Flatten arbitrarily nested arrays.

flatten-arbitrarily-nested-arrays.js

function flatten(arr) {
	return arr.reduce((flat, toFlatten) => {
		return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
	}, []);
}

// usage:
flatten([[1, 2, 3], [4, 5]]); // [1, 2, 3, 4, 5]
flatten([[[1, [1.1]], 2, 3], [4, 5]]); // [1, 1.1, 2, 3, 4, 5]