Improve Warning: a promise was created in a handler but none were returned from it

[bergus]

It seems this warning is happening a lot and confusing quite a few users (see #508, #854, #1054, #1084 and many others).

Here are some ideas how the message could be improved:

• include a link to http://bluebirdjs.com/docs/warning-explanations.html (maybe anchored to some id shorter than #warning-a-promise-was-created-in-a-handler-but-was-not-returned-from-it)
• make the location of the handler function more prominent. An unsuspecting user should be pointed right to the line where the return is missing, the rest of the stack trace is not so important. I bet 80 to 90 percent of cases are "Right I forgot the return here", and only the minority is "but that function doesn't seem to return a promise, why would I need to return?" or "but I do return the result of the function, why is it not a promise?" that would make it necessary to look at the stack trace.
• the stack trace itself should include the lines "promise created by new Promise/then/whatever" and "then/catch/whatever calls the handler"

[benjamingr]

I think we can fix #1 (PR #1207 created) - about the other two:

• I think the stack trace is crucial since the users might return a promise conditionally (that is often the case) so I disagree with Add kew to cujojs benchmark #2.
• The warning message itself already includes the "promise created by " + name, feel free to improve on it.

[bergus]

By 2) I didn't mean that the stack trace should be omitted, rather that the line should be repeated.

For example:

function a() { new Promise(()=>{}); }
function b() { return a(); }
function d() {
    return x().then(function c(res) {
         res += …;
         b();
     })
}
d()

Now I'd love to get

Warning: a promise was created in the handler at test.js line 6
but was not returned from it, see http://goo.gl/rRqMUw
    at new Promise (bluebird.js)
    at a (test.js:1)
    at b (test.js:2)
    at c (test.js:6)
    at callThenHandler (bluebird.js)
from previous event:
    at d (test.js:4)
    at global (test.js:9)

[petkaantonov]

The merged PR only added the link, new PR welcome that adds the line too

[petkaantonov]

Addressed both additional requirements @bergus.

"use strict";

var Promise = require("./js/debug/bluebird");

Promise.config({warnings: true})

function x() {
    return Promise.delay(3);
}
function a() { Promise.delay() }
function b() { return a(); }
function d() {
    return x().then(function c(res) {
         res += 3;
         b();
     }).then(function(){})
}
d()

Now gives:

Warning: a promise was created in a handler at /home/petka/bluebird/throwaway.js:15:10 but was not returned from it, see http://goo.gl/rRqMUw
    at Function.Promise.delay (/home/petka/bluebird/js/debug/timers.js:30:13)
    at a (/home/petka/bluebird/throwaway.js:10:24)
    at b (/home/petka/bluebird/throwaway.js:11:23)
    at c (/home/petka/bluebird/throwaway.js:15:10)
    at tryOnTimeout (timers.js:232:11)
    at Timer.listOnTimeout (timers.js:202:5)
From previous event:
    at d (/home/petka/bluebird/throwaway.js:13:16)
    at Object.<anonymous> (/home/petka/bluebird/throwaway.js:18:1)
    at Module._compile (module.js:556:32)
    at Object.Module._extensions..js (module.js:565:10)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
    at Module.runMain (module.js:590:10)
    at run (bootstrap_node.js:394:7)
    at startup (bootstrap_node.js:149:9)
    at bootstrap_node.js:509:3

"use strict";

var Promise = require("./js/debug/bluebird");

Promise.config({warnings: true})

function x() {
    return Promise.delay(3);
}
function a() { new Promise(function() {}) }
function b() { return a(); }
function d() {
    return x().then(function c(res) {
         res += 3;
         b();
     }).then(function(){})
}
d()

Now gives:

(node:13673) Warning: a promise was created in a handler at /home/petka/bluebird/throwaway.js:15:10 but was not returned from it, see http://goo.gl/rRqMUw
    at new Promise (/home/petka/bluebird/js/debug/promise.js:78:14)
    at a (/home/petka/bluebird/throwaway.js:10:16)
    at b (/home/petka/bluebird/throwaway.js:11:23)
    at c (/home/petka/bluebird/throwaway.js:15:10)
    at tryOnTimeout (timers.js:232:11)
    at Timer.listOnTimeout (timers.js:202:5)
From previous event:
    at d (/home/petka/bluebird/throwaway.js:13:16)
    at Object.<anonymous> (/home/petka/bluebird/throwaway.js:18:1)
    at Module._compile (module.js:556:32)
    at Object.Module._extensions..js (module.js:565:10)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
    at Module.runMain (module.js:590:10)
    at run (bootstrap_node.js:394:7)
    at startup (bootstrap_node.js:149:9)
    at bootstrap_node.js:509:3

[petkaantonov]

Released in 3.4.3

Also, in node v6 without --trace-warnings the warning is now quite useful even without stack:

(node:14142) Warning: a promise was created in a handler at /home/petka/bluebird/throwaway.js:15:10 but was not returned from it, see http://goo.gl/rRqMUw
    at new Promise (/home/petka/bluebird/js/debug/promise.js:78:14)

[bergus]

Gorgeous, thank you!