此ES6函数声明如何有效？

Question

How is this function declaration valid in ES6? What does it mean? I've found it in the source code of a React Native project.

'use strict';

import React, { Component } from 'React';
// some more imports...

function setup():React.Component {

    class Root extends Component {

        // constructor and render functions...   

    }
    return Root
}

export default setup;

I have removed a few lines that are not important to see my issue.

I could not find anything related to ES6 or Javascript regarding function declaration followed by a colon.

Answer 1

That code is typed with flow , Facebook's static type checker (akin to TypeScript). It's not valid ES6, it's a superset of the ES6 syntax.