flatten.js

//@flow *Ecmascript 6 syntax & require flow syntax checking before build;

function flatten (source:Array<any>) : Array<number> {
	return source.reduce((previous, current) => {
		let next;

		if (Array.isArray(current)) {
			next = flatten(current);
		} else if (typeof current == 'number') {
			next = [current];
		} else {
			throw 'Not a number';
		}

		return [...previous, ...next];
	}, []);
}

console.log(flatten([[1,2,[3], [4]],5]));