Implement strStr()

Problem statement

Implement strStr().

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

Example 3:

Input: haystack = "", needle = ""Output: 0

Constraints:

• 0 <= haystack.length, needle.length <= 5 * 104
• haystack and needle consist of only lower-case English characters.

My solution

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function(haystack, needle) {
    if ((haystack === "" && needle === "") || needle === "") {
        return 0;
    }
    
    let idx = -1;
    
    for (let i = 0; i < haystack.length; i++) {
        if (haystack.charAt(i) === needle.charAt(0) && needle === haystack.slice(i, i + needle.length)) {
            idx = i;
            break;
        }
    }
    
    return idx;
};