Kind of like btoa/atob, but doesn't trip on Unicode.

hex-string-encode-decode.js

/**
 * Encode a UTF-16 string as a string of hexadecimal letters
 */
function encodeIntoHex (string) {
    var hex;
    var hexString = "";

    for (var i = 0, ilen = string.length; i < ilen; i++) {
        hex = string.charCodeAt(i).toString(16);
        hexString += ("000" + hex).slice(-4);
    }

    return hexString
}

/**
 * Decode a string of hexadecimal letters back into a UTF-16 string
 */
function decodeFromHex (hexString) {
    var hexes = hexString.match(/.{1,4}/g) || [];
    var decodedString = "";

    for (var i = 0, ilen = hexes.length; i < ilen; i++) {
        decodedString += String.fromCharCode(parseInt(hexes[i], 16));
    }

    return decodedString;
}