Implement strStr().js

/**
Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
*/

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function(haystack, needle) {
    var len1 = haystack.length,
        len2 = needle.length,
        i,
        j;
    
    if (len2 === 0) {
        return 0;
    }
    
    if (len1 === 0 || len1 < len2) {
        return -1;
    }
    
    i = 0;
    while (i <= len1 - len2) {
        if (haystack.substring(i, i + len2) === needle) {
            return i;
        }
        i++;
    }
    
    return -1;
};