Export an es6 default class inline with definition or at end of file?

问题

What is the difference if any between exporting an es6 default class inline with its definition versus at the end of the file after its definition?

Following are two examples I came across in React tutorials.

Ex. 1: Inline with Class

export default class App extends React.Component {
    // class definition omitted
}

Ex. 2: End of file

class App extends React.Component [
    // class definition omitted
}

export default App; // is this different somehow?

If there is no difference, then it seems the first example is more efficient and concise.

回答1:

The only significant difference is that, when exporting something other than a class or a named function declaration, declaring the expression and then exporting it afterwards allows you to reference it elsewhere in the same module.

Class names and (non-arrow, named) function declarations have their name put into the module's scope as a variable:

<script type="module">
export default class App {
    // class definition omitted
}
console.log(typeof App);
</script>

<script type="module">
export default function fn() {
    
}
console.log(typeof fn);
</script>

But this isn't possible for other sorts of expressions, like plain objects or arrow function expressions, which don't intrinsically have a name associated with them:

<script type="module">
export default {
  prop: 'val'
  // There's no way to reference the exported object in here
}
// or out here
</script>

<script type="module">
export default () => 'foo';
// There's no way to reference the exported function elsewhere in this module
</script>

Well, there's one way to get back a reference to it, which is by importing the current module that you're in:

// foo.js
import foo from './foo';
export default () => {
  console.log('foo is', typeof foo);
};

But that looks really ugly and confusing.

So if you're default-exporting something which isn't a class nor a function declaration, you can't easily reference what you're exporting unless you put it into a standalone variable before exporting it.

Keep in mind that this is not the case for named exports, which can easily be referenced by the name of their export:

<script type="module">
export const obj = {
  prop: 'val'
};
console.log(typeof obj);

export const fn = () => 'foo';
console.log(typeof fn);
</script>

来源：https://stackoverflow.com/questions/60106063/export-an-es6-default-class-inline-with-definition-or-at-end-of-file

标签

javascript

ecmascript-6

es6-modules