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How to prevent frequent JavaScript mistakes

By Lucien Bénié —

When writing JavaScript, I spend a lot of time fixing simple mistakes. Unlike compiled languages you are more likely to make mistakes. It is easy for syntax errors to sneak into your code without realizing it until you actually try and run your code.

How many times have I got an undefined variable because I refactored some code and forgot to rename that variable.

Even though it has been more than 5 years since I wrote my first Hello World. The feeling remains the same – Why did I make this mistake again ?

To help me fix some of those mistakes, I tried a few linting tools over the years. From the overly strict JSLint to the more flexible variant JSHint and JSCS. I recently discovered ESLint and fell in love with its extensibility and features.

JSCS and ESLint have merged since April 14th you can check their blog posts JSCS and ESLint.


Linting is a process of checking the source code for programmatic as well as stylistic errors. A Lint or a Linter is a program that supports linting. They are available for most languages like CSS, Python, JavaScript, HTML, etc…


ESLint is the most recent of the four linting tools previously mentioned. It was created by Nicholas C. Zakas in june 2013.

Its goal is to provide a pluggable linting utility for JavaScript.

Designed to be heavily extensible, it comes with a large set of custom rules and it is really easy to install. It gives precise and concise output by including the rule name out of the box. You are always aware of which rule was causing an error.

ESLint provides good documentation for its rules. It is easy to follow and is grouped into logical categories. Each rule gives details about what it enforces or not and provides examples of good and bad written code for it.


  • Customizable: every rule can be toggled, and many rules have extra settings that can be tweaked
  • ES6/JSX support out of the box
  • Supports custom reporters
  • Has many plugins available and is very extensible
  • Include many rules not available in other linters


  • Slow, compared to JSHint or JSCS, but faster than these two if combined
  • Requires some configuration to get started


Since ESLint is extremely extensible, I have created a shareable config rule set to help my fellow colleagues here at Coveo who write Pure JavaScript.


eslint-config-coveo started as an internal project for my team (Salesforce integration) then we decided – Hey… why not open-source it?

With that in mind, I created a gulp task wrapper that uses our rule set defined from that shareable config. You can find the project pretty-javascript here.

Want to be a JavaScript high priest of digital innovation?

For anyone who wants to write JavaScript like we do it at Coveo follow these simple steps to get you up and running.

  • Install pretty-javascript and gulp packages from npm
npm install --save-dev pretty-javascript gulp
  • Create an eslint configuration file


  extends: coveo
    ... (they can be overriden)



  "extends": "coveo",
  "rules": {
    ... (they can be overriden)
  • Create a gulp task to run eslint


var gulp = require('gulp');
var linter = require('pretty-javascript');

gulp.task('lint', function() {

Want to be a TypeScript tech-wizard-in-residence?

For TypeScript lovers, Coveo also provides a gulp task wrapper to lint TypeScript. The project pretty-typescript can be found here (huge kudos to Dominique Bégin).

Similarly to pretty-javascript, you only have to include pretty-typescript and gulp from npm.

npm install --save-dev pretty-typescript gulp

Then simply add a task to lint your TypeScript code in your gulpfile as follows :

var linter = require('pretty-typescript');
var gulp = require('gulp');

gulp.task('lint', function() {

Sit back, relax and enjoy watching your silly mistakes from your terminal output!

Written by Lucien Bénié
Salesforce integration wizard