How do I tell if an object is a Promise?
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?
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+-compliant then 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
检查是否有必要承诺的东西使代码复杂化，只需使用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.
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