var parts = document.location.href.split("/");
var id = parts[parts.length-1];

var url = document.URL.split('/');

var id = url.pop();

var id = url[url.lengh-1];

function isNumeric(n) {
    return !isNaN(parseFloat(n)) && isFinite(n);
}

function GetIdIfExists() {
    var parts = document.location.href.split("/");
    var id = parts[parts.length - 1];

    if (isNumeric(id)) {
        return parseInt(id);
    } else {
        return null;
    }
}

//url = https://stackoverflow.com/questions/15761950/how-do-get-optional-parameter-value-using-javascript-in-mvc
> GetIdIfExists()
< null

//url = https://stackoverflow.com/questions/15761950/how-do-get-optional-parameter-value-using-javascript-in-mvc/0
> GetIdIfExists()
< 0