什么是.d.ts文件以及为什么需要它

Question

I'm reviewing a source code for angular-ui-router and there is the angular-ui-router.d.ts file inside api folder with the following content:

declare module ng.ui {

    interface IState {
        name?: string;
        template?: string;
        templateUrl?: any; // string || () => string
        templateProvider?: any; // () => string || IPromise<string>
    }

    interface ITypedState<T> extends IState {
        data?: T;
    }

I've read that is file is TypeScript type definitions. What is it? Why is it needed?

Answer 1

It's a declaration file :

When a TypeScript script gets compiled there is an option to generate a declaration file (with the extension .d.ts) that functions as an interface to the components in the compiled JavaScript. In the process the compiler strips away all function and method bodies and preserves only the signatures of the types that are exported. The resulting declaration file can then be used to describe the exported virtual TypeScript types of a JavaScript library or module when a third-party developer consumes it from TypeScript.

The concept of declaration files is analogous to the concept of header file found in C/C++.

Answer 2

Here is the definition . The d.ts files describe the shape of 3rd party library, and make the TypeScript compiler know how to deal with this 3rd party code (supply type information to compiler)