What s the difference between returning value or Promise resolve from then

Question

What is the difference between    x000D   x000D  new Promise function res  rej    x000D      res  aaa    x000D       x000D     then function result    x000D      return  bbb   x000D       x000D     then function result    x000D      console log result   x000D        x000D   x000D   x000D    and this    x000D   x000D  new Promise function res  rej    x000D      res  aaa    x000D       x000D     then function result    x000D      return Promise resolve  bbb    x000D       x000D     then function result    x000D      console log result   x000D        x000D   x000D   x000D    I m asking as I m getting different behaviour Using Angular and  http service with chaining  then    A bit too much code hence first the example above

User · Answer

The rule is  if the function that is in the then handler returns a value  the promise resolves rejects with that value  and if the function returns a promise  what happens is  the next then clause will be the then clause of the promise the function returned  so  in this case  the first example falls through the normal sequence of the thens and prints out values as one might expect  in the second example  the promise object that gets returned when you do Promise resolve  quot bbb quot   s then is the then that gets invoked when chaining for all intents and purposes   The way it actually works is described below in more detail  Quoting from the Promises A  spec   The promise resolution procedure is an abstract operation taking as input a promise and a value  which we denote as   Resolve   promise  x   If x is a thenable  it attempts to make promise adopt the state of x  under the assumption that x behaves at least somewhat like a promise  Otherwise  it fulfills promise with the value x  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   The key thing to notice here is this line   if x is a promise  adopt its state  3 4  link  https   promisesaplus com  point-49

User · Answer

Both of your examples should behave pretty much the same   A value returned inside a then   handler becomes the resolution value of the promise returned from that then    If the value returned inside the  then  is a promise  the promise returned by then   will  adopt the state  of that promise and resolve reject just as the returned promise does   In your first example  you return  bbb  in the first then   handler  so  bbb  is passed into the next then   handler   In your second example  you return a promise that is immediately resolved with the value  bbb   so  bbb  is passed into the next then   handler   The Promise resolve   here is extraneous    The outcome is the same   If you can show us an example that actually exhibits different behavior  we can tell you why that is happening

User · Answer

In simple terms  inside a then handler function   A  When x is a value  number  string  etc     return x is equivalent to return Promise resolve x  throw x is equivalent to return Promise reject x    B  When x is a Promise that is already settled  not pending anymore     return x is equivalent to return Promise resolve x   if the Promise was already resolved  return x is equivalent to return Promise reject x   if the Promise was already rejected    C  When x is a Promise that is pending    return x will return a pending Promise  and it will be evaluated on the subsequent then     Read more on this topic on the Promise prototype then   docs

User · Answer

You already got a good formal answer  I figured I should add a short one   The following things are identical with Promises A  promises    Calling Promise resolve  In your Angular case that s  q when  Calling the promise constructor and resolving in its resolver  In your case that s new  q  Returning a value from a then callback   Calling Promise all on an array with a value and then extract that value     So the following are all identical for a promise or plain value X   Promise resolve x   new Promise function resolve  reject   resolve x       Promise resolve   then function    return x      Promise all  x   then function arr   return arr 0         And it s no surprise  the promises specification is based on the Promise Resolution Procedure which enables easy interoperation between libraries  like  q and native promises  and makes your life overall easier  Whenever a promise resolution might occur a resolution occurs creating overall consistency

User · Answer

The only difference is that you re creating an unnecessary promise when you do return Promise resolve  bbb    Returning a promise from an onFulfilled   handler kicks off promise resolution  That s how promise chaining works

[javascript] What's the difference between returning value or Promise.resolve from then()

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