How to use a javascript library without a type definition file

While some existing JavaScript libraries have type definition files (opens new window), there are many that don't.

TypeScript offers a couple patterns to handle missing declarations.

Make a module that exports a default any

For more complicated projects, or in cases where you intend to gradually type a dependency, it may be cleaner to create a module.

Using JQuery (although it does have typings available (opens new window)) as an example:

// place in jquery.d.ts
declare let $: any;
export default $;

And then in any file in your project, you can import this definition with:

// some other .ts file
import $ from "jquery";

After this import, $ will be typed as any.

If the library has multiple top-level variables, export and import by name instead:

// place in jquery.d.ts
declare module "jquery" {
   let $: any;
   let jQuery: any;

   export { $ };
   export { jQuery };
}

You can then import and use both names:

// some other .ts file
import {$, jQuery} from "jquery";

$.doThing();
jQuery.doOtherThing();

Declare an any global

It is sometimes easiest to just declare a global of type any, especially in simple projects.

If jQuery didn't have type declarations (it does (opens new window)), you could put

declare var $: any;

Now any use of $ will be typed any.

Use an ambient module

If you just want to indicate the intent of an import (so you don't want to declare a global) but don't wish to bother with any explicit definitions, you can import an ambient module.

// in a declarations file (like declarations.d.ts)
declare module "jquery";    // note that there are no defined exports

You can then import from the ambient module.

// some other .ts file
import {$, jQuery} from "jquery";

Anything imported from the declared module (like $ and jQuery) above will be of type any