I have a JS array so:

```
var car_type_1 = [12,16,23,31,33,34,39,45,49,54,59,62,62,62,63,63,64,66,69,71,73,75,78,80,82,85,87,89,91,94,95];
var car_type_2 = [17,20,28,35,37,38, and so on];

```

I created another array which stores the subtracted values between the consecutive numbers in the old array:

```
    function diff(arr) {
      return arr.slice(1).map(function(n, i) { return n - arr[i]; });
    }

var car_type_1_diff = diff(car_type_1);
//so on

```

Understandly, the new array has 1 less element compared to the old array. What I want to do is add the first number of the old array to the new array, so it should be like:

```
  car_type_1_diff = [**12**, 4, 7, 8, 2, 1, 5, 6, 4, 5, 5, 3, 0, 0, 1, 0, 1, 2, 3, 2, 2, 2, 3, 2, 2, 3, 2, 2, 2, 3, 1] 

```

12 should be pushed to the beginning of the new array 1. How can I do this?