Transpile Async Await proposal with Babel.js?

问题:

There is a proposal for introducing C# style async-await. I know Babel.js transpiles ES6 to ES5, but is there any way to make it transpile async-await to ES5?

回答1:

Babel v6

As of Babel v6, Babel doesn't contain any transformers itself anymore. You have to explicitly specify any feature you want to transform.

Presets - non ES2015 environment

The quickest way to get this working is to use presets which already contain the set of plugins needed to transform ES2015 and newer proposals. For async, you will need the es2015 and es2017 presets and the runtime plugin (don't forget to install babel-runtime as described in the documentation):

{
  "presets": [
    "es2015",
    "es2017"
  ],
  "plugins": [
    "transform-runtime"
  ]
}

Presets - ES2015 environment

If you run the code in an environment that supports ES2015 (more specifically, generators and Promises), then all you need is the es2017 preset:

{
  "presets": [
    "es2017"
  ]
}

Custom

To only transform the async functions, you will need the following plugins.

syntax-async-functions is needed in any every case to be able to parse async functions

In order to run the async function, you either need to use

• transform-async-to-generator: Converts the async function into a generator. This will use Babel's own "co-routine" implementation.
• transform-async-to-module-method: Also converts the async function to a generator, but passes it to the module and method specified in the configuration instead of Babel's own method. This allows you to use external libraries such as bluebird.

If your code runs in an environment that supports generators, then there is nothing left to do. However, if the target environment does not support generators, you will also have to transform the generator. This is done via the transform-regenerator transform. This transform depends on runtime functions, so you will also need Babel's transform-runtime transform (+ the babel-runtime package).

Examples:

Async to generator

{
  "plugins": [
    "syntax-async-functions",
    "transform-async-to-generator"
  ]
}

Async to module method

{
  "plugins": [
    "syntax-async-functions",
   ["transform-async-to-module-method", {
     "module": "bluebird",
     "method": "coroutine"
   }]
  ]
}

Async to generator + regenerator

{
  "plugins": [
    "syntax-async-functions",
    "transform-async-to-generator",
    "transform-regenerator",
    "transform-runtime"
  ]
}

---

Babel v4 and older

Yes, you have to enable the experimental transformers. Babel uses regenerator.

Usage

$ babel --experimental

---

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

回答2:

This solution may have changed since (Feb 25 Felix Kling) or perhaps there is still more than one way to use async await.

What worked for us was to run Babel like so

$ npm install babel-runtime

---

$ babel inputES7.js -o outputES5.js --optional runtime

回答3:

I got this working as-of today by doing an additional npm install babel-preset-stage-0 and using it like

var babel = require("babel-core");
var transpiled = babel.transform(code, { "presets": ["stage-0"] });

See

• http://babeljs.io/docs/usage/options/#options
• http://babeljs.io/docs/plugins/preset-stage-0/

回答4:

Perhaps even more up-to-date now; just put the babel stuff in a separate file:

'use strict';

require('babel/register'); // Imports babel - auto transpiles the other stuff
require('./app'); // this is es6 - gets transpiled

See my code at how-can-i-use-es2016-es7-async-await-in-my-acceptance-tests-for-a-koa-js-app for some more details.

回答5:

The approved answer seems to be outdated now. The experimental flag has been deprecated in favor of stage.

http://babeljs.io/blog/2015/03/31/5.0.0/#experimental-option

Usage

$ babel --stage 0

---

babel.transform("code", { stage: 0 });

Stage 0

• es7.classProperties
• es7.comprehensions

Stage 1

• es7.asyncFunctions
• es7.decorators
• es7.exportExtensions
• es7.objectRestSpread

Stage 2 (Stage 2 and above are enabled by default)

• es7.exponentiationOperator