How can I determine if a jQuery object is deferred?

Question

If I have a function that sometimes returns a deferred object but sometimes a non-deferred object. How can I tell which one it is?

Answer 1

Since jQuery Deferreds are created by copying the methods of a hidden object instead of calling the new operator on a function, you cannot proof that the object is indeed an instance of jQuery.Deferred. I think you're gonna need to go with Duck-Typing:

"When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck." – James Whitcomb Riley

Depending on what objects might otherwise be returned (what properties must be expected), check if particular properties / methods are present:

var x = getMysteriousObject();
if (x.promise) {
    // Deferred
} else {
    // Not a deferred
}

You can detailed this check if required:

if ($.isFunction(x.promise)) {
    // Deferred
}

or (to distinguish between Deferred objects and other implementations of the Promise interface)

if (x.promise && x.resolve) {
    // Deferred
}