Any comments on improvement welcomed!

```
function sumOfLeftLeaves(root, sum = 0, isLeft = false) {
if (!root) {
return sum;
}
sum = sumOfLeftLeaves(root.left, sum, true);
if (isLeft && !root.left && !root.right) {
sum += root.val;
}
sum = sumOfLeftLeaves(root.right, sum, false);
return sum;
}
```