String IndexOf example

str-indexof.js

function strIndexOf(val, str) {
    val = String(val);
    var strLen = str.length;
    if (strLen >= val.length) {
        var head = val[0]
        var hasHead = -1;
        var matchLen = 0;

        for (var i = 0; i < strLen; i++) {
            var v = str[i];
            if (hasHead >= 0) {
                if (v === val[matchLen - 1]) {
                    matchLen++;
                } else {
                    hasHead = -1;
                    matchLen = 0;
                }
            } else {
                if (head == v) {
                    hasHead = i;
                    matchLen = 1;
                }
            }

            if (matchLen === val.length) {
                return hasHead;
            }
        }
    }
    return -1
}