Char to Hex in javascript

function string_as_unicode_escape(str){
    return str.split("").map(function(s){
        return "\\u"+("0000" + s.charCodeAt(0).toString(16)).slice(-4);
    }).join("");
}

You can loop through the characters and use the charCodeAt function to get their UTF-16 values, then constructing a string with them.

Here's some code I constructed that is much better than the code on the site you've linked, and should be easier to understand:

function string_as_unicode_escape(input) {
    function pad_four(input) {
        var l = input.length;
        if (l == 0) return '0000';
        if (l == 1) return '000' + input;
        if (l == 2) return '00' + input;
        if (l == 3) return '0' + input;
        return input;
    }
    var output = '';
    for (var i = 0, l = input.length; i < l; i++)
        output += '\\u' + pad_four(input.charCodeAt(i).toString(16));
    return output;
}

Let's break it down.

1. string_as_unicode_escape takes one argument, input, which is a string.
2. pad_four is an internal function that does one thing; it pads strings with leading '0' characters until the length is at least four characters long.
3. Start off by defining output as an empty string.
4. For each character in the string, append \u to the output string. Take the UTF-16 value of the character with input.charCodeAt(i), then convert it to a hexadecimal string with .toString(16), then pad it with leading zeros, then append the result to the output string.
5. Return the output string.

As Tim Down commented, we can also add 0x10000 to the charCodeAt value and then .slice(1) the string resulting from calling .toString(16), to achieve the padding effect.