How come resolving a settled Promise doesn't throw?

[Jordan Harband]

Although now that I think about it, it wouldn't have to care about the
promise state, `resolve` and `reject` could just throw if they're invoked
more than once.

On Tue, Feb 28, 2017 at 1:35 PM, Jordan Harband <ljharb at gmail.com> wrote:

> That seems like it would allow synchronous observation of Promise state -
> consider:
>
> ```js
>
> function isResolved(promise) {
>   try {
>     new Promise((resolve) => {
>       resolve(promise);
>       resolve();
>     });
>   } catch (e) {
>     return true;
>   }
>   return false;
> }
>
> ```
>
> On Tue, Feb 28, 2017 at 12:58 PM, Isiah Meadows <isiahmeadows at gmail.com>
> wrote:
>
>> Also, making promise resolution idempotent makes dealing with things way
>> easier. Similarly, most deferred libraries ensure their resolution is
>> idempotent.
>>
>> On Tue, Feb 28, 2017, 13:20 Tab Atkins Jr. <jackalmage at gmail.com> wrote:
>>
>>> On Tue, Feb 28, 2017 at 10:12 AM, /#!/JoePea <joe at trusktr.io> wrote:
>>> > f.e.
>>> >
>>> > ```js
>>> > let resolve
>>> > let p = new Promise(r => resolve = r)
>>> >
>>> > resolve(5) //  resolves the promise.
>>> > resolve(4) // noop (in Chrome), but why not throw an error?
>>> > ```
>>> >
>>> > I only tested in Chrome, and I'm assuming it follows spec, but I could
>>> be
>>> > wrong.
>>> >
>>> > I'm asking because it seems that throwing an error will prevent shots
>>> in the
>>> > foot, so that code doesn't assume that resolving on an already resolved
>>> > Promise worked, although it didn't. It seems like it can lead to
>>> unexpected
>>> > failures.
>>>
>>> That's correct behavior, yes.  In general, it's because the internal
>>> state of a promise is meant to be unobservable unless you're
>>> specifically listening to it.
>>>
>>> ~TJ
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>>>
>>
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