Await implemented in ES6.

await.es6.js

function await(generatorFunction) {
	let gen = generatorFunction();

	/**
	 * @param  {any?}   err    The error to throw in the generator, where yield was last called.
	 * @param  {any?}   result The result to pass to the genarator for the last call to yield
	 */
	function next(err, result) {
		// If the last promise that was yielded was rejected,
		// trigger an error inside the generator where yield was last called
		if (err) {
			gen.throw(
				// Make sure that the error we're passing is actually an error and not just a string
				err instanceof Error ? err : new Error(err)
			);
		}

		// Get the next promise from the generator
		let nextResult = gen.next(
			result
		);
		let value = nextResult.value;

		// If we're done with the generator, no need to keep going.
		if (nextResult.done) {
			return;
		}

		value.then(
			// We have a value, call next again to pass it back into the generator and keep going
			result => next(null, result),
			// We have an error, throw it in the generator
			e => next(e)
		);
	}

	next();
}

function generateRandomNumber() {
	return new Promise(resolve => setTimeout(() => resolve(Math.random()), 2000));
}

function makeError() {
	return new Promise((r, reject) => reject('Some error'));
}

await(function*() {
	try {
		console.log('Starting random number generator');
		console.log('Random number is', yield generateRandomNumber());
		console.log('Starting another random number generator');
		console.log('Another random number is', yield generateRandomNumber());
		console.log('Throwing error');
		yield makeError();
	} catch (e) {
		console.error('Caught error', e);
	}
});