How to add custom “typings” in typescript 2.0 / 3.0

You can create local custom typings just for your project, where you can declare types for JS libraries. For that, you need to:

1. Create directory structure to keep your type declaration files so that your directory structure looks similar to this:

    .
    ├── custom_typings
    │   └── some-js-lib
    │       └── index.d.ts
    └── tsconfig.json
   

2. In the index.d.ts file, add a declaration for your JS library:

    declare module 'some-js-lib' {
      export function hello(world: string): void
    }
   

3. (Optional: skip if you have TypeScript >= 4.x) Add a reference to this type declaration in the compilerOptions section of your tsconfig.json:

    {
      "compilerOptions": {
        ...
        "typeRoots": ["./node_modules/@types", "./custom_typings"]
      },
      ...
    }
   

4. Use the declared module in your code:

    import { hello } from 'some-js-lib'
   
    hello('world!')