LeetCode - 28 - Find the Index of the First Occurrence in a String

 /**

 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function(haystack, needle) {
    var start = "";
    for (var i = 0; i < needle.length - 1 ; i++) {
        start += haystack[i];
    }
    for (var i = needle.length - 1 ; i < haystack.length ; i++) {
        start += haystack[i];
        if (start == needle) {
            return i - needle.length + 1;
        }
        start = start.slice(1);
    }
    return -1;
};