Module

TypeScript · Reference cheat sheet

📖 Overview

TypeScript modules follow ECMAScript import/export syntax. Compiler options (module, moduleResolution, verbatimModuleSyntax) control emit and how imports resolve in TS 5.x projects.

🧩 Core concepts

• ES modules — export / import / export default / export * from.
• Type-only imports — import type \{ User \} from "./user" erases at compile time.
• verbatimModuleSyntax — requires type-only imports/exports to be marked; mirrors runtime ESM.
• Resolution — moduleResolution: "bundler" | "node16" | "nodenext" (avoid legacy "node" in new apps).
• paths / baseUrl — path aliases in tsconfig (bundler must mirror them).
• CommonJS interop — esModuleInterop / allowSyntheticDefaultImports smooth CJS default imports.

💡 Examples

// math.ts
export function add(a: number, b: number) {
  return a + b;
}
export type Sum = ReturnType<typeof add>;

// app.ts
import { add } from "./math.js"; // nodenext: include .js extension in relative paths
import type { Sum } from "./math.js";

const total: Sum = add(1, 2);

// Re-exports
export { add as sum } from "./math.js";
export type { Sum as Total } from "./math.js";

// Namespace import
import * as MathUtils from "./math.js";
MathUtils.add(1, 2);

// Dynamic import
const mod = await import("./math.js");

package.json for Node ESM:

{
  "type": "module"
}

⚠️ Pitfalls

• Under nodenext/node16, relative imports often need the .js extension that will exist after emit.
• Mixing export = (CJS) with ESM requires esModuleInterop and careful defaults.
• Path aliases are not rewritten by tsc alone — configure the bundler/runtime.
• Side-effect imports (import "./polyfill") must not be type-only.

🔗 Related

• Config
• Command line
• Types
• Object types
• Class