text: function array_collapse(arr) {
  	var collapse_arr = [];
  
  	for (var i=0; i<arr.length; i++) {
  		for (var j=0; j<arr[i].length; j++) {
  			collapse_arr.push(arr[i][j]);
  		}
  	}
  
  	return collapse_arr;
  }
  
  console.log(array_collapse([[1, 2, 3, 4], [5, 6], ["hello", "world"]]));
  
  