.d.ts Declaration Files
Last updated: 2026-10-22
.d.ts Declaration Files
Declaration files (.d.ts) describe what a library looks like without containing any implementation, so TypeScript can type-check plain JS libraries. Types come from three sources: bundled with the library (best), community @types packages (npm i -D @types/xxx), or hand-written. The core of hand-writing is the declare keyword — it asserts "this value/function is implemented elsewhere" and the compiler emits no code for it.
| Group | Engineering & Practice |
|---|---|
| Kind | Config |
| Strict mode | Not required |
ℹ️ Info The editor transpiles TypeScript to JavaScript and runs it (no type checking); see each example's Output for type-check results.
📝 Syntax
// my-lib.d.ts — declaration file: types only, erased after compilation
declare const APP_VERSION: string;
declare function log(msg: string): void;
interface Config {
host: string;
port: number;
debug?: boolean;
}
// Install community types: npm i -D @types/lodash⚙️ Parameters
| declare const / let | Declares a value provided by the host (a library, a CDN global, ...); the type comes from your description |
|---|---|
| declare function | Declares a function signature for an existing implementation — types only, no body |
| @types/* | Community-maintained declaration packages (DefinitelyTyped); install into devDependencies and they take effect automatically |
Returns：None — .d.ts files emit no JS after compilation; they only feed type information to the compiler.
▶ Example
Edit the code below and click Try it to see the transpiled result:
Output：
info app booted
[10:24:00] OK db connected
(Runtime output after transpilation; the LogOptions shape comes from the .d.ts — passing { timestamps: "yes" } yields error TS2322)💡 Notes
🏷️ Related Engineering & Practice
💡 Related cases
📚 Related tutorials
❓ FAQ
