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

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

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

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

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); } }

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;

• 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

• 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.

• 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