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example of how to debug mocha v4 if hanging
Raw
README.md

Here's an example of how to debug Mocha v4 if it hangs.

Ensure you're using a Node.js 8 or newer (or any version with async_hooks support).

If you run your test, you'll notice it hangs:

$ mocha test.js


  how to debug Mocha when it hangs
    ✓ should complete, but Mocha should not exit


  1 passing (25ms)

You can include a module like async-dump.js to print information about what's happening. First, add this hook somewhere in the describe block:

after(function () {
  global.asyncDump();
});

Then, you can --require async-dump.js with Mocha:

$ mocha --require async-dump test.js


  how to debug Mocha when it hangs
    ✓ should complete, but Mocha should not exit

STUFF STILL IN THE EVENT LOOP:
Type: SIGNALWRAP
Error
    at AsyncHook.init (async-dump.js:12:62)
    at Signal.emitInitNative (async_hooks.js:458:43)
    at process.<anonymous> (internal/process.js:209:20)
    at startup (bootstrap_node.js:201:16)
    at bootstrap_node.js:626:3


Type: TCPWRAP
Error
    at AsyncHook.init (async-dump.js:12:62)
    at TCP.emitInitNative (async_hooks.js:458:43)
    at createServerHandle (net.js:1267:14)
    at Server.setupListenHandle [as _listen2] (net.js:1310:14)
    at listenInCluster (net.js:1391:12)
    at Server.listen (net.js:1474:7)
    at Context.<anonymous> (debug-hanging-mocha.js:9:12)


Type: TCPWRAP
Error
    at AsyncHook.init (async-dump.js:12:62)
    at TCP.emitInitNative (async_hooks.js:458:43)
    at Socket.connect (net.js:997:40)
    at Object.connect (net.js:104:35)
    at Context.<anonymous> (debug-hanging-mocha.js:17:22)


Type: DNSCHANNEL
Error
    at AsyncHook.init (async-dump.js:12:62)
    at ChannelWrap.emitInitNative (async_hooks.js:458:43)
    at new Resolver (dns.js:249:20)
    at dns.js:380:25
    at NativeModule.compile (bootstrap_node.js:614:7)
    at NativeModule.require (bootstrap_node.js:559:18)
    at lazyDns (net.js:1376:11)
    at lookupAndConnect (net.js:1026:15)
    at Socket.connect (net.js:1019:5)
    at Object.connect (net.js:104:35)
    at Context.<anonymous> (debug-hanging-mocha.js:17:22)


Type: TCPWRAP
Error
    at AsyncHook.init (async-dump.js:12:62)
    at TCP.emitInitNative (async_hooks.js:458:43)


Type: Immediate
Error
    at AsyncHook.init (async-dump.js:12:62)
    at emitInitNative (async_hooks.js:458:43)
    at emitInitScript (async_hooks.js:361:3)


  1 passing (25ms)

These are not Errors; everything that's printed above are async tasks that have been "started" but not "finished".

From the above, you will notice that the TCPWRAP resource(s) are interesting. Within the stack trace, you can see where exactly these happen in your code:

Type: TCPWRAP
Error
    at AsyncHook.init (async-dump.js:12:62)
    at TCP.emitInitNative (async_hooks.js:458:43)
    at createServerHandle (net.js:1267:14)
    at Server.setupListenHandle [as _listen2] (net.js:1310:14)
    at listenInCluster (net.js:1391:12)
    at Server.listen (net.js:1474:7)
    at Context.<anonymous> (debug-hanging-mocha.js:9:12)

On line 9, we're listening on a port, but we never shut down the server.

async-dump.js could certainly be improved upon, but this is a rough guide.

The Node.js inspector (use --inspect-brk, etc.) will also give you similar information, but it's not so easily filtered.

why-is-node-running can also be used with some fiddling (do node --expose-internals /path/to/_mocha test.js then require('why-is-node-running')() at the top of test.js), but YMMV.

Raw
async-dump.js
'use strict';
const {createHook} = require('async_hooks');
const {stackTraceFilter} = require('mocha/lib/utils');
const allResources = new Map();
// this will pull Mocha internals out of the stacks
const filterStack = stackTraceFilter();
const hook = createHook({
init(asyncId, type, triggerAsyncId) {
allResources.set(asyncId, {type, triggerAsyncId, stack: (new Error()).stack});
},
destroy(asyncId) {
allResources.delete(asyncId);
}
}).enable();
global.asyncDump = module.exports = () => {
hook.disable();
console.error(`
STUFF STILL IN THE EVENT LOOP:`)
allResources.forEach(value=> {
console.error(`Type: ${value.type}`);
console.error(filterStack(value.stack));
console.error('\n');
});
};
Raw
mytest.js
'use strict';
const net = require('net');
const assert = require('assert');
describe('how to debug Mocha when it hangs', function () {
before(function (done) {
const server = net.createServer();
server.listen(10101, done);
});
it('should complete, but Mocha should not exit', function(done) {
const sock = net.createConnection(10101, () => {
assert.deepEqual(sock.address().family, 'IPv4');
done();
});
});
});
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Author

...and if you're spinning your wheels, nobody is telling you that you can't just add --exit and move on to something else!
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nuest commented Oct 27, 2017

This was very helpful - thanks!
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Great work!
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I am spinning my wheels and --exit has not worked for me... it still hangs at the end of the tests and I have to kill the process manually.
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@boneskull --exit works for me. Thank you!
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@boneskull thank you
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RdC1965 commented Jun 5, 2019

Thanks a lot!
--exit works for me. In package.json:
"test": "(dropdb --if-exists myDb && createdb myDb) && NODE_ENV=test mocha --exit"
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UPDATE: try https://www.npmjs.com/package/wtfnode instead
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RdC1965 commented Jun 5, 2019 via email

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