How do I tell if an object is a Promise

Question

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

User · Answer

This is how graphql-js package detects promises:

function isPromise(value) {
  return Boolean(value && typeof value.then === 'function');
}

value is the returned value of your function. I'm using this code in my project and have no problem so far.

User · 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 thenable which is basically  quot an object with a then method quot   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 thenables 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 thenable 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

User · Answer

Update  This is no longer the best answer  Please vote up my other answer instead   obj instanceof Promise   should do it  Note that this may only work reliably with native es6 promises   If you re using a shim  a promise library or anything else pretending to be promise-like  then it may be more appropriate to test for a  thenable   anything with a  then method   as shown in other answers here

User · Answer

In case you are using Typescript  I d like to add that you can use the  type predicate  feature  Just should wrap the logical verification in a function that returns x is Promise lt any gt  and you won t need to do typecasts  Below on my example  c is either a promise or one of my types which I want to convert into a promise by calling the c fetch   method   export function toPromise c  Container lt any gt    Promise lt any gt    Promise lt any gt        if  c    null  return Promise resolve        return isContainer c    c fetch     c     export function isContainer val  Container lt any gt    Promise lt any gt    val is Container lt any gt        return val  amp  amp    lt Container lt any gt  gt val  fetch     undefined     export function isPromise val  Container lt any gt    Promise lt any gt    val is Promise lt any gt        return val  amp  amp    lt Promise lt any gt  gt val  then     undefined      More info  https   www typescriptlang org docs handbook advanced-types html

User · Answer

const isPromise    value    gt      return         value  amp  amp      value then  amp  amp      typeof value then      function   amp  amp      value  constructor  name      Promise         As for me - this check is better  try it out

User · Answer

Checking if something is promise unnecessarily complicates the code  just use Promise resolve  Promise resolve valueOrPromiseItDoesntMatter  then function value

User · Answer

if  typeof thing  then      function            probably a promise   else          definitely not a promise

User · Answer

I use this function as a universal solution   function isPromise value      return value  amp  amp  value then  amp  amp  typeof value then      function

User · Answer

after searching for a reliable way to detect Async functions or even Promises  i ended up using the following test          gt  fn constructor name      Promise     fn constructor name      AsyncFunction

User · Answer

ES6   const promise   new Promise resolve   gt  resolve  ol        console log promise toString   includes  Promise       true

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Not an answer to the full question but I think it s worth to mention that in Node js 10 a new util function called isPromise was added which checks if an object is a native Promise or not   const utilTypes   require  util   types const b Promise   require  bluebird    utilTypes isPromise Promise resolve 5      true utilTypes isPromise b Promise resolve 5      false

User · Answer

Here is the code form https   github com ssnau xkit blob master util is-promise js    obj  amp  amp   typeof obj      object     typeof obj      function    amp  amp  typeof obj then      function     if an object with a then method  it should be treat as a Promise

User · Answer

it  should return a promise   function         var result   testedFunctionThatReturnsPromise        expect result  toBeDefined           3 slightly different ways of verifying a promise     expect typeof result then  toBe  function        expect result instanceof Promise  toBe true       expect result  toBe Promise resolve result

User · Answer

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

User · Answer

If you are in an async method you can do this and avoid any ambiguity   async myMethod promiseOrNot     const theValue   await promiseOrNot       If the function returns promise  it will await and return with the resolved value   If the function returns a value  it will be treated as resolved   If the function does not return a promise today  but tomorrow returns one or is declared async  you will be future-proof

User · Answer

To see if the given object is a ES6 Promise  we can make use of this predicate   function isPromise p      return p  amp  amp  Object prototype toString call p        object Promise        Calling toString directly from the Object prototype returns a native string representation of the given object type which is   object Promise   in our case  This ensures that the given object   Bypasses false positives such as      Self-defined object type with the same constructor name   Promise    Self-written toString method of the given object   Works across multiple environment contexts  e g  iframes  in contrast to instanceof or isPrototypeOf    However  any particular host object  that has its tag modified via Symbol toStringTag  can return   object Promise    This may be the intended result or not depending on the project  e g  if there is a custom Promise implementation      To see if the object is from a native ES6 Promise  we can use   function isNativePromise p      return p  amp  amp  typeof p constructor      function       amp  amp  Function prototype toString call p constructor  replace                         Function prototype toString call   native object  Function         replace  Function    Promise      replacing Identifier        replace                     removing possible FormalParameterList      According to this and this section of the spec  the string representation of function should be       function Identifier   FormalParameterListopt     FunctionBody      which is handled accordingly above  The FunctionBody is  native code  in all major browsers   MDN  Function prototype toString  This works across multiple environment contexts as well

[javascript] How do I tell if an object is a Promise?

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