2.6 Reserved Words

Hi folks, in this lesson we're going to take a quick look at reserved words in JavaScript. Reserved words are words that have special meaning in JavaScript and so can't be used as identifier names. In JavaScript an identifier is the name given to a variable, an object property, a function, or a class. It identifies the thing so that we can reference it in other places in our code. As we saw earlier, there are some rules for identifier names. They must start with either a letter, an underscore, or the dollar sign. We can use these characters as well as numbers within the identifier name, but we can't start an identifier with them. So my thing one is a valid identifier but 1thing, as in the number 1 followed by the word thing, is an invalid identifier name. The casing of identifier names is also important. Because the casing between these two identifier names is different, they are not treated the same by JavaScript. In addition to these rules, identifiers must not be reserved words. This is generally okay because most code editors these days have code highlighting. So if we try to use a reserved word as an identifier, generally our IDE will tell us. So we can see that there is some green underlining in the IDE that I'm using here which, by the way, is Microsoft Visual Studio. And if we hover over that, we get a fairly generic message to say that a variable declaration is expected. And we can also see that the 1thing variable, which we declared earlier, also has this green underlining. The message that it gives us is equally generic. But with a bit of experience, we'll come to recognize the fact that the identifier that we're trying to use is illegal in JavaScript. There are about 60 different reserved words in JavaScript, and these are mostly keywords that have syntactical meaning, like function, if, else, new, type of, and things like that. As well as the reserved words that are actually in the official JavaScript specification, there are also a huge number of words that browsers use to make certain global objects available to JavaScript. Things like window, document, and element. Generally, it's best to avoid using these as well, because other scripts running on the page may depend upon them. There are many, many more of these not quite reserved words. And there are also a range of event handlers whose names we should avoid overwriting, like on click as well as operator precedence and associativity. The list of reserved words in JavaScript is generally just something that you will get to know with experience. You can look them up at your own time using a website like Javascripter.net, for example. And this lists all of the reserved words as well as the words that browsers generally use down below. So as I said, you'll get to learn these over time with experience, and you can also come and look them up whenever you need to. So in this lesson we learned about reserved words and Identifiers in JavaScript. We saw that there are many words that we cannot use as identifier names. And a lot more words that are best avoided for identifier names because the browser makes use of them. In the next lesson, we're going to look at Strict Mode. Thanks for watching.