How do I tell if an object is a Promise?
How a promise library decides
If it has a .then
function - that's the only standard promise libraries use.
The Promises/A+ specification has a notion called then
able which is basically "an object with a then
method". Promises will and should assimilate anything with a then method. All of the promise implementation you've mentioned do this.
If we look at the specification:
2.3.3.3 if
then
is a function, call it with x as this, first argument resolvePromise, and second argument rejectPromise
It also explains the rationale for this design decision:
This treatment of
then
ables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliantthen
method. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.
How you should decide
You shouldn't - instead call Promise.resolve(x)
(Q(x)
in Q) that will always convert any value or external then
able into a trusted promise. It is safer and easier than performing these checks yourself.
really need to be sure?
You can always run it through the test suite :D
Checking if something is promise unnecessarily complicates the code, just use Promise.resolve
Promise.resolve(valueOrPromiseItDoesntMatter).then(function(value) {
})
Here's my original answer, which has since been ratified in the spec as the way to test for a promise:
Promise.resolve(obj) == obj
This works because the algorithm explicitly demands that Promise.resolve
must return the exact object passed in if and only if it is a promise by the definition of the spec.
I have another answer here, which used to say this, but I changed it to something else when it didn't work with Safari at that time. That was a year ago, and this now works reliably even in Safari.
I would have edited my original answer, except that felt wrong, given that more people by now have voted for the altered solution in that answer than the original. I believe this is the better answer, and I hope you agree.