New Features in TypeScript 5.0: Decorators, Const Type Parameters, Module Resolution, JSDoc Enhancements and Performance Improvements

On March 17, 2023 TypeScript 5.0 was officially released, bringing a series of language and tooling improvements aimed at making TypeScript smaller, simpler, and faster.

Major language updates

New decorator standard : decorators can now be written as functions that wrap methods, enabling logging, binding, and other cross‑cutting concerns. function loggedMethod(originalMethod: any, _context: any) { return function replacementMethod(this: any, ...args: any[]) { console.log("LOG: Entering method."); const result = originalMethod.call(this, ...args); console.log("LOG: Exiting method."); return result; }; }

Const type parameters : the const modifier can be used on generic type parameters to preserve literal types without needing as const . type HasNames = { names: readonly string[] }; function getNamesExactly<const T extends HasNames>(arg: T): T["names"] { return arg.names; } const names = getNamesExactly({ names: ["Alice", "Bob", "Eve"] });

Multiple extends in tsconfig.json : a configuration file can now inherit from an array of base configs, merging their compiler options. { "extends": ["@tsconfig/strictest/tsconfig.json", "../../../tsconfig.base.json"], "compilerOptions": { "outDir": "../lib" } }

Enhanced module resolution : new "moduleResolution": "bundler" option mirrors the behavior of modern bundlers, while node16 and nodenext continue to support strict ESM rules. { "compilerOptions": { "target": "esnext", "moduleResolution": "bundler" } }

Custom parsing flags : flags such as --allowImportingTsExtensions , --resolvePackageJsonExports , and --allowArbitraryExtensions give finer control over how TypeScript resolves imports and package exports.

Verbatim module syntax : the --verbatimModuleSyntax flag preserves all non‑type imports/exports in the emitted JavaScript, simplifying inter‑op with CommonJS and ESM.

JSDoc enhancements : new @satisfies and @overload tags allow JavaScript files to benefit from the same type‑checking capabilities as TypeScript. /** @satisfies {ConfigSettings} */ let myConfig = { compilerOptions: { strict: true, outDir: "../lib" }, extends: ["@tsconfig/strictest/tsconfig.json"] };

Other improvements

TypeScript 5.0 also adds case‑insensitive import sorting in editors, a more robust switch/case completion experience, and substantial speed, memory, and package‑size optimizations that reduce the overall npm package size by over 26 MB.

These changes make TypeScript a more powerful and performant tool for both frontend and backend development.