Adler 32 checksum implementation in JS

adler32.js

function hash(str) {
    for (var i = 0, len = str.length, s1 = 1, s2 = 0; i < len; i++) {
        s1 = (s1 + str.charCodeAt(i)) % 65521;
        s2 = (s2 + s1) % 65521;
    }
    return (s2 << 16) + s1;
}