A JSLint Primer for JavaScript Developers

Have you ever written JavaScript and had it not work? Have you ever seen a giant wall of red in the console, but had no idea how to begin solving the problem? Are you constantly writing if(typeof x === ‘undefined’) or if(!x)?

If you’ve answered yes to any of these questions, then this blog is for you.

In this blog I will teach you what JSLint is, the basics of how it works and why it’s important. We will also learn how to pass your code through JSLint and use its warnings as a way to find bugs and avoid common mistakes.

JSLint is a tool for encouraging programmers to write programs that conform to a particular style. It was created by Douglas Crockford and is built in JavaScript, but you can use it to analyze code in any language. It’s open source, so you can download it from http://www.jslint.com/lint.js and run it yourself on your own computer if you prefer.

The purpose of this article is to introduce JSLint to developers who may not be familiar with it, answer some common questions about how JSLint works, and explain how it can help you write better JavaScript code.

Without a doubt, JavaScript is the most popular language for client-side Web development. Unfortunately, it is also the most misunderstood and abused language in existence today. This is mainly because of the lack of understanding on how to use JavaScript effectively and efficiently. Thus, there are many developers out there who are writing JavaScript code that is riddled with poor coding practices and bugs.

JSLint is a tool written by Douglas Crockford which attempts to address this problem by analyzing your code and pointing out potential problem areas.

JSLint works by looking at every token (literals, identifiers, and operators) in your source code and then checking it against a list of guidelines. These guidelines are based on Crockford’s best practices for writing high-quality JavaScript code. In addition, JSLint will also look for common coding mistakes such as missing semicolons and unreachable code.

Think of JSLint as an automated version of yourself (only much more pedantic). It will be able to discover syntax errors that you would normally miss during initial testing, thus saving you time from having to track them down later when they become actual bugs.

JSLint is a JavaScript code quality tool, written by Douglas Crockford, which can be found online at http://www.jslint.com.

JSLint is a very strict linting tool that enforces a set of coding guidelines to help you write better code. It will only allow you to do things in your code that are “safe” and “efficient.”

To put it simply, JSLint will check your code thoroughly for syntax errors and bad JavaScript practices. For example, it will check for the following:

Missing semi-colons

Errors in your if and for statements

Uninitialized variables

Too many parameters for functions

Undefined variables

If you are anything like me, you’ve probably downloaded a bunch of jQuery plugins and scripts that you were going to use in your next project. However, before you get started, the first thing you should do is run those files through JSLint.

JSLint was created by Douglas Crockford (the guy who discovered JSON) back in 2002. There are many JavaScript linting tools available (JSLint, JSHint, ESLint), but for this article we’re going to focus on JSLint.

JSLint will analyze a JavaScript source file, and check for problems. It is different from a normal code quality tool in that it does not attempt to enforce any particular style or convention. Instead, it looks for problematic patterns or signs of bad coding practice that could lead to bugs further down the road.

Why should I use it?

I use JSLint because it helps me catch bugs before they make their way into production code. It can catch a wide variety of common mistakes such as implicit type coercion, missing semicolons, and even poorly written regular expressions. As an added bonus, you can also configure JSLint with your own custom options too!

How do I use it?

There are two ways

I recently picked up on a coding practice called JSLint. If you are unfamiliar with JSLint, it is a static code analysis tool for JavaScript. It was created by Douglas Crockford and is available at jslint.com.

JSLint works by analyzing your JavaScript source code, looking for potential problems and offering suggestions as to how these problems may be resolved. The tool can be helpful in catching errors quickly, as well as ensuring that your code is readable and easier to understand.

JSLint is not a replacement for testing, but rather an additional step in the development process. As noted in the documentation: “JSLint takes a strict view of JavaScript that is not shared by all JavaScript programmers.” All of the options available with JSLint are listed on the site’s documentation page, with full descriptions of what they do and how they operate. I highly recommend taking time to familiarize yourself with these options before running JSLint on your code.

While there is no single correct way to write JavaScript, here are some ideas I have found helpful when using JSLint on my own projects:

JSLint is a JavaScript program that looks for problems in JavaScript programs. It is a code quality tool.

JSLint takes a JavaScript source and scans it. If it finds a problem, it returns a message describing the problem and an approximate location within the source. The problem is not necessarily a syntax error, although it often is. JSLint looks at some style conventions as well as structural problems. It does not prove that your program is correct. It just provides another set of eyes to help spot problems.

The messages that JSLint produces are intended to be self-explanatory and are listed in alphabetical order so that they can be used as a checklist for your code reviews. For example:


‘===’ should be ‘==’.

‘x’ was used before it was defined.


Confusing minuses.

Comma warnings can be turned off with ‘lax comma’.

Missing ‘use strict’ statement.

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