babel-plugin-transform-async-to-promises

Babel plugin to transform async functions containing await expressions to the equivalent chain of Promise calls with use of minimal helper functions.

Input:

async function fetchAsObjectURL(url) {
    const response = await fetch(url);
    const blob = await response.blob();
    return URL.createObjectURL(blob);
}

Output:

const fetchAsObjectURL = _async(function(url) {
	return _await(fetch(url), function(response) {
		return _await(response.blob(), URL.createObjectURL);
	});
});

Output with `hoist` enabled:

function _response$blob(response) {
	return _await(response.blob(), URL.createObjectURL);
}
const fetchAsObjectURL = _async(function(url) {
	return _await(fetch(url), _response$blob);
});

Output with `inlineHelpers` enabled:

const fetchAsObjectURL = function(url) {
	try {
		return Promise.resolve(fetch(url)).then(function(response) {
			return Promise.resolve(response.blob()).then(URL.createObjectURL);
		});
	} catch(e) {
		return Promise.reject(e);
	}
}

Output with `externalHelpers` enabled:

In the normal case, helpers are added to the top of the file for the _async and _await functions (as well as others). This can cause bloat in a codebase due to duplication of helper code in every file. To avoid this, enable externalHelpers and those will be imported instead:

import { _async } from "babel-plugin-transform-async-to-promises/helpers";
import { _await } from "babel-plugin-transform-async-to-promises/helpers";

const fetchAsObjectURL = _async(function(url) {
	return _await(fetch(url), function(response) {
		return _await(response.blob(), URL.createObjectURL);
	});
});

export default fetchAsObjectURL;

JavaScript Language Features

Full Support

async/await
for/while/do loops (including loops that would exhaust stack if dispatched recursively)
switch statements (including fallthrough and default cases)
conditional expressions
logical expressions
try/catch
break/continue statements (on both loops and labeled statements)
throw expressions
Function hoisting
Variable hoisting
Arrow functions
Methods
arguments
this
Proper member dereference order of operations
Standards-compliant event loop scheduling

Partial Support

Function.length: async functions will often return a length of 0 (when the _async wrapper is used)
Top level await support is experimental with compatible module bundler. Set topLevelAwait option to return when using SystemJS.

No support

eval: impossible to support without deep hooks into the runtime
Async generator functions: not implemented or planned
Function.name: rewrite pass removes function name instrumentation
new AsyncFunction(...): impossible to support without shipping babel and the plugin in the output

Current Tags

0.8.18 ... latest (3 years ago)