Implement strStr.js

/*Implement strStr().

Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

 

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2
Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1
 

Constraints:

1 <= haystack.length, needle.length <= 104
haystack and needle consist of only lowercase English characters.*/

var strStr = function(haystack, needle) {
    let index = -1;
    for(let i = 0; i < haystack.length; i++) {
        if(haystack[i] === needle[0] && index === -1) {
            index = i
            for(let j = 0; j < needle.length; j++) {
                if(haystack[i+j] !== needle[j]) {
                   index = -1
                   break
                }
            }
        }
    }
    return needle ? index : 0
};