Whether it’s an ES6 Promise
or a Bluebird Promise
, Q Promise
, etc.
How do I test to see if a given object is a Promise
?
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Answer
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 😀