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Modules in TypeScript

Use ES modules for modern TypeScript. Prefer named exports; avoid default exports in libraries for better refactoring.

Import and export

// math.ts
export function add(a: number, b: number) { return a + b }
export const PI = Math.PI;

// app.ts
import { add, PI } from "./math.js"; // with bundlers, extension often optional
console.log(add(1, 2), PI);

Default exports (use sparingly)

export default function main() {}
import main from "./main.js";

Default exports can complicate named refactors; prefer named exports in libraries.

Re-exports and barrels

// index.ts
export * from "./math.js";
export { add as plus } from "./math.js";

Module resolution

  • Use module: ESNext and moduleResolution: Bundler (or NodeNext for Node)
  • Configure path aliases in tsconfig.json:
{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": { "@/utils/*": ["src/utils/*"] }
  }
}

Then import { x } from "@/utils/x".

Ambient modules and globals

Declare types for untyped modules or globals in *.d.ts.

// env.d.ts
declare const VERSION: string;

CommonJS interop

Set esModuleInterop: true and use default import syntax for CJS in many cases. For precise interop, use import = require() with module: commonjs.

Summary

  • Prefer named exports and ES module settings
  • Use path aliases and barrel files thoughtfully; declare ambient types where needed
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