function flatten(arr){
  var flatArr = [];
  
  var recursive = function(arr2) {
    if(arr2.length == 0) {
      return;
    } else if (Array.isArray(arr2[0])) {
      recursive(arr2[0]);
    } else {
      flatArr.push(arr2[0]); 
    }
    recursive(arr2.slice(1));
  }
  
  recursive(arr);
  return flatArr;
}

var test1 = JSON.stringify(flatten([])) === JSON.stringify([]);
var test2 = JSON.stringify(flatten([1])) === JSON.stringify([1]);
var test3 = JSON.stringify(flatten([[1,2]])) === JSON.stringify([1,2]);
var test4 = JSON.stringify(flatten([[1,2],3])) === JSON.stringify([1,2,3]);
var test5 = JSON.stringify(flatten([[1,2,[3]],4])) === JSON.stringify([1,2,3,4]);