Test

flatten.js

var a =  [[1,2,[3]],4]; // -> [1,2,3,4]
var newA = [];

function flatten(a) {
  for (var i=0; i<a.length; i++) {
     if (typeof a[i] != 'number') {
        newA.concat(flatten(a[i]));
     }
    else {
      newA.push(a[i]);
    }
  }
  return newA;
}

flatten(a);