var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]];

​

// Solution 1

var myNewArray1 = [].concat.apply([], myArray);

console.log(myNewArray1);

​

// Solution 2

var myNewArray2 = myArray.reduce(function(a, b) {

  return a.concat(b);

});

console.log(myNewArray2);

​

// Solution 3

var myNewArray3 = [];

for (var i = 0; i < myArray.length; ++i) {

  for (var j = 0; j < myArray[i].length; ++j)

    myNewArray3.push(myArray[i][j]);

}

console.log(myNewArray3);

​

var myNewArray4 = [].concat(...myArray);

console.log(myNewArray4);