Question

I found two functions to get cookie data with Javascript, one on w3schools.com and one on quirksmode.org
I would like to know which one I should use?

For example I believe I read somewhere that there was a problem with some browsers splitting the ; semicolon?

w3schools:

function getCookie(c_name) {
    if (document.cookie.length > 0) {
        c_start = document.cookie.indexOf(c_name + "=");
        if (c_start != -1) {
            c_start = c_start + c_name.length + 1;
            c_end = document.cookie.indexOf(";", c_start);
            if (c_end == -1) c_end = document.cookie.length;
            return unescape(document.cookie.substring(c_start, c_end));
        }
    }
    return "";
}

quirksmode:

function readCokie(name) {
    var nameEQ = name + "=";
    var ca = document.cookie.split(';');
    for(var i = 0; i < ca.length; i++) {
        var c = ca[i];
        while (c.charAt(0) == ' ') c = c.substring(1, c.length);
        if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length, c.length);
    }
    return null;
}

Answer 1

The function from W3CSchool is wrong. It fails if there are multiple cookies that have the same suffix like:

ffoo=bar; foo=baz

When you search for foo it will return the value of ffoo instead of foo.

Now here is what I would do: First of all, you need get to know the syntax of how cookies are transported. Netscape’s original specification (there are only copies available like this one at haxx.se) uses semicolons to separate multiple cookies while each name/value pair has the following syntax:

NAME=VALUE
This string is a sequence of characters excluding semi-colon, comma and white space. If there is a need to place such data in the name or value, some encoding method such as URL style %XX encoding is recommended, though no encoding is defined or required.

So splitting document.cookie string at semi-colons or commas is a viable option.

Besides that, RFC 2109 does also specify that cookies are separated by either semi-colons or commas:

cookie          =       "Cookie:" cookie-version
                        1*((";" | ",") cookie-value)
cookie-value    =       NAME "=" VALUE [";" path] [";" domain]
cookie-version  =       "$Version" "=" value
NAME            =       attr
VALUE           =       value
path            =       "$Path" "=" value
domain          =       "$Domain" "=" value

Although both are allowed, commas are preferred as they are the default separator of list items in HTTP.

Note: For backward compatibility, the separator in the Cookie header
is semi-colon (;) everywhere. A server should also accept comma (,)
as the separator between cookie-values for future compatibility.

Furthermore, the name/value pair has some further restrictions as the VALUE can also be a quoted string as specified in RFC 2616:

attr        =     token
value       =     token | quoted-string

So these two cookie versions need to be treated separately:

if (typeof String.prototype.trimLeft !== "function") {
    String.prototype.trimLeft = function() {
        return this.replace(/^\s+/, "");
    };
}
if (typeof String.prototype.trimRight !== "function") {
    String.prototype.trimRight = function() {
        return this.replace(/\s+$/, "");
    };
}
if (typeof Array.prototype.map !== "function") {
    Array.prototype.map = function(callback, thisArg) {
        for (var i=0, n=this.length, a=[]; i<n; i++) {
            if (i in this) a[i] = callback.call(thisArg, this[i]);
        }
        return a;
    };
}
function getCookies() {
    var c = document.cookie, v = 0, cookies = {};
    if (document.cookie.match(/^\s*\$Version=(?:"1"|1);\s*(.*)/)) {
        c = RegExp.$1;
        v = 1;
    }
    if (v === 0) {
        c.split(/[,;]/).map(function(cookie) {
            var parts = cookie.split(/=/, 2),
                name = decodeURIComponent(parts[0].trimLeft()),
                value = parts.length > 1 ? decodeURIComponent(parts[1].trimRight()) : null;
            cookies[name] = value;
        });
    } else {
        c.match(/(?:^|\s+)([!#$%&'*+\-.0-9A-Z^`a-z|~]+)=([!#$%&'*+\-.0-9A-Z^`a-z|~]*|"(?:[\x20-\x7E\x80\xFF]|\\[\x00-\x7F])*")(?=\s*[,;]|$)/g).map(function($0, $1) {
            var name = $0,
                value = $1.charAt(0) === '"'
                          ? $1.substr(1, -1).replace(/\\(.)/g, "$1")
                          : $1;
            cookies[name] = value;
        });
    }
    return cookies;
}
function getCookie(name) {
    return getCookies()[name];
}