Is it possible to have long stack traces?

[amasad]

In addition to that there are some errors that are very cryptic:

Example snippet:

TypeError: Cannot read property 'length' of null
    at Promise._94 

Are there any plans to enhance the UX?

cc @ForbesLindesay

[ForbesLindesay]

Yes, I would love to see long stack traces implemented. We can use Q as a good example of a library that already implements them. We should make them disabled by default and allow people to require('promise/lib/long-stack-traces') if they want them enabled.

[amasad]

Can you help me understand this stacktrace:

 ERROR  Cannot read property 'length' of null
TypeError: Cannot read property 'length' of null
    at Promise._94 (react-native-github/node_modules/promise/lib/core.js:138:31)
    at Promise._82 (react-native-github/node_modules/promise/lib/core.js:129:8)
    at react-native-github/node_modules/promise/lib/core.js:97:24
    at flush (react-native-github/node_modules/promise/node_modules/asap/raw.js:50:29)
    at process._tickCallback (node.js:415:13)

[amasad]

Apparently it's a race. After tracing back this call stack to the original source it seems that asap is calling twice (and finalizing) on the same promise. If I change a part of my code from Promise.all to a serial chain this works fine.

[ForbesLindesay]

Interesting. Can you share the code to reproduce this? It sounds like this might be related to #93 but I was never able to get code to reproduce that.

[amasad]

let me try and boil it down to something reproducible.
Another random thought, could this be an issue with interop with a different promise library like bluebird?

[ForbesLindesay]

It's definitely possible. Either way I really want to get to the bottom of it. If there's a bug in asap or this lib then finding it would be a huge win.

[ForbesLindesay]

I've reconstructed as much as I can of what the original stack trace should look like. Unfortunately it doesn't tell me much extra:

TypeError: Cannot read property 'length' of null
    at Promise._finale (react-native-github/node_modules/promise/lib/core.js:138:31)
    at Promise._resolve (react-native-github/node_modules/promise/lib/core.js:129:8)
    at react-native-github/node_modules/promise/lib/core.js:97:24
    at flush (react-native-github/node_modules/promise/node_modules/asap/raw.js:50:29)
    at process._tickCallback (node.js:415:13)
From previous event:
    at Promise._handle (react-native-github/node_modules/promise/lib/core.js:87)

[amasad]

I couldn't reproduce outside of the codebase. I'm going to have to add you to my (internal) diff and you only have to change from bluebird to promise to start reproing

[ForbesLindesay]

It turns out this is (as you suspected) caused by asap sometimes calling back twice and sometimes not at all. The offending issue is kriskowal/asap#55, which has now been resolved. Updating asap should be sufficient, which should happen automatically if you delete any caches and re-install.

I will release an update of promise that requires the latest version of asap though to encourage people to update faster.