[1,2,-3].reduce((a, b) => a - b, 0)

The output of this code is -4.

Here's how it works:

- The reduce() method is called on the array [1, 2, -3].
- The arrow function (a, b) => a - b is used as the callback function for reduce(). This function takes two arguments: a (the accumulator) and b (the current value being processed in the array).
- The initial value of the accumulator is set to 0 (the second argument passed to reduce()).
- The function subtracts b from a (a - b) and returns the result. This value becomes the new value of the accumulator for the next iteration.
- The reduce() method continues to iterate over each element in the array, applying the callback function until all elements have been processed.
- The final value of the accumulator (which is also the output of the reduce() method) is -4. This is the result of subtracting all the elements in the array from the initial value of 0.

So, in summary, the code subtracts all the elements in the array from 0 and returns the result (-4).