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.