Flatten array

flatten.js

/**
 * @param xs array 
 * @param oneLevel boolean -> pass true to flatten only one level
 */
let flatten = (xs, oneLevel) => 
  oneLevel ? xs.reduce( (acc, xs) => acc.concat(xs), []) :
  xs.reduce( (acc, x) => Array.isArray(x) ? acc.concat(flatten(x)) : acc.concat(x), []);

let nums = [1, [2, [3, 4, [5] ]]]

flatten(nums)
// -> [1, 2, 3, 4, 5]


// flatten one level deep
nums = [1, [2, [3, 4] ], 5]

flatten(nums, true)
// -> [1, 2, [3, 4], 5]