pretty far along, some constructs were still not fully checked yet. For many developers, the most notable omission was JSX. While Corsa was able to parse JSX, it would mostly just pass over JSX expressions when type-checking and note that JSX was not-yet supported.

Since then, we’ve actually added type-checking support for JSX and we can get a better sense of how fast a real JSX project can be built. As an example codebase, we looked at the codebase for Sentry. If you run TypeScript 5.8 from the repository root with --extendedDiagnostics --noEmit, you’ll get something like the following:

$ tsc -p . --noEmit --extendedDiagnostics
Files: 9306
Lines of Library: 43159
Lines of Definitions: 352182
Lines of TypeScript: 1113969
Lines of JavaScript: 1106
Lines of JSON: 304
Lines of Other: 0
Identifiers: 1956007
Symbols: 3563371
Types: 999619
Instantiations: 3675199
Memory used: 3356832K
Assignability cache size: 944737
Identity cache size: 43226
Subtype cache size: 110171
Strict subtype cache size: 430338
I/O Read time: 1.40s
Parse time: 3.48s
ResolveModule time: 1.88s
ResolveTypeReference time: 0.02s
ResolveLibrary time: 0.01s
Program time: 7.78s
Bind time: 1.77s
Check time: 63.26s
printTime time: 0.00s
Emit time: 0.00s
Total time: 72.81s

$ tsgo -p . --noEmit --extendedDiagnostics
...
Files: 9292
Lines: 1508361
Identifiers: 1954236
Symbols: 5011565
Types: 1689528
Instantiations: 6524885
Memory used: 3892267K
Memory allocs: 61043466
Parse time: 0.712s
Bind time: 0.133s
Check time: 5.882s
Emit time: 0.012s
Total time: 6.761s

There are some discrepancies in results, but Corsa brings build times down from over a minute to just under 7 seconds on the same machine. Your results may vary, but in general we’ve seen consistent speed-ups of over 10x on this specific example. You can try introducing an error in existing JSX code and tsgo will catch it.

You can see more over at our PR to port JSX type-checking, plus some follow-on support for tslib and JSX factory imports.

JavaScript Checking

TypeScript supports parsing and type-checking JavaScript files. Because valid JavaScript/ECMAScript doesn’t support type-specific syntax like annotations or interface and type declarations, TypeScript looks at JSDoc comments in JS source code for type analysis.

The native previews of TypeScript now also support type-checking JS files. In developing JS type-checking for Corsa, we revisited our early decisions in our implementation. JavaScript support was built up in a very organic way and, in turn, analyzed very specific patterns that may no longer be (or may never have been) in widespread use. In order to simplify the new codebase, JavaScript support has been rewritten rather than ported. As a result, there may be some constructs that may need to be rewritten, or to use a more idiomatic/modern JS style.

If this causes difficulties for your project, we are open to feedback on the issue tracker.

Editor Support & LSP Progress

When we released the Corsa codebase, it included a very rudimentary LSP-based language server. While most tangible development has been on the compiler itself, we have been iterating on multiple fronts to port our editor functionality in this new system. Because the Strada codebase communicates with editors via the TSServer format that predates LSP, we are not aiming to do a perfect 1:1 port between the codebases. This means that porting code often requires more manual porting and has required a bit more up-front thought in how we generate type definitions that conform to LSP. Gathering errors/diagnostics, go-to-definition, and hover work in very early stages.

Most recently, we have hit another milestone: we have enabled completions! While auto-imports and other features around completions are not fully ported, this may be enough for many teams in large codebases. Going forward, our priorities are in porting over our existing language server test suite, along with enabling find-all-references, rename, and signature help.

API Progress

A big challenge as part of this port will be continuity with API consumers of TypeScript. We have the initial foundation of an API layer that can be leveraged over standard I/O. This work means that API consumers can communicate with a TypeScript process through IPC regardless of the consuming language. Since we know that many API consumers will be writing TypeScript and JavaScript code, we also have JavaScript-based clients for interacting with the API.

Because so much TypeScript API usage today is synchronous, we wanted to make it possible to communicate with this process in a synchronous way. Node.js unfortunately doesn’t provide an easy way to communicate synchronously with a child process, so we developed a native Node.js module in Rust (which should make lots of people happy) called libsyncrpc.

We are still in the early days of API design here, but we are open to thoughts and feedback on the matter. More details about the current API server are available here.

Known and Notable Differences

As we’ve mentioned, the Corsa compiler and language service may still have some differences from Strada. There are some differences that you may hit early on in trying out tsgo and the Native Preview VS Code extension.

Some of those come from eventual TypeScript 6.0 deprecations, like node/node10 resolution (in favor of node16, nodenext, and bundler). If you use --moduleResolution node or --module commonjs, you may see some errors like:
Cannot find module 'blah' or its corresponding type declarations.
Module '"module"' has no exported member 'Thing'.

You will get consistent errors if you switch your tsconfig.json settings to use
{
"compilerOptions": {
// ...
"module": "preserve",
"moduleResolution": "bundler",
}
}
or
{
"compilerOptions": {
// ...
"module": "nodenext"
}
}

These can be manually fixed depending on your configuration, though often you can remove the imports and leverage auto-imports to do the right thing.

Beyond deprecations, downlevel emit to older targets is limited, and JSX emit only works as far as preserving what you wrote. Declaration emit is currently not supported either. --build mode and language service functionality around project references is still not available, though project dependencies can be built through tsc, and the native preview language service can often leverage generated .d.ts files.

What’s Next?

By later this year, we will aim to have a more complete version of our compiler with major features like --build, along with most language service features for editors.

But we don’t expect you to wait that long! As TypeScript Native Previews are published nightly, we’ll aim to provide periodic updates on major notable developments. So give the native previews a shot!

Happy Hacking!

— Daniel Rosenwasser and the TypeScript Team

Original source Report a problem

Today I’m excited to announce the next steps we’re taking to radically improve TypeScript performance.
The core value proposition of TypeScript is an excellent developer experience. As your codebase grows, so does the value of TypeScript itself, but in many cases TypeScript has not been able to scale up to the very largest codebases. Developers working in large projects can experience long load and check times, and have to choose between reasonable editor startup time or getting a complete view of their source code. We know developers love when they can rename variables with confidence, find all references to a particular function, easily navigate their codebase, and do all of those things without delay. New experiences powered by AI benefit from large windows of semantic information that need to be available with tighter latency constraints. We also want fast command-line builds to validate that your entire codebase is in good shape.
To meet those goals, we’ve begun work on a native port of the TypeScript compiler and tools. The native implementation will drastically improve editor startup, reduce most build times by 10x, and substantially reduce memory usage. By porting the current codebase, we expect to be able to preview a native implementation of tsc capable of command-line typechecking by mid-2025, with a feature-complete solution for project builds and a language service by the end of the year.
You can build and run the Go code from our new working repo, which is offered under the same license as the existing TypeScript codebase. Check the README for instructions on how to build and run tsc and the language server, and to see a summary of what’s implemented so far. We’ll be posting regular updates as new functionality becomes available for testing.

How Much Faster?

Our native implementation is already capable of loading many popular TypeScript projects, including the TypeScript compiler itself. Here are times to run tsc on some popular codebases on GitHub of varying sizes:

Codebase | Size (LOC) | Current | Native | Speedup
VS Code | 1,505,000 | 77.8s | 7.5s | 10.4x
Playwright | 356,000 | 11.1s | 1.1s | 10.1x
TypeORM | 270,000 | 17.5s | 1.3s | 13.5x
date-fns | 104,000 | 6.5s | 0.7s | 9.5x
tRPC (server + client) | 18,000 | 5.5s | 0.6s | 9.1x
rxjs (observable) | 2,100 | 1.1s | 0.1s | 11.0x

While we’re not yet feature-complete, these numbers are representative of the order of magnitude performance improvement you’ll see checking most codebases.
We’re incredibly excited about the opportunities that this massive speed boost creates. Features that once seemed out of reach are now within grasp. This native port will be able to provide instant, comprehensive error listings across an entire project, support more advanced refactorings, and enable deeper insights that were previously too expensive to compute. This new foundation goes beyond today’s developer experience and will enable the next generation of AI tools to enhance development, powering new tools that will learn, adapt, and improve the coding experience.

Editor Speed

Most developer time is spent in editors, and it’s where performance is most important. We want editors to load large projects quickly, and respond quickly in all situations. Modern editors like Visual Studio and Visual Studio Code have excellent performance as long as the underlying language services are also fast. With our native implementation, we’ll be able to provide incredibly fast editor experiences.
Again using the Visual Studio Code codebase as a benchmark, the current time to load the entire project in the editor on a fast computer is about 9.6 seconds. This drops down to about 1.2 seconds with the native language service, an 8x improvement in project load time in editor scenarios. What this translates to is a faster working experience from the time you open your editor to your first keystroke in any TypeScript codebase. We expect all projects to see this level of improvement in load time.
Overall memory usage also appears to be roughly half of the current implementation, though we haven’t actively investigated optimizing this yet and expect to realize further improvements. Editor responsiveness for all language service operations (including completion lists, quick info, go to definition, and find all references) will also see significant speed gains. We’ll also be moving to the Language Server Protocol (LSP), a longstanding infrastructural work item to better align our implementation with other languages.

Versioning Roadmap

Our most recent TypeScript release was TypeScript 5.8, with TypeScript 5.9 coming soon. The JS-based codebase will continue development into the 6.x series, and TypeScript 6.0 will introduce some deprecations and breaking changes to align with the upcoming native codebase.
When the native codebase has reached sufficient parity with the current TypeScript, we’ll be releasing it as TypeScript 7.0. This is still in development and we’ll be announcing stability and feature milestones as they occur.
For the sake of clarity, we’ll refer to them simply as TypeScript 6 (JS) and TypeScript 7 (native), since this will be the nomenclature for the foreseeable future. You may also see us refer to “Strada” (the original TypeScript codename) and “Corsa” (the codename for this effort) in internal discussions or code comments.
While some projects may be able to switch to TypeScript 7 upon release, others may depend on certain API features, legacy configurations, or other constraints that necessitate using TypeScript 6. Recognizing TypeScript’s critical role in the JS development ecosystem, we’ll still be maintaining the JS codebase in the 6.x line until TypeScript 7+ reaches sufficient maturity and adoption.
Our long-term goal is to keep these versions as closely aligned as possible so that you can upgrade to TypeScript 7 as soon as it meets your requirements, or fall back to TypeScript 6 if necessary.

Next Steps

In the coming months we’ll be sharing more about this exciting effort, including deeper looks into performance, a new compiler API, LSP, and more. We’ve written up some FAQs on the GitHub repo to address some questions we expect you might have. We also invite you to join us for an AMA at the TypeScript Community Discord at 10 AM PDT | 5 PM UTC on March 13th.
A 10x performance improvement represents a massive leap in the TypeScript and JavaScript development experience, so we hope you are as enthusiastic as we are for this effort!

Original source Report a problem

Today we’re excited to announce the release of TypeScript 5.8!

If you’re not familiar with TypeScript, it’s a language that builds on top of JavaScript by adding syntax for types. Writing types in our code allows us to explain intent and have other tools check our code to catch mistakes like typos, issues with null and undefined, and more. Types also power TypeScript’s editor tooling like the auto-completion, code navigation, and refactorings that you might see in editors like Visual Studio and VS Code. In fact, TypeScript and its ecosystem powers the JavaScript experience in both of those editors as well!

To get started using TypeScript, you can install it through npm with the following command:

npm install -D typescript

Let’s take a look at what’s new in TypeScript 5.8!

What’s New Since the Beta and RC?

Since our beta release, we have had to pull back some work on how functions with conditional return types are checked. Based on some of the limitations and changes we wanted to make, we decided to iterate on the feature with the goal of shipping it in TypeScript 5.9. However, as part of this work, we added more granular checks for branches within return expressions. This enhancement was not documented in the beta post, but will remain in TypeScript 5.8.
No new major changes have been added since the release candidate.

Granular Checks for Branches in Return Expressions

Consider some code like the following:

declare const untypedCache: Map<any, any>;
function getUrlObject(urlString: string): URL {
  return untypedCache.has(urlString) ? untypedCache.get(urlString) : urlString;
}

The intent of this code is to retrieve a URL object from a cache if it exists, or to create a new URL object if it doesn’t. However, there’s a bug: we forgot to actually construct a new URL object with the input. Unfortunately, TypeScript generally didn’t catch this sort of bug.
When TypeScript checks conditional expressions like cond ? trueBranch : falseBranch, its type is treated as a union of the types of the two branches. In other words, it gets the type of trueBranch and falseBranch, and combines them into a union type. In this case, the type of untypedCache.get(urlString) is any, and the type of urlString is string. This is where things go wrong because any is so infectious in how it interacts with other types. The union any | string is simplified to any, so by the time TypeScript starts checking whether the expression in our return statement is compatible with the expected return type of URL, the type system has lost any information that would have caught the bug in this code.
In TypeScript 5.8, the type system special-cases conditional expressions directly inside return statements. Each branch of the conditional is checked against the declared return type of the containing functions (if one exists), so the type system can catch the bug in the example above.
This change was made within this pull request, as part of a broader set of future improvements for TypeScript.

Support for require() of ECMAScript Modules in --module nodenext

Support for require() of ECMAScript Modules in --module nodenext
For years, Node.js supported ECMAScript modules (ESM) alongside CommonJS modules. Unfortunately, the interoperability between the two had some challenges.

• ESM files could import CommonJS files
• CommonJS files could not require() ESM files
  In other words, consuming CommonJS files from ESM files was possible, but not the other way around. This introduced many challenges for library authors who wanted to provide ESM support. These library authors would either have to break compatibility with CommonJS users, “dual-publish” their libraries (providing separate entry-points for ESM and CommonJS), or just stay on CommonJS indefinitely. While dual-publishing might sound like a good middle-ground, it is a complex and error-prone process that also roughly doubles the amount of code within a package.
  Node.js 22 relaxes some of these restrictions and permits require("esm") calls from CommonJS modules to ECMAScript modules. Node.js still does not permit require() on ESM files that contain a top-level await, but most other ESM files are now consumable from CommonJS files. This presents a major opportunity for library authors to provide ESM support without having to dual-publish their libraries.
  TypeScript 5.8 supports this behavior under the --module nodenext flag. When --module nodenext is enabled, TypeScript will avoid issuing errors on these require() calls to ESM files.
  Because this feature may be back-ported to older versions of Node.js, there is currently no stable --module nodeXXXX option that enables this behavior; however, we predict future versions of TypeScript may be able to stabilize the feature under node20. In the meantime, we encourage users of Node.js 22 and newer to use --module nodenext, while library authors and users of older Node.js versions should remain on --module node16 (or make the minor update to --module node18).
  For more information, see our support for require(“esm”) here.

--module node18

--module node18
TypeScript 5.8 introduces a stable --module node18 flag. For users who are fixed on using Node.js 18, this flag provides a stable point of reference that does not incorporate certain behaviors that are in --module nodenext. Specifically:

• require() of ECMAScript modules is disallowed under node18, but allowed under nodenext
• import assertions (deprecated in favor of import attributes) are allowed under node18, but are disallowed under nodenext
  See more at both the --module node18 pull request and changes made to --module nodenext.

The --erasableSyntaxOnly Option

The --erasableSyntaxOnly Option
Recently, Node.js 23.6 unflagged experimental support for running TypeScript files directly; however, only certain constructs are supported under this mode. Node.js has unflagged a mode called --experimental-strip-types which requires that any TypeScript-specific syntax cannot have runtime semantics. Phrased differently, it must be possible to easily erase or “strip out” any TypeScript-specific syntax from a file, leaving behind a valid JavaScript file.
That means constructs like the following are not supported:

• enum declarations
• namespaces and modules with runtime code
• parameter properties in classes
• import = aliases
  Here are some examples of what does not work:

// ❌ error: A namespace with runtime code.
namespace container {
  foo.method();
  export type Bar = string;
}
// ❌ error: An `import =` alias
import Bar = container.Bar;
class Point {
  // ❌ error: Parameter properties
  constructor(public x: number, public y: number) { }
}
// ❌ error: An enum declaration.
enum Direction {
  Up, Down, Left, Right,
}

Similar tools like ts-blank-space or Amaro (the underlying library for type-stripping in Node.js) have the same limitations. These tools will provide helpful error messages if they encounter code that doesn’t meet these requirements, but you still won’t find out your code doesn’t work until you actually try to run it.
That’s why TypeScript 5.8 introduces the --erasableSyntaxOnly flag. When this flag is enabled, TypeScript will error on most TypeScript-specific constructs that have runtime behavior.
Typically, you will want to combine this flag with the --verbatimModuleSyntax, which ensures that a module contains the appropriate import syntax, and that import elision does not take place.
For more information, see the implementation here.

The --libReplacement Flag

The --libReplacement Flag
In TypeScript 4.5, we introduced the possibility of substituting the default lib files with custom ones. This was based on the possibility of resolving a library file from packages named @typescript/lib-*. For example, you could lock your dom libraries onto a specific version of the @types/web package with the following package.json:

{
  "devDependencies": {
    "@typescript/lib-dom": "npm:@types/[email protected]"
  }
}

When installed, a package called @typescript/lib-dom should exist, and TypeScript will currently always look it up when dom is implied by your settings.
This is a powerful feature, but it also incurs a bit of extra work. Even if you’re not using this feature, TypeScript always performs this lookup, and has to watch for changes in node_modules in case a lib-replacement package begins to exist.
TypeScript 5.8 introduces the --libReplacement flag, which allows you to disable this behavior. If you’re not using --libReplacement, you can now disable it with --libReplacement false. In the future --libReplacement false may become the default, so if you currently rely on the behavior you should consider explicitly enabling it with --libReplacement true.
For more information, see the change here.

Preserved Computed Property Names in Declaration Files

In an effort to make computed properties have more predictable emit in declaration files, TypeScript 5.8 will consistently preserve entity names (bareVariables and dotted.names.that.look.like.this) in computed property names in classes.
For example, consider the following code:

export let propName = "theAnswer";
export class MyClass {
  [propName] = 42;
  //  ~~~~~~~~~~
  // error!
  // A computed property name in a class property declaration must have a simple literal type or a 'unique symbol' type.
}

Previous versions of TypeScript would issue an error when generating a declaration file for this module, and a best-effort declaration file would generate an index signature.

export declare let propName: string;
export declare class MyClass {
  [x: string]: number;
}

In TypeScript 5.8, the example code is now allowed, and the emitted declaration file will match what you wrote:

export declare let propName: string;
export declare class MyClass {
  [propName]: number;
}

Note that this does not create statically-named properties on the class. You’ll still end up with what is effectively an index signature like [x: string]: number, so for that use case, you’d need to use unique symbols or literal types.
Note that writing this code was and currently is an error under the --isolatedDeclarations flag; but we expect that thanks to this change, computed property names will generally be permitted in declaration emit.
Note that it’s possible (though unlikely) that a file compiled in TypeScript 5.8 may generate a declaration file that is not backward compatible in TypeScript 5.7 or earlier.
For more information, see the implementing PR.

Optimizations on Program Loads and Updates

TypeScript 5.8 introduces a number of optimizations that can both improve the time to build up a program, and also to update a program based on a file change in either --watch mode or editor scenarios.
First, TypeScript now avoids array allocations that would be involved while normalizing paths. Typically, path normalization would involve segmenting each portion of a path into an array of strings, normalizing the resulting path based on relative segments, and then joining them back together using a canonical separator. For projects with many files, this can be a significant and repetitive amount of work. TypeScript now avoids allocating an array, and operates more directly on indexes of the original path.
Additionally, when edits are made that don’t change the fundamental structure of a project, TypeScript now avoids re-validating the options provided to it (e.g. the contents of a tsconfig.json). This means, for example, that a simple edit might not require checking that the output paths of a project don’t conflict with the input paths. Instead, the results of the last check can be used. This should make edits in large projects feel more responsive.

Notable Behavioral Changes

This section highlights a set of noteworthy changes that should be acknowledged and understood as part of any upgrade. Sometimes it will highlight deprecations, removals, and new restrictions. It can also contain bug fixes that are functionally improvements, but which can also affect an existing build by introducing new errors.

lib.d.ts

lib.d.ts
Types generated for the DOM may have an impact on type-checking your codebase. For more information, see linked issues related to DOM and lib.d.ts updates for this version of TypeScript.

Restrictions on Import Assertions Under --module nodenext

Restrictions on Import Assertions Under --module nodenext
Import assertions were a proposed addition to ECMAScript to ensure certain properties of an import (e.g. “this module is JSON, and is not intended to be executable JavaScript code”). They were reinvented as a proposal called import attributes. As part of the transition, they swapped from using the assert keyword to using the with keyword.
// An import assertion ❌ - not future-compatible with most runtimes.
import data from "./data.json" assert { type: "json" };
// An import attribute ✅ - the preferred way to import a JSON file.
import data from "./data.json" with { type: "json" };
Node.js 22 no longer accepts import assertions using the assert syntax. In turn when --module nodenext is enabled in TypeScript 5.8, TypeScript will issue an error if it encounters an import assertion.

What’s Next?

The next version of TypeScript will be TypeScript 5.9, and we’ll have more details through an upcoming iteration plan on our issue tracker. That will have precise target dates and more details on upcoming features that we plan to work on. In the meantime, you can try out early versions of TypeScript 5.9 today by installing our nightly builds from npm with

npm install typescript@next

or by using the VS Code TypeScript Nightly extension.
Until then, TypeScript 5.8 is here and ready to use, and we’re excited for you to install it today! We hope that this release makes your day-to-day coding a joy.
Happy Hacking!
– Daniel Rosenwasser and the TypeScript Team

Original source Report a problem

Release Candidate of TypeScript 5.8

Today we are excited to announce the Release Candidate (RC) of TypeScript 5.8!
To get started using the Release Candidate, you can get it through npm with the following command:

npm install -D typescript@rc

Let’s take a look at what’s new in TypeScript 5.8!

What’s New Since the Beta?

Since our beta release, we have had to pull back some work on how functions with conditional return types are checked. Based on some of the limitations and changes we wanted to make, we decided to iterate on the feature with the goal of shipping it in TypeScript 5.9. However, as part of this work, we added more granular checks for branches within return expressions. This enhancement was not documented in the beta post, but will remain in TypeScript 5.8.

Granular Checks for Branches in Return Expressions

Consider some code like the following:

declare const untypedCache: Map<any, any>;
function getUrlObject(urlString: string): URL {
  return untypedCache.has(urlString) ? untypedCache.get(urlString) : urlString;
}

The intent of this code is to retrieve a URL object from a cache if it exists, or to create a new URL object if it doesn’t. However, there’s a bug: we forgot to actually construct a new URL object with the input. Unfortunately, TypeScript generally didn’t catch this sort of bug.
When TypeScript checks conditional expressions like cond ? trueBranch : falseBranch, its type is treated as a union of the types of the two branches. In other words, it gets the type of trueBranch and falseBranch, and combines them into a union type. In this case, the type of untypedCache.get(urlString) is any, and the type of urlString is string. This is where things go wrong because any is so infectious in how it interacts with other types. The union any | string is simplified to any, so by the time TypeScript starts checking whether the expression in our return statement is compatible with the expected return type of URL, the type system has lost any information that would have caught the bug in this code.
In TypeScript 5.8, the type system special-cases conditional expressions directly inside return statements. Each branch of the conditional is checked against the declared return type of the containing functions (if one exists), so the type system can catch the bug in the example above.
This change was made within this pull request, as part of a broader set of future improvements for TypeScript.

#### Support for require() of ECMAScript Modules in --module nodenext
For years, Node.js supported ECMAScript modules (ESM) alongside CommonJS modules. Unfortunately, the interoperability between the two had some challenges.
- ESM files could import CommonJS files
- CommonJS files could not require() ESM files

In other words, consuming CommonJS files from ESM files was possible, but not the other way around. This introduced many challenges for library authors who wanted to provide ESM support. These library authors would either have to break compatibility with CommonJS users, "dual-publish" their libraries (providing separate entry-points for ESM and CommonJS), or just stay on CommonJS indefinitely. While dual-publishing might sound like a good middle-ground, it is a complex and error-prone process that also roughly doubles the amount of code within a package.
Node.js 22 relaxes some of these restrictions and permits require("esm") calls from CommonJS modules to ECMAScript modules. Node.js still does not permit require() on ESM files that contain a top-level await, but most other ESM files are now consumable from CommonJS files. This presents a major opportunity for library authors to provide ESM support without having to dual-publish their libraries.
TypeScript 5.8 supports this behavior under the --module nodenext flag. When --module nodenext is enabled, TypeScript will avoid issuing errors on these require() calls to ESM files.
Because this feature may be back-ported to older versions of Node.js, there is currently no stable --module nodeXXXX option that enables this behavior; however, we predict future versions of TypeScript may be able to stabilize the feature under node20. In the meantime, we encourage users of Node.js 22 and newer to use --module nodenext, while library authors and users of older Node.js versions should remain on --module node16 (or make the minor update to --module node18).
For more information, see our support for require("esm") here.

#### --module node18
TypeScript 5.8 introduces a stable --module node18 flag. For users who are fixed on using Node.js 18, this flag provides a stable point of reference that does not incorporate certain behaviors that are in --module nodenext. Specifically:
- require() of ECMAScript modules is disallowed under node18, but allowed under nodenext
- import assertions (deprecated in favor of import attributes) are allowed under node18, but are disallowed under nodenext
See more at both the --module node18 pull request and changes made to --module nodenext.

### The --erasableSyntaxOnly Option
Recently, Node.js 23.6 unflagged experimental support for running TypeScript files directly; however, only certain constructs are supported under this mode. Node.js has unflagged a mode called --experimental-strip-types which requires that any TypeScript-specific syntax cannot have runtime semantics. Phrased differently, it must be possible to easily erase or "strip out" any TypeScript-specific syntax from a file, leaving behind a valid JavaScript file.
That means constructs like the following are not supported:
- enum declarations
- namespaces and modules with runtime code
- parameter properties in classes
- import = aliases
Here are some examples of what does not work:
```ts
// ❌ error: A namespace with runtime code.
namespace container {
  foo.method();
  export type Bar = string;
}
// ❌ error: An `import =` alias
import Bar = container.Bar;
class Point {
  // ❌ error: Parameter properties
  constructor(public x: number, public y: number) { }
}
// ❌ error: An enum declaration.
enum Direction {
  Up, Down, Left, Right,
}

Similar tools like ts-blank-space or Amaro (the underlying library for type-stripping in Node.js) have the same limitations. These tools will provide helpful error messages if they encounter code that doesn’t meet these requirements, but you still won’t find out your code doesn’t work until you actually try to run it.
That’s why TypeScript 5.8 introduces the --erasableSyntaxOnly flag. When this flag is enabled, TypeScript will error on most TypeScript-specific constructs that have runtime behavior.

class C {
  constructor(public x: number) { }
  //          ~~~~~~~~~~~~~~~~
  // error! This syntax is not allowed when 'erasableSyntaxOnly' is enabled.
}

Typically, you will want to combine this flag with the --verbatimModuleSyntax, which ensures that a module contains the appropriate import syntax, and that import elision does not take place.
For more information, see the implementation here.

The --libReplacement Flag

In TypeScript 4.5, we introduced the possibility of substituting the default lib files with custom ones. This was based on the possibility of resolving a library file from packages named @typescript/lib-*. For example, you could lock your dom libraries onto a specific version of the @types/web package with the following package.json:

{
  "devDependencies": {
    "@typescript/lib-dom": "npm:@types/[email protected]"
  }
}

When installed, a package called @typescript/lib-dom should exist, and TypeScript will currently always look it up when dom is implied by your settings.
This is a powerful feature, but it also incurs a bit of extra work. Even if you’re not using this feature, TypeScript always performs this lookup, and has to watch for changes in node_modules in case a lib-replacement package begins to exist.
TypeScript 5.8 introduces the --libReplacement flag, which allows you to disable this behavior. If you’re not using --libReplacement, you can now disable it with --libReplacement false. In the future --libReplacement false may become the default, so if you currently rely on the behavior you should consider explicitly enabling it with --libReplacement true.
For more information, see the change here.

Preserved Computed Property Names in Declaration Files

In an effort to make computed properties have more predictable emit in declaration files, TypeScript 5.8 will consistently preserve entity names (bareVariables and dotted.names.that.look.like.this) in computed property names in classes.
For example, consider the following code:

export let propName = "theAnswer";
export class MyClass {
  [propName] = 42;
  //  ~~~~~~~~~~
  // error!
  // A computed property name in a class property declaration must have a simple literal type or a 'unique symbol' type.
}

Previous versions of TypeScript would issue an error when generating a declaration file for this module, and a best-effort declaration file would generate an index signature.

export declare let propName: string;
export declare class MyClass {
  [x: string]: number;
}

In TypeScript 5.8, the example code is now allowed, and the emitted declaration file will match what you wrote:

export declare let propName: string;
export declare class MyClass {
  [propName]: number;
}

Note that this does not create statically-named properties on the class. You’ll still end up with what is effectively an index signature like [x: string]: number, so for that use case, you’d need to use unique symbols or literal types.
Note that writing this code was and currently is an error under the --isolatedDeclarations flag; but we expect that thanks to this change, computed property names will generally be permitted in declaration emit.
Note that it’s possible (though unlikely) that a file compiled in TypeScript 5.8 may generate a declaration file that is not backward compatible in TypeScript 5.7 or earlier.
For more information, see the implementing PR.

Optimizations on Program Loads and Updates

TypeScript 5.8 introduces a number of optimizations that can both improve the time to build up a program, and also to update a program based on a file change in either --watch mode or editor scenarios.
First, TypeScript now avoids array allocations that would be involved while normalizing paths. Typically, path normalization would involve segmenting each portion of a path into an array of strings, normalizing the resulting path based on relative segments, and then joining them back together using a canonical separator. For projects with many files, this can be a significant and repetitive amount of work. TypeScript now avoids allocating an array, and operates more directly on indexes of the original path.
Additionally, when edits are made that don’t change the fundamental structure of a project, TypeScript now avoids re-validating the options provided to it (e.g. the contents of a tsconfig.json). This means, for example, that a simple edit might not require checking that the output paths of a project don’t conflict with the input paths. Instead, the results of the last check can be used. This should make edits in large projects feel more responsive.

Notable Behavioral Changes

This section highlights a set of noteworthy changes that should be acknowledged and understood as part of any upgrade. Sometimes it will highlight deprecations, removals, and new restrictions. It can also contain bug fixes that are functionally improvements, but which can also affect an existing build by introducing new errors.

lib.d.ts

lib.d.ts
Types generated for the DOM may have an impact on type-checking your codebase. For more information, see linked issues related to DOM and lib.d.ts updates for this version of TypeScript.

Restrictions on Import Assertions Under --module nodenext

Restrictions on Import Assertions Under --module nodenext
Import assertions were a proposed addition to ECMAScript to ensure certain properties of an import (e.g. "this module is JSON, and is not intended to be executable JavaScript code"). They were reinvented as a proposal called import attributes. As part of the transition, they swapped from using the assert keyword to using the with keyword.

// An import assertion ❌ - not future-compatible with most runtimes.
import data from "./data.json" assert { type: "json" };
// An import attribute ✅ - the preferred way to import a JSON file.
import data from "./data.json" with { type: "json" };

Node.js 22 no longer accepts import assertions using the assert syntax. In turn when --module nodenext is enabled in TypeScript 5.8, TypeScript will issue an error if it encounters an import assertion.

What’s Next?

At this point, we anticipate very few changes to TypeScript 5.8 apart from critical bug fixes to the compiler and minor bug fixes to the language service. In the next few weeks, we’ll be releasing the first stable version of TypeScript 5.8. Keep an eye on our iteration plan for target release dates and more if you need to coordinate around that.
Otherwise, our main focus is on developing our next version of TypeScript, and we’ll have the iteration plan available in the coming days (including scheduled release dates). On top of that, we make it easy to use nightly builds of TypeScript on npm, and there is an extension to use those nightly releases in Visual Studio Code.
So give the RC or our nightlies a try! We encourage you to provide any feedback you might have on GitHub.
Happy Hacking!
– Daniel Rosenwasser and the TypeScript Team

Original source Report a problem