Flatten Nested Array ES6

flatten.js

/**
 * @summary function that will flatten nested array into regular array
 * @param Array - nested array (can be in different level)
 */
 
const flatten = nestedArray => nestedArray.reduce(
    (a, next) => a.concat(Array.isArray(next) ? flatten(next) : next), []
);

// Test
var numbers = [[1,2,[3]],4,[5,[6,[7,8]]]];
console.log(flatten(numbers)); // [1, 2, 3, 4, 5, 6, 7, 8]