Menu
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?
263
The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value. To learn about the way promises work and how you can use them, we advise you to read Using promises first.
A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation, and its resulting value. var promise = new Promise(function(resolve, reject) { // do thing, then…
The Promise. resolve() method "resolves" a given value to a Promise . If the value is a promise, that promise is returned; if the value is a thenable, Promise. resolve() will call the then() method with two callbacks it prepared; otherwise the returned promise will be fulfilled with the value.
If it has a .then function - that's the only standard promise libraries use.
The Promises/A+ specification has a notion called thenable 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
thenis 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
thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliantthenmethod. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.
You shouldn't - instead call Promise.resolve(x) (Q(x) in Q) that will always convert any value or external thenable into a trusted promise. It is safer and easier than performing these checks yourself.
You can always run it through the test suite :D
64
Checking if something is promise unnecessarily complicates the code, just use Promise.resolve
Promise.resolve(valueOrPromiseItDoesntMatter).then(function(value) { }) If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
