Simple async functionality for native style nodeJS functions

async-nodeJS.js

// The normal async programming flow in nodeJS can get hairy, quickly:

function readAndParse(filename, callback){
	fs.readFile(filename, 'utf8', (err, data) => {
		if(err){
			callback(err, null);
			return;
		}

		myParser.doParsing(data, (err, result) => {
			if(err){
				callback(err, null);
				return;
			}

			callback(null, result);
		});
	});
}

// With a small helper function, this nested callback construction can be flattened to this:

async function readAndParse(filename){
	const src = await asPromise(fs.readFile, filename, 'utf8');
	const result = await asPromise(myParser.doParsing, src);
	return result;
}

// This is the wrapper function `asPromise`, that can take any nodeJS callback style function and wrap it into a promise:

/**
 * Converts a nodeJS native style function and calls it as a promise.
 * @param {function} normalFunc The original nodeJS function to wrap in a promise.
 * @param {any...} normalArguments Any arguments you want to pass to the wrapped function.
 * @returns {Promise<any>}
 */
const asPromise = (normalFunc, ...normalArguments) => new Promise((resolve, reject) => {
	const combinedArguments = [...normalArguments, function () {
		const [err, ...rest] = arguments;
		if (err) {
			reject(err);
		}
		resolve.apply(normalFunc, rest);
	}];

	normalFunc.apply(normalFunc, combinedArguments);
});