first-last-in-array.js

function occurences(n, arr) {
    var first = -1;
    var last = -1;
    if (arr === null || arr.length === 0 || arr[0] > n || arr[arr.length - 1] < n) {
    	return [first, last];
    }
    
	for (var i = 0; i < arr.length; i++) {
    	var num = arr[i];
        if (num === n) {
        	if (first < 0) {
            	first = i;
            } else {
            	last = i;
            }
        } else if (num > n) {
        	return [first, last];
        }
    }
    return [first, last];
}
console.log(occurences(7, [1, 2, 3, 6, 7, 7, 8]));