What is "export default" in JavaScript?

It's part of the ES6 module system, described here. There is a helpful example in that documentation, also:

If a module defines a default export:

// foo.js
export default function() { console.log("hello!") }

then you can import that default export by omitting the curly braces:

import foo from "foo";
foo(); // hello!

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Update: As of June 2015, the module system is defined in §15.2 and the export syntax in particular is defined in §15.2.3 of the ECMAScript 2015 specification.

What is “export default” in JavaScript?

In default export the naming of import is completely independent and we can use any name we like.

I will illustrate this line with a simple example.

Let’s say we have three modules and an index.html file:

• modul.js
• modul2.js
• modul3.js
• index.html

File modul.js

export function hello() {
    console.log("Modul: Saying hello!");
}

export let variable = 123;

File modul2.js

export function hello2() {
    console.log("Module2: Saying hello for the second time!");
}

export let variable2 = 456;

modul3.js

export default function hello3() {
    console.log("Module3: Saying hello for the third time!");
}

File index.html

<script type="module">
import * as mod from './modul.js';
import {hello2, variable2} from './modul2.js';
import blabla from './modul3.js'; // ! Here is the important stuff - we name the variable for the module as we like

mod.hello();
console.log("Module: " + mod.variable);

hello2();
console.log("Module2: " + variable2);

blabla();
</script>

The output is:

modul.js:2:10   -> Modul: Saying hello!
index.html:7:9  -> Module: 123
modul2.js:2:10  -> Module2: Saying hello for the second time!
index.html:10:9 -> Module2: 456
modul3.js:2:10  -> Module3: Saying hello for the third time!

---

So the longer explanation is:

'export default' is used if you want to export a single thing for a module.

So the thing that is important is "import blabla from './modul3.js'" - we could say instead:

"import pamelanderson from './modul3.js" and then pamelanderson();. This will work just fine when we use 'export default' and basically this is it - it allows us to name it whatever we like when it is default.

---

P.S.: If you want to test the example - create the files first, and then allow CORS in the browser -> if you are using Firefox type in the URL of the browser: about:config -> Search for "privacy.file_unique_origin" -> change it to "false" -> open index.html -> press F12 to open the console and see the output -> Enjoy and don't forget to return the CORS settings to default.

P.S.2: Sorry for the silly variable naming

More information is in link2medium and link2mdn.

export default is used to export a single class, function or primitive from a script file.

The export can also be written as

export default function SafeString(string) {
  this.string = string;
}

SafeString.prototype.toString = function() {
  return "" + this.string;
};

This is used to import this function in another script file

Say in app.js, you can

import SafeString from './handlebars/safe-string';

A little about export

As the name says, it's used to export functions, objects, classes or expressions from script files or modules

Utiliites.js

export function cube(x) {
  return x * x * x;
}
export const foo = Math.PI + Math.SQRT2;

This can be imported and used as

App.js

import { cube, foo } from 'Utilities';
console.log(cube(3)); // 27
console.log(foo);    // 4.555806215962888

Or

import * as utilities from 'Utilities';
console.log(utilities.cube(3)); // 27
console.log(utilities.foo);    // 4.555806215962888

When export default is used, this is much simpler. Script files just exports one thing. cube.js

export default function cube(x) {
  return x * x * x;
};

and used as App.js

import Cube from 'cube';
console.log(Cube(3)); // 27