This snippet of javascript code will flatten an array with nested arrays of any arbitrary depth.

flattenNested.js

var arr = [1,2,3,[2,[9,9],5],[4,5,5]];

function flatten(input) {
  var output = [];
  for (var i = 0; i < input.length; i++) {
    output = Array.isArray(input[i])
              ? [...output, ...flatten(input[i])]
              : [...output, ...input[i]]; 
  }
  return output;
}

console.log(flatten(arr));