The JavaScript Code Quality Tool

What is JSLint?

JSLint is a JavaScript program that looks for problems in JavaScript programs. It is a code quality tool.
When C was a young programming language, there were several common programming errors that were not caught by the primitive compilers, so an accessory program called lint was developed that would scan a source file, looking for problems.
As the language matured, the definition of the language was strengthened to eliminate some insecurities, and compilers got better at issuing warnings. lint is no longer needed.
JavaScript is a young-for-its-age language. It was originally intended to do small tasks in webpages, tasks for which Java was too heavy and clumsy. But JavaScript is a very capable language, and it is now being used in larger projects. Many of the features that were intended to make the language easy to use are troublesome when projects become complicated. A lint for JavaScript is needed: JSLint, a JavaScript syntax checker and validator.
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.
JSLint defines a professional subset of JavaScript, a stricter language than that defined by Third Edition of the ECMAScript Programming Language Standard. The subset is related to recommendations found in Code Conventions for the JavaScript Programming Language.
JavaScript is a sloppy language, but inside it there is an elegant, better language. JSLint helps you to program in that better language and to avoid most of the slop. JSLint will reject programs that browsers will accept because JSLint is concerned with the quality of your code and browsers are not. You should accept all of JSLint's advice.
JSLint can operate on JavaScript source, HTML source, CSS source, or JSON text.

Reference:
http://www.jslint.com/