TypeScript with JavaScript Libraries

You do not need to abandon JavaScript to use TypeScript.

Using JS files in TS project

You can mix:

• .ts files
• .js files

TypeScript understands JS.

Installing type definitions

Many JS libraries provide types via DefinitelyTyped.

Example:

npm install lodash
npm install --save-dev @types/lodash

Using the library

import _ from "lodash";

const nums = [1, 2, 3];
const doubled = _.map(nums, n => n * 2);

Now you get:

• autocomplete
• type checking

Gradual migration idea

Start with:

• allowJs: true in tsconfig
• convert files slowly from .js to .ts

Remember

• TypeScript works with JS ecosystem
• types give safety on top of JS libraries
• migration can be gradual