Question

From within a function like:

function eventHandler(e) {
  // ...
}

is there a reliable and efficient way to determine whether e is a DOM event?

Answer 1

I don’t believe there is a reliable way to determine if a given object is NOT a DOM event.

1. typeof e will always return 'object' for genuine Event objects, which is not helpful.
2. Any property that you might check for on the object can exist in both a genuine Event object or any non-Event object.
3. You might think that the prototype chain could be of use in determining this, but it has the same problem as #2 (can easily be replicated).
4. The contructor property might seem promising, but one could do this:

function DummyEvent(){
    this.constructor.toString = function(){
        return "function MouseEvent() { [native code] }";
    }
}

This ends up making console.log(e.constructor) print "function MouseEvent() { [native code] }"

So, is there a “reliable” way to tell if an object is an event? No.

Edit — Note that all of this is irrelevant if you want to prevent event spoofing as you can create real events easily.

var evt = new Event('click');
var evt2 = document.createEvent('MouseEvents');
evt2.initMouseEvent('click', ...);
//etc

Edit2 — I’ve created a test jsFiddle trying to find some way to distinguish objects, but I haven’t found anything definite yet. Note that I didn’t bother to specify properties on the DummyEvent because those can obviously easily be spoofed.