What is "export default" in JavaScript?

javascript

export-default

javascript-features

performance-optimization

byNikita Barsukov·Oct 7, 2024

export default in JavaScript ES6 allows you to export a single entity (like a function, class, object) from a module. When you import this entity in another file, you don't need curly braces. Here's an uncluttered example:

// myModule.js
export default () => 'Hello, world!'; //Extraterrestrial greeting

// main.js
import greet from './myModule.js'; 
console.log(greet()); //logs 'Hello, world!' like a casual Martian greeting

With export default, greet can be imported just by using its filename, bringing about a more streamlined syntax and usage in main.js.

When to use default and named exports

Default exports serve well when your module only has a single entity to be exported. Named exports, on the other hand, are more suitable for utility modules or libraries that offer a range of functionalities.

Optimizing Performance with Dynamic Imports

Combining export default with dynamic imports can enable effective code splitting. This is how we can do it:

// dynamicImport.js
export default function dynamicImport() {
  return import('./myModule.js').then(module => module.default()); //like a promise to a starving dev
}

Here, modules are loaded on-demand, which can significantly minimize the initial load time of your application - cue the applause for performance!

Transpiling `export default` for Older Browsers

Need to support older browsers? Not a problem! export default works smoothly with transpilers like Babel which convert your hipster ES6 code into the golden olds of CommonJS or AMD, ensuring a wide browser compatibility.

Improving Readability and Maintainability

export default significantly elevates the readability of your code as it's clear what the primary functionality of the module is. This makes for a smooth reading and maintaining experience for developers, letting them grasp the module's main purpose quickly.

Understanding the Quirks and Gotchas

While export default is a powerful tool in your JS arsenal, be aware of these edge cases:

export { default as something }: This is non-standard and may confuse developers.
Anonymous function as default export: Debugging can be challenging if you export anonymous functions/classes as it makes naming tough when debugging.
Exception of Single Default Export: A module can only have one default export. Less is more, folks!

Creative Usage with Higher-Order Components

export default can be paired with higher-order components in frameworks like React for a more exciting development experience:

// withLogging.js
const withLogging = Component => {
  return class extends React.Component {
    // more logic than a Spock convention
  }
}
export default withLogging;