strStr.js

/*Implement strStr().
Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
Example 1:
Input: haystack = "hello", needle = "ll"
Output: 2
Example 2:
Input: haystack = "aaaaa", needle = "bba"
Output: -1
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().*/
/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var haystack = 'hello', needle = 'll';
var strStr = function (haystack, needle) {   // 28
    if (!needle.length) {
      return 0;
    }

    for (let i = 0; i <= haystack.length - needle.length; i++) {
      if (haystack[i] === needle[0]) {
        if (haystack.substring(i, i + needle.length) === needle) {
          return i;
        }
      }
    }
    return -1;    
};

console.log(strStr(haystack, needle));