What is the 'target' option?

In TypeScript, the 'target' option in your tsconfig.json file tells the TypeScript compiler which version of JavaScript your code should be converted (transpiled) into. Think of it like this: TypeScript understands modern JavaScript features, but older browsers or environments might not. The 'target' option bridges this gap by converting your TypeScript code into a version that the target environment understands.

Why is 'target' important?

• Compatibility: Ensures your code runs correctly in the browsers or environments you intend to support. Older browsers, for example, might not understand ES6 features like arrow functions or classes.
• Feature Usage: Allows you to use modern JavaScript features in your TypeScript code, knowing that the compiler will convert them into equivalent code that works on your target environment.
• Performance: Choosing the correct target can sometimes impact the performance of your JavaScript code in different environments. While newer targets can use more efficient features, they require the environment to support them.

How to Specify the 'target' Option

The 'target' option is set in your tsconfig.json file. Here's a basic example:

 {
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",  // Choose a module system (see below)
    "strict": true,       // Enable strict type checking
    "esModuleInterop": true, // Allows importing CommonJS modules easily
    "skipLibCheck": true,  // Skips type checking of declaration files (*.d.ts)
    "forceConsistentCasingInFileNames": true // Enforces consistent capitalization
  }
} 

The key part is the "target": "es5" line. This tells the compiler to convert your TypeScript code into ES5-compatible JavaScript.

Common 'target' Values

Here are some common values for the 'target' option:

• "es5": This is a widely supported target, compatible with older browsers (including Internet Explorer). A good choice if you need to support older environments.
• "es6" (also known as "es2015"): A good balance between modern features and browser support. Most modern browsers support ES6.
• "es2016", "es2017", "es2018", "es2019", "es2020", "es2021", "es2022", "es2023": These target specific ECMAScript versions. Choose the newest version that your target environment supports to leverage the latest JavaScript features and potential performance improvements.
• "esnext": This targets the latest version of JavaScript that TypeScript understands. It includes features that are still in proposal stages. Use with caution, as these features might change before they are officially standardized. Usually used for libraries or tools, not necessarily web applications intended to run on a wide variety of browsers.

Choosing the Right 'target'

The best 'target' depends on your project's requirements:

• Consider your target environment: Which browsers or JavaScript engines will your code run on? Use tools like "Browserlist" to determine which target will work for the browsers you must support.
• Balance compatibility and features: A lower target provides broader compatibility but limits you to older JavaScript features. A higher target allows you to use modern features but might require polyfills (code that provides missing features in older browsers).
• Start with ES6/ES2015: This is generally a good starting point for modern web development, as it's widely supported. Then, increase or decrease if required based on browser needs.
• Testing is key: Always test your code in your target environments to ensure everything works as expected.

Related Compiler Options

The target option works together with other compiler options. Here are two very important related options:

• module: This option specifies the module system to use (e.g., commonjs, amd, es6/es2015, esnext, system, umd, node16, nodenext). It determines how your code is organized into modules and how those modules are loaded. For example, commonjs is commonly used in Node.js, while es6/es2015 is used for modern JavaScript modules. You may need to adjust this in conjunction with your target to ensure it is compatible.
• lib: This option specifies which built-in TypeScript libraries (e.g., ES5, ES6, DOM, WebWorker) to include in your project. This helps TypeScript understand the built-in objects and methods available in your target environment. If you're targeting ES5 and using DOM APIs, you'll typically include "lib": ["es5", "dom"]. If you're using a higher target like ES2015, TypeScript will often include the necessary libraries automatically, but you may need to explicitly include "dom" or other specialized libraries if you are interacting with the browser's document object model.

Using Polyfills

If you want to use modern JavaScript features with a lower target, you might need to use polyfills. A polyfill is a piece of code that provides the functionality that's missing in older environments. For example, if you want to use Array.includes() in a browser that doesn't support it, you can include a polyfill that adds this method to the Array prototype. Libraries like core-js are commonly used for polyfilling. When using polyfills, you also must have the appropriate lib in your tsconfig so that Typescript can properly type check your code.