Bluebird forgotten return warning is missing

Question

I expect Bluebird forgotten return warning to appear but it doesn't work for some reason.

A demo:

const Bluebird = require('bluebird');

Bluebird.config({
    warnings: true
})

Bluebird.resolve(1)
.then(() => {
    Bluebird.resolve(2); // should warn about forgotten return
})
.then(two => console.log(two));

How can it be fixed to output a warning?

I suspect I already encountered this problem before but I don't remember what was the solution.

Answer 1

It seems that long stack traces need to be enabled for the warnings to show. You can use the config object to enable them (docs) (demo):

Bluebird.config({
    warnings: true,
    longStackTraces: true
});

Or environment variables (docs) (demo):

In Node.js you may configure warnings and long stack traces for the entire process using environment variables:

BLUEBIRD_LONG_STACK_TRACES=1 BLUEBIRD_WARNINGS=1 node app.js

Both features are automatically enabled if the BLUEBIRD_DEBUG environment variable has been set or if the NODE_ENV environment variable is equal to "development".

and

To enable long stack traces and warnings in node development:

$ NODE_ENV=development node server.js

To enable long stack traces and warnings in node production:

$ BLUEBIRD_DEBUG=1 node server.js

See Environment Variables.

---

Edit as to why this is necessary:

It seems that both warnings and long stack traces are disabled by default, and are only enabled if a development environment is detected, see here:

Note that even though false is the default here, a development environment might be detected which automatically enables long stack traces and warnings.

To get warnings to show in a production environment, not only do you have to enable warnings, you have to enable long stack traces as well, see here and here.

Answer 2

You can configure the warning for checking forgotten return statements with a combination between wForgottenReturn and longStackTraces properties from the config object. wForgottenReturn is a property of warning and must be set to true and is the only warning type that can be separately configured. The corresponding environmental variable key is BLUEBIRD_W_FORGOTTEN_RETURN. You may check the documentation for more info.

const Bluebird = require('bluebird');

Bluebird.config({
    warnings: {
        wForgottenReturn: true
    }, longStackTraces: true,
});


Bluebird.resolve(1).then(() => {
   Bluebird.resolve(2);
}).then(two => console.log(two));

Running the program in the console gives me:

Warning: a promise was created in a handler at /home/adrianpop/test/bb.js:11:13 but was not returned from it, see 
    at Function.Promise.cast (/home/adrianpop/Downloads/Test/node_modules/bluebird/js/release/promise.js:196:13)
undefined

which is the desired output by you.

You can also run the application as:

BLUEBIRD_LONG_STACK_TRACES=1 BLUEBIRD_WARNINGS=1 node app.js, producing the same result.

Cheers!

Edit:

From this issue on github, we have that:

So the problem is that by default Nodejs 6.x does not display stack traces for warnings. There is a command line option (--trace-warnings) to enable them. Without this option Bluebird warnings are a lot less useful. Without the call stack, it can be very difficult to figure out where the warning originated.

More info can also be found:

• here (github issue)

• this (github PR)

• this SO question.