flatten.js

/**
 * Returns a nested array into a flatten single level array
 * @param  {Array} input a multiple level array
 * @return {Array}  an one level array of all the elements inside
 */

const result = [];

const flatten = array => {
	for (var i = 0; i < array.length; i++) {
		if (Array.isArray(array[i])) {
			flatten(array[i]);
		} else {
			result.push(array[i]);
		}
    }
	return result;
}

const test = [[1,2,[3]],4];

console.log(flatten(test));