These are the three ways to merge a multidimensional array into a single array.

array_flatten.js

/**
 * Given:
 * var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]];
 *
 * Result:
 * // [1, 2, 3, 4, 5, 6, 7, 8, 9]
 */
 
// Using concat() and apply()
var myNewArray1 = [].concat.apply([], myArray);

// Using reduce()
var myNewArray2 = myArray.reduce(function(prev, curr) {
  return prev.concat(curr);
});

// Using a for loop
var myNewArray3 = [];
for (var i = 0; i < myArray.length; ++i) {
  for (var j = 0; j < myArray[i].length; ++j) {
    myNewArray3.push(myArray[i][j]);
  }
}