Start type checking your JavaScript without changing your code

#javascript #typescript #webdev

Index

Introduction

Enable type checking

Type checking at project level

Type checking at per file

Adding types

Inferring types

Manually specifying types

On variables

On function parameters

Define your own type

Importing type definitions

The full list of JSDOC annotations

Introduction

Now this post is not intended to persuade you to begin type checking your JavaScript. Rather if you're interested in type checking but don't want to transform your code to typescript yet, this article will inform you how to do it.
A key benefit of this approach is that you can add type checking gradually, per file.

Enable JavaScript type checking

Install typescript

npm install -g typescript

Visual Studio Code can do the checks without needing to install typescript separately.

On other editors, you may need to configure the Typescript language server to include js files.

Otherwise, use the tsc cli: tsc filename.js --allowJs --checkJs --noEmit.

Type checking at project level

Create a jsconfig.json file at project root:

{
  "compilerOptions": {
    "checkJs": true
  }
}

This also allows you to specify other options

{
  "compilerOptions": {
    "checkJs": true,
    "noEmit": true, // Do not transpile (only type checking)
    "jsx": "react",
    "target": "es2016",
    "moduleResolution": "node",
    "lib": ["es2016", "dom"],
    "strict": true,
    "noImplicitAny": false,
    "baseUrl": "src"
  },
  "exclude": ["node_modules", "public"]
}

Type checking at per file

Add // @ts-check comment on top of the file you want to check

Adding types

Inferring types

Whenever possible, Typescript will infer the type of the variables automatically and provide relevant autocompletion and error reporting.

Manually specifying types

You can specify types using JSDOC comments

On variables

/** @type {number | null} */
let a; // a will be assumed to be a number or null

/** @type {{ prop1: string, prop2: number[] }} */
let b; // Object b will have two properties prop1 which is a string and prop2 which would be a number array

On function parameters

/**
 * @param {Object} options - Object type
 * @param {string} options.prop1 - Property of Object options
 * @param {number} options.prop2 - Property of Object options
 * @param {string} arg1 - A string param.
 * @param {string} [arg2] - Optional param.
 * @param {string} [arg3="defaultValue"] - Optional param with default value
 * @return {string} return type
 */
function demo(options, arg1, arg2, arg3) {
  return arg1 + arg2 + arg3;
}

Define your own type

/**
 * @typedef {Object} MyObject - Define Object type MyObject
 * @property {string} prop1 - Property of Object MyObject
 * @property {number} prop2 - Property of Object MyObject
 */
/** @type {MyObject} */
let MyObject;

Importing type definitions

This is the really cool part. You can import type definitions form d.ts files and get type information for your npm packages.

/**
 * @typedef {import('webpack').Configuration} WebpackConfig
 * @typedef {import('webpack').RuleSetRule} RuleSetRule
 * @typedef {import('webpack').RuleSetLoader} RuleSetLoader
 */

/**
 * @type {WebpackConfig}
 */
const config = getConfig(); // config has the structure of the Webpack config object!