What TypeScript removes during compilation

TypeScript strips all type-only constructs: interface declarations, type aliases, type annotations on variables and parameters, generic type parameters, type assertions (as Type), access modifiers (public/private/protected), the readonly keyword, implements clauses, declare statements, and non-null assertions (!). These exist only for the type checker and have zero runtime impact.

// TypeScript type stripping — remove types, keep logic

// Before (TypeScript)
interface User {
  name: string;
  age: number;
}
function greet(user: User): string {
  return `Hello, ${user.name}`;
}
const users: User[] = [{ name: "Alice", age: 30 }];

// After (JavaScript) — types removed, logic preserved
function greet(user) {
  return `Hello, ${user.name}`;
}
const users = [{ name: "Alice", age: 30 }];

// Node.js 22+ supports --experimental-strip-types
// node --experimental-strip-types app.ts

What TypeScript keeps or transforms

Enums are converted to JavaScript objects. Decorators are transformed to function calls. Class fields with initializers are preserved. Optional chaining (?.) and nullish coalescing (??) are kept as-is for modern targets or polyfilled for older targets. Import statements are rewritten based on the module format (CommonJS vs ESM). JSX is either preserved or compiled to React.createElement calls depending on the jsx compiler option.

Type-only imports and exports

TypeScript 3.8+ supports 'import type' and 'export type' syntax, which are completely erased during compilation. Regular imports of types are also removed if the compiler can determine they are type-only. This is called 'import elision' and helps produce cleaner JavaScript output with no unused imports.