flatten array in js

flatten.js

/**
 * Flattens nested arrays
 *
 * @param {array} arr - The nested array
 * @returns {array} Returns flattened array
 *
 * @example
 * // returns [ 1, 2, 3, 4 ]
 * flattenArray( [ [ 1, 2, [ 3 ] ], 4 ] );
 *
 * checks if param is array
 * if so, runs concat on expanded members of map of array, recusrively calling function until it no longer hits an array
 * if not, returns the non-array param
 */

const flattenArray = arr => Array.isArray( arr ) ? [].concat( ...arr.map( flattenArray ) ) : arr;