1039153 - Success handler for promises from "core/promise" fails unexpectedly

Status: RESOLVED INVALID

(Add-on SDK Graveyard :: General, defect)

Description

[Alan]

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:30.0) Gecko/20100101 Firefox/30.0 (Beta/Release)
Build ID: 20140605174243

Steps to reproduce:

Certain success handler for promise.then() will cause its returned promise to fail.

Run the following:

let arr = [];
let d = defer();
d.resolve(true);
d.promise.
    then(arr.push).
    then(r => console.log("Success, result is " + r), console.log);


Actual results:

a TypeError and a nasty trace is logged. (from the error handler of the second then())


Expected results:

instead of logging "Success, result is true"

Comment 1

[Alan]

Corrections, the expected result should be "Success, result is 1" (return value of arr.push)

here is a version that will work expectedly

let arr = [];
let d = defer();
d.resolve(true);
d.promise.
    then(r => arr.push(r)).
    then(r => console.log("Success, result is " + r), console.log);

Comment 2

[Matteo Ferretti [:zer0] [:matteo]]

It's actually correct that is failing.: you have an exception on the first `then`, that is properly catch in the following one.

With `then(arr.push)` you're basically passing only the reference to `push`, so the `this` object internally won't point to `arr`. It's like doing:

  let arr = [];
  let f = arr.push;
  f(12); // exception

You have to bind `push` with `arr`:
 
  …
  then(arr.push.bind(arr)).
  …

Or, as you did already in your second comment, call it as method of `arr`.

Comment 3

[Alan]

(In reply to Matteo Ferretti [:matteo] [:zer0] from comment #2)
> It's actually correct that is failing.: you have an exception on the first
> `then`, that is properly catch in the following one.
> 
> With `then(arr.push)` you're basically passing only the reference to `push`,
> so the `this` object internally won't point to `arr`. It's like doing:
> 
>   let arr = [];
>   let f = arr.push;
>   f(12); // exception
> 
> You have to bind `push` with `arr`:
>  
>   …
>   then(arr.push.bind(arr)).
>   …
> 
> Or, as you did already in your second comment, call it as method of `arr`.

Thanks a lot for the insight! I'm still pretty new to JS and this taught me a lot! I would of never thought it is related to prototypical inheritance.