Function to add and subtract alternate array elements in JavaScript

Took 5 minutes and 10 seconds to code JavaScript method to add and subtract alternate elements in an array. Here is the code snippet.

Code:

arr = [1,2,3,4,5,6,7];

function alternateAddSubtract(arr) {

var sum = 0;

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

if (i % 2 == 0) {

sum += arr[i];

} else {

sum -= arr[i];

}

}

return sum;

}

document.write(alternateAddSubtract(arr));

Result:

4