In this detailed announcement, Daniel Rosenwasser introduces the TypeScript 5.7 RC, outlining major feature enhancements and important changes for TypeScript developers.

Announcing TypeScript 5.7 RC

By Daniel Rosenwasser and the TypeScript Team

Today, we are announcing the availability of the release candidate (RC) for TypeScript 5.7. To try out the RC, simply run:

npm install -D typescript@rc

Below, we highlight key updates and new features in this release.

Checks for Never-Initialized Variables

TypeScript now offers improved checks for variables that have never been initialized. While it already warned about possibly uninitialized variables, previous analysis had some blind spots, such as variables accessed from inner functions. TypeScript 5.7 can now catch more of these scenarios, improving reliability and preventing runtime errors. See PR #55887 for details.

Example:

function foo() {
  let result: number;
  // No assignment to 'result'
  function printResult() {
    console.log(result); // error: Variable 'result' is used before being assigned.
  }
}

Path Rewriting for Relative Paths

Modern tools like ts-node, tsx, Deno, and Bun allow in-place execution of TypeScript files. To maximize compatibility, when importing via relative paths with TypeScript extensions (e.g., .ts, .tsx), developers often use imports like ./foo.ts. While TypeScript previously supported this using --allowImportingTsExtensions, generating .js files from .ts files caused issues as paths weren’t rewritten.

A new compiler option, --rewriteRelativeImportExtensions, addresses this. When enabled, relative imports ending in TypeScript extensions will be rewritten to their JavaScript equivalents on build.

Example:

// main.ts
import * as foo from "./foo.ts"; // Will be rewritten to ./foo.js

Limitations:

  • Only relative paths are rewritten.
  • Dynamic imports or imports involving custom resolution (like baseUrl, paths, or package exports) are not rewritten automatically.

See PR #59767 for more details.

Support for --target es2024 and --lib es2024

TypeScript 5.7 adds support for the ES2024 target, unlocking features such as SharedArrayBuffer, ArrayBuffer improvements, Object.groupBy, Map.groupBy, Promise.withResolvers, and more. There’s also a change to TypedArrays, which are now generic over ArrayBufferLike. This may require updates to @types/node in some codebases.

Example:

interface Uint8Array<TArrayBuffer extends ArrayBufferLike = ArrayBufferLike> {
  // ...
}

Thanks to Kenta Moriuchi for contributions here. See PR #59417.

TypeScript’s language server (TSServer) now extends its search for relevant tsconfig.json files up the directory tree, providing better support for monorepos and nested project configurations. Instead of stopping at the first tsconfig.json encountered, TSServer continues searching, aiding scenarios with multiple configuration files (e.g., test configs, workspace setups).

This enhances flexibility for project organization and configuration structure. Learn more in PR #57196 and PR #59688.

Faster Editor Project Ownership Checks for Composite Projects

Performance for large, composite projects improves: when opening files, TypeScript can now quickly determine project ownership without fully parsing unrelated projects. This leverages the composite setting’s guarantee that root input files are known up-front. See PR #59688 for more information.

Validated JSON Imports in --module nodenext

TypeScript now validates JSON imports when using --module nodenext. You must specify type: "json" as an import attribute for such imports.

Example:

import myConfig from "./myConfig.json" with { type: "json" }; // ✅

JSON imports no longer generate named exports, and only the default export is provided. For further details, see PR #60019.

Node.js V8 Compile Caching Support

TypeScript 5.7 utilizes the new module.enableCompileCache() API in Node.js 22, enabling significant speed-ups for subsequent executions by caching compilation work (up to 2.5x faster in tests). See PR #59720.

Notable Behavioral Changes

  • lib.d.ts Updates: Changes to types generated for the DOM may affect type-checking. See relevant issues.
  • TypedArray Generics: All TypedArrays are now generic; issues may require updating type declarations (see above).
  • Index Signatures for Symbol-Named Methods: Methods declared with non-literal computed property names now correctly contribute to type information as index signatures (see PR #59860).
  • Implicit ‘any’ Errors: Improved detection of implicit any return types in functions returning null or undefined within generic signatures, especially under noImplicitAny (see PR #59661).

What’s Next?

TypeScript 5.7 is nearly complete, with the focus now shifting to bug fixes ahead of the stable release. To follow along or coordinate upgrades, check the iteration plan for upcoming dates.

You can also try daily/nightly builds using nightly npm packages or the VS Code Nightly Extension.

We encourage you to test this RC in your projects and provide feedback!

Happy hacking!

— Daniel Rosenwasser and the TypeScript Team

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