JavaScript is probably the world's most popular and misunderstood programming languages and for good reason.

When a developer claims to know JavaScript this generally equates to one or all of the following:

• "I know what JavaScript is, I can learn JavaScript in 30 minutes"
• "JavaScript is for kids, it's a toy language, it's easy"
• "I just copy and paste my JavaScript snippets / scripts from the internet"
• "JavaScript is just another programming language like C++, C#, VB.NET, or Java"

If thoughts similar to these have crossed your mind when sizing up your JavaScript knowledge, then you probably don't know JavaScript.

I would argue that if you've never programmed in a functional programming language (like: Lisp, Scheme, F#), never read a GOOD book on JavaScript, or never watched a video on JavaScript, then you probably don't understand JavaScript at all. JavaScript is starkly different than any other mainstream programming language.

When it comes to JavaScript, if you don't understand the fundamentals then you're only punishing yourself.

Here are some things every JavaScript developer should probably understand:

• JavaScript, Mocha, LiveScript, JScript, and ECMAScript are all essentially the same.
• JavaScript is a prototypically inherited (prototype-based programming) language which is very different than the classically inherited (class-based programming) languages like C++, Java, and most of the .NET languages - you know the difference between a prototype-based programming and class-based programming language.
• JavaScript is a loosely typed language - you know the difference between strongly-typed programming languages and weak-typed or loosely typed languages.
• JavaScript is a functional lambda language - you understand lambdas and that functions in JavaScript can be lambdas.
• JavaScript functions can be defined inside functions, inside functions, inside functions, and so on - you can demonstrate this.
• JavaScript uses function scope, has no curly brace '{}' block scope - you understand that this is common attribute of most functional programming languages.
• JavaScript has no private, public, protected assessors, no classes, no enums, no structs - you understand that all these language features can be achieved through objects and function closures.
• JavaScript is classless, but instead it uses objects as general containers.
• JavaScript makes use of garbage collection.
• JavaScript uses truthy and falsy values - you understand which values are truth and falsy.
• JavaScript makes use of the 'nan' (not a number) and 'undefined' values - you understand the difference between these values.
• JavaScript has pretty good debuggers - you understand how to debug JavaScript from both IE (link) and Firefox (link).
• You appreciate recursion, and understand that recursion is a useful technique in JavaScript.
  
• You understand that JavaScript Namespaces should be used - you understand the importance of namespaces, and know how to create one (Namespacing Your JavaScript).
• You know how to avoid the JavaScript pseudo-protocol (<a href="Javascript:alert('hello');"), and appreciate avoiding it.
• You know how to avoid embedding DOM Level 0 JavaScript events (<a onclick=''alert('hello');") in your mark-up / document structure, and appreciate this technique.
• You know that JavaScript can make use of two different event models (event capturing and bubbling), but we should only ever make use of bubbling.
• You understand what JSON is and how it's used.
• You know how to keep you JavaScript, CSS, document structure separate, and appreciate non obtrusive JavaScript.

If any of these points seem foreign, then it may be time to learn JavaScript. With all the hype and buzz around Web 2.0 and AJAX it could be a great way to augment your career while broadening your programming language vocabulary.

Great resources for learning more about JavaScript:

Videos:

• The JavaScript Programming Language: Part 1, Part 2, Part 3, Part 4
• An Inconvenient API: The Theory of the DOM: Part 1, Part 2, Part 3
• Advanced JavaScript: Part 1, Part 2, Part 3
• JavaScript: The Good Stuff: Link
• All these videos are available on the YUI Theater - all the videos are great, watch them all!

Books:

• JavaScript: The Definitive Guide by David Flanagan
• JavaScript: The Good Parts by Douglas Crockford
  

Links:

• MDC: A re-introduction to JavaScript
  
• Code Conventions for the JavaScript Programming Language
• JavaScript: The World's Most Misunderstood Programming Language
• What ASP.NET Developers Should Know About JavaScript
• Best Practices: Implementing JavaScript for rich internet applications
• The seven rules of Unobtrusive JavaScript