本文档源自：https://github.com/babel/website/blob/5.0.0/docs/usage/external-helpers.md

Details

Babel has a few helper functions that'll be placed at the top of the generated code if needed so it's not inlined multiple times throughout that file. This may become an issue if you have multiple files, especially when you're sending them to the browser. gzip alleviates most of this concern but it's still not ideal.

You can tell Babel to not place any declarations at the top of your files and instead just point them to a reference contained within the external helpers.

Usage

$ babel --external-helpers

babel.transform("code", { externalHelpers: true });

Getting the external helpers

$ babel-external-helpers [options]

or

require("babel").buildExternalHelpers();

or from an npm release in external-helpers.js from the babel directory.

Options

Output formats

global

global output format sets helpers as global variable by adding babelHeleprs to global or this.

umd

umd output format wraps helpers in UMD compatible with browsers, CommonJS and AMD.

var

var outputs variable babelHelpers (var babelHelpers = {}) and helpers are assigned to it. This output format is suitable for additional processing.

Injecting the external helpers

Node

require("babel/external-helpers");

This injects the external helpers into global.

Browser

<script type="application/javascript" src="your-path-to/babel/external-helpers.js"></script>

In a browser environment you can use a <script> tag to inject the babelHelpers into the window object.

Selective builds

You can pass the option metadataUsedHelpers to babel.transform() in order to get a list of helpers that were used for that file:

require("babel").transform("code", { metadataUsedHelpers: true }).metadata.usedHelpers;

This will be an array of helpers that you can then pass to buildExternalHelpers like so:

require("babel").buildExternalHelpers(usedHelpers);