– You’re Using Global Variables
If you’re just getting started with JavaScript, you probably think it’s a great thing that all variables are global. Actually, if you’re just getting started, you might not know what that means. Global variables are variables that are accessible from anywhere in your JavaScript, even in different files loaded on the same page. Sounds great, doesn’t it? Any variable you might every want to change is always accessible.
Actually, no.
The reason this is a bad idea is because it’s easy to overwrite values unintentionally. Say you’ve got a web store, and you’re using JavaScript to display the price of all the items in the shopping cart (of course, you’ll recalculate this on the server side; this just enhances the user experience).
– You’re Not Using Semicolons
Every statement in JavaScript must end with a semicolon. It’s that simple. The issue here is that is you don’t put it in, the compiler will: this is called semicolon insertion. So the question is, if the compiler will insert them for you, why waste your time?
Well, in some places, it’s absolutely necessary; for example, you must put semicolons between the statements in a for-loop’s condition, or you’ll get a syntax error. But what about at the end of lines?
The JavaScript community is really divided on this. I’ve read very well-respected professionals on both sides of the debate. Here’s my argument: whenever you’re relying on the JavaScript compiler to change your code (even in what seems like a small way), you’re in dangerous waters.
– You’re Using ==
If you left your computer right now and walked until you met any random JavaScript developer (that might take a while), and asked him/her to give you one common JavaScript mistake, this is probably what he/she would say: “using double-equals instead of triple-equals.” What’s this mean?
– You’re using Type Wrapper Objects
JavaScript kindly (um?) gives us some type wrappers for easy (um?) creation of primitive types:
– You’re not Property-Checking when Using For-In
We’re all familiar with iterating over arrays; however, you’ll probably find yourself wanting to iterate over the properties of an object. (Digression: the items in an array are actually just numbered properties in an object.) If you’ve done this before, you’ve used a for-in loop:
– You’re Using with or eval
Thankfully, most sources for learning JavaScript today don’t teach you about with or eval. But if you’re using some older material—or using a less-than-reputable source (because sometimes good material is hard to find on the web)—you might have found with and eval and given them a try. Terrible move, web developer.
Let’s start with with. Two main reasons not to use it:
It really slows down the execution of your JavaScript.
– You’re Not Using a Radix When Using parseInt
JavaScript has a great little helper function called parseInt that allows you to convert a string that contains a number to a number:
– You’re Not Using Braces on if and while statements
One of the most obvious beauties of JavaScript is its flexibility. But sometimes, that can come back to fight you. That’s certainly the case with braces on if- and while-statement blocks. These braces are optional if you only have one line of code in the block:
– You’re Adding Elements to the DOM Individually
All right, all right: this isn’t really JavaScript itself. But, in 99 of 100 cases, JavaScript means using the DOM. While there’s a lot of mistakes you can make when working with the DOM, this is a big one.
I fondly remember the day when I inserted my first DOM element via JavaScript. It’s fun to do, and oh-so-useful, but it unfortunately is a strain on the page: inserting a DOM element forces the browser to completely repaint the page, so if you have a whole bunch of elements to add, adding them one by one is a bad idea:
– You’re Not Learning JavaScript
Many people don’t take the time to learn JavaScript right.
JavaScript does not equal jQuery. Did I just shock your sock off? If you found yourself guilty of committing several of the mistakes listed above, you probably need to do some serious JavaScript studying. JavaScript is a language that you can use almost without learning it, which means that so many people don’t take the time to learn it right. Don’t be one of those people: there are so many awesome JavaScript tutorials out there that you have no excuse for not learning the language. If all you know is jQuery (or Mootools, or whatever), you’re really putting yourself in a bad spot.