How to add TypeScript Declaration Files without TypeScript

No code completion for your JavaScript library? No type-checking?

Declaration files are files that describe the form of an existing JavaScript codebase to TypeScript.

By using declaration files (.d.ts) you get among other things a code completion in your editor and simplify the use of the library.

But how can I create declaration files for my JavaScript library? I'll give you a short instruction.

Library function

File: src/unit/index.js

/**
  * Converts celsius (C) to fahrenheit (F)
  *
  * @param   {number} value Temperature in celsius
  * @returns {number}       Temperature in fahrenheit
  */
export const cToF = (value) => (value * 9 / 5) + 32

Declaration file

File: src/unit/index.d.ts

export function cToF(value: number): number

In the declaration file you describe only the parameters and the return type.

webpack configuration

The declaration files must be delivered with the library. For this I use the webpack plugin copy-webpack-plugin.

File: config/webpack/webpack.config.js

const CopyPlugin = require('copy-webpack-plugin')

module.exports = {
  ...
  plugins: [
    new CopyPlugin({
      patterns: [
        {
          from: path.join(__dirname, '..', '..', 'src/**/*.d.ts'),
          to: path.join(__dirname, '..', '..', 'dist', '[folder].d.ts'),
          flatten: true
        }
      ]
    })
  ]
  ...
}

Ready

With tools like dtslint you can tests declaration files for style and correctness.

The TypeScript Handbook provides detailed documentation on declaration files.