Typescript + Webpack library generates "ReferenceError: self is not defined"

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06-10-2024

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"ReferenceError: self is not defined" in your TypeScript + Webpack Library: A Common Problem and Its Solution

Problem: You've built a fantastic TypeScript library using Webpack, but when you try to use it in another project, you're greeted with the dreaded "ReferenceError: self is not defined" error.

Rephrased: Imagine trying to open a door with a key that doesn't fit. Your library is like that key – it's built correctly but doesn't work in the environment it's intended for. The "self is not defined" error is your library saying, "Hey, I can't find the environment I need to function properly!"

Scenario and Code Example:

Let's say you have a simple library that uses window to access browser functionality:

// my-library.ts
export function logToConsole() {
  console.log("This message is from your library!", window.location.href);
} 

And you build it with Webpack:

// webpack.config.js
const path = require('path');

module.exports = {
  entry: './src/index.ts',
  output: {
    filename: 'my-library.js',
    path: path.resolve(__dirname, 'dist'),
    library: {
      name: 'MyLibrary',
      type: 'umd',
    },
  },
  module: {
    rules: [
      {
        test: /\.tsx?$/,
        use: 'ts-loader',
        exclude: /node_modules/,
      },
    ],
  },
  resolve: {
    extensions: ['.tsx', '.ts', '.js'],
  },
};

Now, when you try to use this library in a browser environment, you might get the "ReferenceError: self is not defined" error.

Analysis and Explanation:

The "self is not defined" error typically arises when you're using variables like window, self, or global in your library code, which are global objects in the browser environment but not necessarily available in other environments. This is especially common when building Universal JavaScript libraries that need to work in both the browser and Node.js.

Solution:

To fix this, you need to use an environment-agnostic way to access the global object. This is where the globalThis object comes in.

Solution Code:

// my-library.ts
export function logToConsole() {
  console.log("This message is from your library!", globalThis.location.href);
}

Explanation:

The globalThis object is a standardized way to access the global object in all major environments, including browsers, Node.js, and even Web Workers. By using globalThis.location.href instead of window.location.href, your library will work consistently in all these environments.

Additional Value:

• Understanding "self" and "window": These are closely related global variables in browser environments. self refers to the current window object, and window refers to the global object itself. However, in a Node.js environment, window is not defined.

• Avoiding the "undefined" error: If you need to check if a global variable is available, it's better to use a conditional approach:

  if (typeof window !== 'undefined') {
      // Access window properties here
  }
  

• Using globalThis for better portability: globalThis is a modern and reliable way to access the global object across environments, providing greater compatibility and reduced risk of errors.

Resources:

• MDN Web Docs: globalThis
• Webpack documentation

By incorporating these best practices and using globalThis in your TypeScript libraries, you can avoid the "ReferenceError: self is not defined" error and create libraries that are more robust and compatible with different environments.