Asynchronous stack traces

The asynchronous nature of JavaScript affects the way a stack trace works. In JavaScript, each tick (each time the JavaScript event-loop iterates) has a new stack.

Let's copy our app folder to async-stack-app:

$ cp -fr app async-stack-app

Now let's alter content.js like so:

function content (opts, c = 20) { 
  function produce (cb) { 
    if (--c) setTimeout(produce, 10, cb) 
    cb(null, opts.ohoh) 
  } 
  return produce 
} 
 
module.exports = content 

Then let's alter routes.js in the following way:

const content = require('./content') 
const {Router} = require('express') 
const router = new Router() 
 
router.get('/', (req, res) => { 
  content()((err, html) => {  
    if (err) { 
      res.send(500) 
      return 
    }  
    res.send(html) 
  }) 
}) 
 
module.exports = router 

Now we start our server:

$ node index.js

And make a request:

$ curl http://localhost:3000/

We'll see only a small stack trace descending from timeout specific internal code, as in the following screenshot:

We can obtain asynchronous stack traces with the longjohn module. Let's install it as a development dependency:

$ npm install --save-dev longjohn

Now we can add the following the very top of the index.js file:

if (process.env.NODE_ENV !== 'production') { 
  require('longjohn') 
} 

Let's run our server again:

$ node index.js
  

And make a request:

$ curl http://localhost:3000/

Now we should see the original stack, followed by a line of dashes, followed by the call stack of the previous tick.