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Dan's Bug Hunting Tips & Tricks

for JavaScript

This is a quick rundown on key JavaScript Error and Exception handling pitfalls.

Includes snippets which apply across the JS ecosystem, including Client-side, Back-end and everything in-between!

Everyone from rookies to Node Team members forget this shit all the time. Myself included. This is my attempt to collect my assorted debugging tips, reminders, tricks and magic.

Table of Contents

  1. 👌 Shit
    1. Error Pitfall #1
    2. Error.callStackLimit = 50
    3. FIX: Error to JSON is empty object
  2. Express
    1. Required: Request Logging (morgan, etc)
    2. Required: 404 Not Found Handler
    3. Required: 500 Exception Handler
  3. Promises (when you use knex, axios, fetch, etc it's Promises)
    1. Promise.reject()
    2. Catching Errors
    3. Never use .then's 2nd parameter
  4. React & React Native
    1. Faster Simulator
  5. IDE / Environment
    1. Auto eslint
    2. Auto Complete Tools
    3. Prettier: Auto Formatting

👌 Shit

Error Pitfall #1

HOWTO: Correctly throw/new Error's

Similar to the topic in the Promises section on Promise.reject

Bad & Correct Examples:

throw 'Invalid Usage, May Fail Silently'

// CORRECT: Use `new Error()`
throw new Error('Will have call stack & other exception details provided in the ctx of the callsite')

Using correct method ensures you get best possible stack traces in any runtime/engine.


Error.callStackLimit is a little-known gem of a feature.

Ever struggle to find your code in a stack trace?

Endless functions in node_modules, yet none of your files?

Well, this fixes it 50-75% of the time: add the next line to the top of your entrypoint file (likely index.js or app.js).

// Opt #1: Add to the top of your first file - set before any Error might be created
Error.callStackLimit = 50 // IMPORTANT: Will kill performance - disable in production

// Opt #2: In Node Apps use something like this:
if (process.env.NODE_ENV !== 'production') { Error.stackTraceLimit = 50; }

Error.callStackLimit works on both NodeJS & Browser apps!

Bonus: Here's another way to log a stack trace: console.trace('Print Trace of Function Sequence that got us here')

Error to JSON


JSON.stringifying an Error instance fails to include expected keys.


I've seen this cause problems in Chrome console logging, analytics/tracking libraries, and Express apps' res.send(json) or res.json(json), etc.

Interview Question: What do you think this code returns?

JSON.stringify(new Error('Oh noes'))

Hint: assert(err.message !== 'Oh noes')

[.... think about it ....]








Answer: JSON.stringify(new Error('uh oh')) returns an empty object {}

WHAT is going on??? 💩

Do you know about Object.keys()'s often forgotten cousin Object.getOwnPropertyNames()?

Let's look at some example code.

Examine the 3 different log outputs:

const err = new AppError({httpStatus: 420, httpMessage: 'Traffic too hi',  message: 'Sad Panda', stack: null})

console.log('.getOwnPropertyNames: ', Object.getOwnPropertyNames(err).sort().join(','))
// -> .getOwnPropertyNames: httpMessage,httpStatus,message,stack

console.log('.keys: ', Object.keys(err).sort().join(','))
// -> .keys: httpMessage,httpStatus

console.log('err: ', JSON.stringify(err, null, 1))
// JSON.stringify -> 
// err:  {
//   "httpStatus": 420,
//   "httpMessage": "Traffic too hi"
//  }

Consider how JSON.stringify is likely implemented?

TLDR; quick fixes!

Get the important keys out of the Object/Error manually:

  1. Check out my simple fix function: toPlainObject(obj)
  2. Some more express-specific solution(s) might look like either:
    1. Required: const errorInfo = err => {error: 'Server Error', message: err.message, stack: err.stack}
    2. Pick one: if (err) return next(errorInfo(err))
    3. Pick one: if (err) return res.status(500).json(errorInfo(err))
    4. Pick one: promise.then(fn).catch(err => next(errorInfo(err)))
    5. Plenty alternatives exist


For my complete App.js starter file, click here!

Request Logging

Install the morgan logging lib.

It will log (to terminal) any requests made to your node HTTP server.

For all those times you wondered, "am I even requesting file X correctly?"

Some example code/usage:

See Correct Implementation

Rough usage example:

// Add to your Express App.js entry file
const devMode     = process.env.NODE_ENV !== 'production'
const morgan      = require('morgan')

// then add the following after your body-parsers
app.use(morgan(devMode ? 'dev' : 'combined'))

404 Not Found Handler

The last app.use(req,res[,next]) will become your defacto 'File Not Found' handler.

Makes sense, considering all middleware and routes are applied in the sequence they were setup.

// Add right before the final error handler middleware (see next tip) 
app.use(function _notFound(req, res, next) {
  console.error('File not found:', req.originalUrl);
  res.status(404).send({error: 'Url not found.', url: req.originalUrl});

500 Error Handler

One method is add call a final app.use() with a 4-argument function, like so: app.use((err, req, res, next) => {}). This will become your auto-magic error handler. Makes little sense, but it works.

An approach I've found to be problematic uses something roughly like server.listen(3000).on('error', err => console.error('OnError:', err)).

Note: While the server.on('error') style may seem to work, I've had trouble losing stack traces and getting erroneous messages with certain tool combos (notable, yet junk libs like Q/$q & async are partly to blame).

// Express 4-arg Error Handler:
app.use(function _errorHandler(err, req, res, next) {
  console.error('ERROR', err);
  res.status(500).send({error: err.message, stack: err.stack, url: req.originalUrl});



Make sure you use correct error invocations:

return Promise.reject('Invalid Usage, May Fail Silently')

// CORRECT: Use `new Error()`
return Promise.reject(new Error('Will have call stack & other exception details provided in the ctx of the callsite'))

React / React Native

Faster Simulator


IDE / Environment


Linting enforces per-project code styling rules. This should minimally be used in build steps or deploy/distribution tasks.

eslint will catch bugs in your code and enforce consistency based off .eslintrc config.

standard js is another popular option - quite easy to setup - but it's more geared towards auto-fixable issues.


If you find a high-level debate about the virtue of linting. Ignore it.

It's the same as saying: I'm a great driver, so, not only don't I need headlights, I can skip those silly seatbelts too! Seems like a choking hazard anyway!

Driving habits aside, clearly we should at least avoid coding carelessly.

We might as well catch errors early, upon saving.

Atom Note: If Atom auto-save feature is slowing things down, add a 1-3 sec delay in the settings somewhere.

JS Lint Tools:
  1. ESLint The defacto standard js linter in 2016-2017+
# Install pkgs locally
npm install eslint@3.x babel-eslint@7 --save-dev

Docs: Config options for eslintrc

Example .eslintrc.js:

module.exports = {
  "extends": ["eslint:recommended", "plugin:react/recommended"],
  "plugins": [
  "rules": {
    "no-var": 0,
    "max-len": 0,
    "prettier/prettier": ["error", { "trailingComma": "es5" }]
  "env": {
    "node": true,
    "mocha": true
  "parser": "babel-eslint",
  "parserOptions": {
    "sourceType": "module",
    "allowImportExportEverywhere": false,
    "codeFrame": false,
    "ecmaFeatures": {
      "ecmaVersion": 6,
      "jsx": true,
      "experimentalObjectRestSpread": true,
An alternative to ESLint:
  1. Standard JS Rapidly rising popularity.
    1. Code: https://marketplace.visualstudio.com/items?itemName=chenxsan.vscode-standardjs
    2. Atom: https://atom.io/packages/linter-js-standard-engine
    3. Atom: https://atom.io/packages/standard-formatter
    4. VIM: https://github.com/w0rp/ale

Auto Complete Tools

  1. VS Code has included great auto-complete out-the-box for a while!

  2. Atom has many options... That said, I'd avoid all except atom-ternjs

Custom Errors

IMHO, this is one of few valid use-cases for the ES6 class syntax. Inheritance with class/extends is just harder to mess up compared to older methods: Object.create, Error.prototype, util.inherit, etc.

Lets outline & checkout example code:

Class Hierarchy:

- Error            (built-in, extends `Function` built-in)
  - HttpError      (extends `Error`)
    - AppError     (extends `HttpError`)
    - ServiceError (extends `HttpError`)


class HttpError extends Error {
    constructor(details) {
        super(typeof details === 'string' ? details : 'ERROR: message wasn\'t specified')
        if (typeof details === 'object') Object.assign(this, details)

// Create 2 classes based off of HttpError
class AppError extends HttpError { }
class ServiceError extends HttpError { }

Util Methods


Goal: Merge all instance properties into a fresh POJO - this is inclusive of inherited members/keys. This is a JSON.stringify and res.json friendly method.

TODO toPlainObject()

  1. Circular variable reference detection.
  2. Callback overrides
  3. Key exclusions

Example usage:

toPlainObject(new AppError({aborted: false, external: true, message: 'Test Error'})
// => {aborted: false, external: true, message: 'Test Error', stack: [...]}
Usage & Code: toPlainObject(obj)
// More advanced sample!
const e = new AppError({message: 'Test Error', stack: null})
console.log('message:', e.message)
console.log('   json:', JSON.stringify(e))
console.log('   keys:', Object.keys(e))
console.log('   gopn:', Object.getOwnPropertyNames(e))

console.log('Wrapped with toPlainObject:\n')
console.log('   json:', JSON.stringify(toPlainObject(e)))
console.log('   keys:', Object.keys(toPlainObject(e)))

function toPlainObject(obj) {
	if (obj === null || obj === undefined || typeof obj !== 'object') return obj

	return Object.getOwnPropertyNames(obj)
	.reduce((output, key) => {
		output[key] = _getVal(obj[key])
		return output
	}, {})

function _getVal(v) {
  if (v === null || v === undefined) return v
  if (v instanceof Date) return v.toISOString()
  if (/Number|Boolean|String/.test(v.constructor && v.constructor.name || '')) return v
  if (Array.isArray(v)) return v.map(toPlainObject)
  if (typeof v === 'object') return toPlainObject(v)
  return v


Goal: Merge all instance properties into a fresh POJO - this is inclusive of inherited members/keys. This is a JSON.stringify and res.json friendly method.

Example usage:

getTaxonomy(new AppError('error').join(' -> '))  
// => Error -> HttpError -> AppError
Usage & Code: getTaxonomy(obj)
getTaxonomy(new Error('error'))     // -> [ 'Error' ]
getTaxonomy(new HttpError('error')) // -> [ 'Error', 'HttpError' ]
getTaxonomy(new AppError('error'))  // -> [ 'Error', 'HttpError', 'AppError' ]
getTaxonomy(new AppError('error').join(' -> '))  // => "Error -> HttpError -> AppError"
getTaxonomy(AppError)  // -> [ 'Function' ] 
getTaxonomy(parseInt)  // -> [ 'Function' ]
// [ 'Function' ] Indicates obj NOT an instance

function getTaxonomy(o, taxonomy = []) {
  o = taxonomy.length <= 0 ? o.__proto__.constructor : o && o.__proto__
	return !o || !o.name || taxonomy.indexOf('Function') > -1 ? taxonomy : getTaxonomy(o, [o.name].concat(taxonomy))