Daniel Rosenwasser announces the release of TypeScript 5.7, detailing key upgrades for developers. This update offers improved checks, modern JavaScript support, and faster editor experiences.

Announcing TypeScript 5.7: Key Features and Improvements

Author: Daniel Rosenwasser

TypeScript 5.7 introduces a range of new features and enhancements aimed at improving the developer experience, compiler behavior, and compatibility with modern JavaScript and large codebases. Below is a summary of the main updates and their impact on TypeScript users.

What is TypeScript?

TypeScript is a language developed by Microsoft that builds on JavaScript by adding optional static types. Types make code more readable, catch bugs at compile time, and enable rich tooling support in editors like Visual Studio and VS Code.

Getting Started

To install TypeScript 5.7 via npm:

npm install -D typescript

Main Highlights in TypeScript 5.7

1. Improved Checks for Never-Initialized Variables

TypeScript 5.7 improves detection of variables that are never initialized, catching more errors that might otherwise go unnoticed. Previously, only some cases were flagged—now, uninitialized variables accessed in nested functions or complex flows are reliably reported.

function foo() {
  let result: number;
  // Forgot to assign to 'result'
  function printResult() {
    console.log(result); // error: Variable 'result' is used before being assigned.
  }
}

2. Path Rewriting for Relative Imports

A new compiler flag --rewriteRelativeImportExtensions updates relative import paths when transpiling TypeScript to JavaScript, supporting seamless use in tools like ts-node, Deno, and Node.js with in-place execution. This change is particularly useful in projects distributing both TypeScript and JavaScript output.

  • Only relative paths with TypeScript extensions are rewritten (e.g., import "./foo.ts" → import "./foo.js")
  • Dynamic imports and non-relative/module-based imports remain unchanged

3. Support for --target es2024 and --lib es2024

TypeScript 5.7 supports the latest ECMAScript (2024), allowing use of new JavaScript features and updated library definitions, including new enhancements to SharedArrayBuffer, ArrayBuffer, Object.groupBy, and more. TypedArrays such as Uint8Array now include a generic type parameter for broader compatibility.

4. Project Ownership Improvements in Editors

Editors using TSServer now walk the project tree for the most appropriate tsconfig.json file, improving project resolution in complex repository layouts.

b. Faster Checks with Composite Projects

TypeScript 5.7 optimizes project ownership checks, significantly speeding up the editor experience in large, composite repositories by limiting file scanning to known input files.

5. Validated JSON Imports (in --module nodenext)

JSON imports now require type: "json" attributes, preventing runtime errors and ensuring compatibility with NodeNext module systems. Default imports are used for content access, aligning with ECMAScript module semantics.

import myConfig from "./myConfig.json" with { type: "json" };
let version = myConfig.version;

6. V8 Compile Caching Support in Node.js

TypeScript’s CLI now leverages Node.js 22’s module.enableCompileCache() API, resulting in markedly faster command-line operations, such as running tsc --version.

7. Miscellaneous Behavioral Changes

  • Types generated for the DOM may be updated, potentially impacting codebases that depend on lib.d.ts.
  • Methods in classes declared with non-literal computed property names now contribute to class types as index signatures.
  • Functions returning null or undefined are more strictly checked under noImplicitAny.

Upgrade Guidance

  • Read full release notes on the TypeScript blog
  • Review behavior in projects, especially those using advanced project layouts or relying on strict type checks
  • Consider updating dependencies like @types/node if encountering TypedArray issues

What’s Next?

Stay tuned for upcoming releases and try nightly builds via npm for the latest fixes and features.

For more information, visit:

TypeScript 5.7 is a significant step forward for the language, equipping developers with modern tooling, improved error detection, and a smoother developer experience.

This post appeared first on “Microsoft TypeScript Blog”. Read the entire article here