Here is a revised, simpler version of your code.

    function indexFirstLast(target, arr) {
        let [first, last] = [-1, -1];
        for (let i = 0; i < arr.length && arr[i] <= target; i++) {
            if (arr[i] === target) {
                if (first === -1) {
                    first = i;
                }
                last = i;
            }
        }
        return [first, last];
    }