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