Polyfilling in JavaScript

flatten.js

/**
 * Array Polyfill to flatten out an array upto specified depth
 *
 * @param {number} depth
 * @default 1
 * @returns new flattened out array object
 *
 * @example `[1, 2, [3, 4, [5, 6, [7]]]].flatten() -> [1, 2, 3, 4, [5, 6, [7]]]`
 * @example `[1, 2, [3, 4, [5, 6, [7]]]].flatten(2) -> [1, 2, 3, 4, 5, 6, [7]]`
 * 
 */
Array.prototype.flatten = function (depth = 1) {
    if (typeof depth !== 'number') {
        throw new TypeError('depth must be a number');
    }
    return depth <= 0
        ? this.slice()
        : this.reduce((accu, curr) => (
            accu.concat(Array.isArray(curr) ? curr.flatten(depth - 1) : curr)
        ), []);
};