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Node.js and browsers warn on unhandled promise rejections. You might have seen:

(node:1234) UnhandledPromiseRejectionWarning

When this happens, it's not always obvious what promise is unhandled. The error stacktrace will tell where the error object construction is, not the construction of the promise which left it dangling. It might have travelled through various asynchronous chains before it got to an unhandled promise chain.

trace-unhandled helps with this. It keeps track of promises and when an unhandled promise rejection is logged, the location of both the error object and the promise is logged. This makes it a lot easier to find the bug.

This package is not intended to be used in production, only to aid locating bugs


Consider the following code which creates an error (on line 1) and rejects a promise (on line 3) and "forgets" to catch it on line 9 (the last line). This is an incredibly simple example, and in real life, this would span over a lot of files and a lot of complexity.

1. const err = new Error( "foo" );
2. function b( ) {
3.	return Promise.reject( err );
4. }
5. function a( ) {
6.	return b( );
7. }
8. const foo = a( );
9. foo.then( ( ) => { } );

Without trace-unhandled, you would get something like:

(node:1234) UnhandledPromiseRejectionWarning: Error: foo
    at Object.<anonymous> (/my/directory/test.js:1:13)
    at Module._compile (internal/modules/cjs/loader.js:776:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
    at Module.load (internal/modules/cjs/loader.js:643:32)
    at Function.Module._load (internal/modules/cjs/loader.js:556:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:839:10)
    at internal/main/run_main_module.js:17:11

This is the output of Node.js. You'll see the stacktrace up to the point of the Error err, but that's rather irrelevant. What you want to know is where the promise was used leaving a rejection unhandled (i.e. a missing catch()). With trace-unhandled this is exactly what you get, including the Error construction location:

(node:1234) UnhandledPromiseRejectionWarning
[ Stacktrace altered by ]
Error: foo
    ==== Promise at: ==================
    at Promise.then (<anonymous>)
    at Object.<anonymous> (/my/directory/test.js:9:5)  👈

    ==== Error at: ====================
    at Object.<anonymous> (/my/directory/test.js:1:13)

    ==== Shared trace: ================
    at Module._compile (internal/modules/cjs/loader.js:776:30)
	... more lines below ...

We "used" the promise by appending another .then() to it. This means that the promise was actually "handled", and that the new promise should handle rejections. If we delete the last line (line 9), we see where the promise was last "used":

(node:1234) UnhandledPromiseRejectionWarning
[ Stacktrace altered by ]
Error: foo
    ==== Promise at: ==================
    at b (/my/directory/test.js:3:17)                   👈
    at a (/my/directory/test.js:6:9)                    👈
    at Object.<anonymous> (/my/directory/test.js:8:13)  👈

    ==== Error at: ====================
    at Object.<anonymous> (/my/directory/test.js:1:13)

    ==== Shared trace: ================
    at Module._compile (internal/modules/cjs/loader.js:776:30)
	... more lines below ...

Both these examples show clearly where the promise is left unhandled, and not only where the Error object is constructed.


trace-unhandled can be used in 4 ways.

As a standalone program

trace-unhandled exports a program which can run JavaScript files and shebang scripts. Instead of running your program as node index.js you can do trace-unhandled index.js as long as trace-unhandled is globally installed.

You can also use npx:

npx trace-unhandled index.js

In a website

<head><script src=""></script></head>

To specify a custom logger function, use setTraceUnhandledLogger:

window.setTraceUnhandledLogger( msg => { ... } ); // msg is a string

Programatically - API

require( 'trace-unhandled/register' ); // As early as possible

or if you want to allow some code to execute before you start tracing:

const { register } = require( 'trace-unhandled' );

// ... whenever you want to start tracing
register( );

To specify a custom logger function, use setLogger:

const { setLogger } = require( 'trace-unhandled' );
setLogger( msg => { ... } ); // msg is a string

In unit tests

To use this package when running jest, install the package and configure jest with the following setup:

  setupFiles: [

For mocha you can use --require node_modules/trace-unhandled/register.js.

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