Google Tag Manager custom JavaScript errror

The solution is to use the built-in "Cookie" Variable (go to variables, new, 1st Party Cookie, enter a name for the variable and in the config the name of the cookie). No need for homegrown solutions.

To strictly answer the question, custom javascript variables must be written as an anonymous function with a return value:

function() {
return "something";
}

I'm sure there is a workaround to pass parameters (i.e. the cookie name) but that's not typically how custom javascript variables are used and you do not need this for your use case.

if you insist on your function you could always return it via a custom javascript variable:

function() {
return function (name) {
            var re = new RegExp(name + "=([^;]+)");
            var value = re.exec(document.cookie);
         return (value != null) ? unescape(value[1]) : null;
            }
}

Store it in a custom javascript variable called getCookie and access in in custom HTML tags via {{getCookie}}('xyz'). Not a very good idea but possible.

I usually deal with cookie sets & gets via a custom HTML tag as I believe the custom Javascript requires a return value. So something along the lines of this:

<script>
(function() {

function getCookie(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;
}

console.log(getCookie("xyz"));
</script>

})();

The above would output a log to the console with the value of a cookie named xyz whenever the tag's trigger is fired.

If you wanted to store and use this cookie value within GTM you would then need to pass this variable on and store it, either using the cookie variable or via the dataLayer.