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Transform async/await to somewhat idiomatic JavaScript promise chains
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README.md

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.

Build Status

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);
	}
}

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)

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
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