indexOf

indexOf.js

function indexOf(haystack, needle) {
    // If either is undefined, quit
    if (!haystack || !needle) return -1;
    
    for (var i = 0; i < haystack.length; i++) {
        
        // If there's not enough chars left in
        // `haystack` for a match, quit
        if (haystack.length - i < needle.length) return -1;
        
        // Starting at the current index in `haystack`, iterate
        // over the chars in `needle`, comparing against `haystack`
        for (var j = 0; j < needle.length; j++) {
            
            // If a char in `needle` doesn't match the char in
            // `haystack`, break, going to the next char in `needle`
            if (haystack[i+j] !== needle[j]) break;
            
            // Otherwise, there is a match; if we're at the end of
            // `needle`, we've matched all the chars from `needle`,
            // so return the current index from `haystack`
            if (j+1 === needle.length) return i;
        }
        
    }
    return -1;
}