custom Errors?

[ultimatebuster]

I'm pretty new to JavaScript.

I'm trying to define my custom errors, but it doesn't seems to be working

function MyError(msg){
    this.message = msg
    this.name = "My Error"
}

body.onload = function(){
    try{ 
        er = new MyError("Test Message")

        throw er();
    } catch(e) {
        alert(e.name + ": " + e.message);
    }
}

Getting TypeError, claiming that it's not a function.