flatten.js

function flatten(input) { console.log(input);
	var _flatten = (input, output) => {
		console.debug("flattening %o", input);
		return input.reduce((output, item) => {
			if (typeof item == "object" && item instanceof Array) {
				console.debug("flattening nested array %o", item);
				return _flatten(item, output);
			} else {
				output.push(item);
				return output;
			}
		}, output);
	}
	return _flatten(input, []);
}


flatten([[[[1,2],3],[],4,[5,6],7,8,9]]); // [1 2 3 4 5 6 7 8 9]
flatten([{ 0: "not an array", length: 1}, 1, 2]); // [{ 0: "not an array", length: 1}, 1, 2]
flatten(0, 1, [2, 3]); // Error: input is not an array