What is .d.ts?

Learn what declaration files are.

What is a .d.ts?

A .d.ts file is a TypeScript declaration file. It contains only type information; no executable JavaScript code. It is used to add types to JavaScript libraries on the TypeScript side. The declare keyword tells TypeScript about a value that already exists outside.

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

Difference between .d.ts and .ts

A .ts file holds both types and implementation; when compiled it produces .js. A .d.ts file holds only type declarations and is not turned into .js by the compiler. That is why you use .d.ts for JS libraries that need types.

// app.d.ts (declaration)
export const PI: number;
// app.ts (implementation)
export const PI = 3.14;

Exercise Declare topla ambient function

Declare an ambient function topla that takes two number parameters and returns number. Then print "topla hazır" with console.log.

Show example solution

declare function topla(a: number, b: number): number;
console.log("topla hazır");

Test yourself

Which of the following is found in a .d.ts file?
- Only comment lines
- Type declarations
- Type declarations and implementation
- Only JavaScript code
Show answer

Type declarations — A .d.ts file contains only type declarations; it does not host any implementation or executable code.

Declare the VERSION global

// VERSION globalini ambient olarak bildir
____ const VERSION: string;

Show answer

// VERSION globalini ambient olarak bildir
declare const VERSION: string;

.d.ts concepts

.d.ts

Declaration file extension
declare

Declares an existing value
tür bilgisi

Content without implementation
.ts

Both type and implementation

Next lesson: Ambient module

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