SOURCE

 
var arr = [
 [1,2],
 [4,5],
 [7,8]
];
var sarr = [[]];
for (var i = 0; i < arr.length; i++) {
 var tarr = [];
 for (var j = 0; j < sarr.length; j++)
  for (var k = 0; k < arr[i].length; k++){
   tarr.push(sarr[j].concat(arr[i][k]));
  }
 sarr = tarr;
}
console.log(sarr)