What Are Variables?
http://lifehacker.com/5736011/learn-how-to-co...data-types
You can think of variables as labeled jars that store different types of data. While there are several kinds of variables, today we're only going to look at three:
- String - A string variable is a string of alphanumeric characters and allowed symbols that are contained within quotation marks. For example, "Hello world, I'm 102 years old today!" is an example of a string. Strings can also be contained within single quotes, which is useful if you want to have a string with a quotation like this: '"I hate the snow," Laurel said.' Strings are basically used for storing text.
- Number - A number variable couldn't be more straightforward because all number variables store are numbers. You don't store them within quotes like strings. Instead, numbers can just be written as they are. If you want to store the number 9 in a variable, you just write 9.
- Boolean - A boolean variable is one of two things: true or false . This data type is kind of like an on and off switch, so you can ask true or false questions in your code. For example, you might ask "is the video currently playing?" The response you'd get would be a boolean variable. True would mean the video is currently playing and false would mean it is not.
So how do you putt a variable to your code (or declare a variable, as it's more traditionally called)? In JavaScript, all you need to do is this:
To Var or Not to Var
In JavaScript, you can define a variable as myVariable = "something"; or var myVariable = "something"; , the difference being the word var preceding the statement. When you're declaring variables in a script outside of a function, this distinction is pretty much irrelevant. When you're declaring a variable inside a function and do not use var this creates a global variable. Global variables can be accessed from anywhere in your code, whereas local variables (such as the ones defined in functions) can only be accessed within their own scope (e.g. if a variable is local to a function, only that function can use it). This is not an important distinction right this minute, but when we learn about functions later it'll be good to know.
There are a few of things to notice here. First, the name myVariable. All programming languages have something called reserved words, which means you can't use them as variable names. What they varies, but if the name is sufficiently generic, there's a chance it could be a reserved word. To avoid using reserved words and screwing up your code, just decide on a naming scheme for your variables. I've put "my" in front of my example variable, but you'll probably want to come up with something else. Second, you'll notice a semicolon at the end of the line. A semicolon is like a period at the end of a sentence in many programming languages, and that is definitely the case in JavaScript. In nearly every situation, you need to end your code sentences with a semicolon so your computer doesn't get confused when reading it. The semicolon tells the computer, "Okay I'm all done with this statement." (Note: JavaScript is forgiving, and sometimes you can get away without the semicolon, but it's good practice.)
One more thing to note is that JavaScript is a loosely-typed language. There are (basically) two kinds of languages: loosely-typed and strictly-typed. An example of a strictly-typed language is ActionScript (the language Flash apps are written in), and the same variable declaration we just wrote would look like this in ActionScript 3:
The additions you're seeing are the word var and the word String (with a colon in front of it). The word var tells the computer we're about to declare a variable. The :String attached to the variable's name tells the computer what type of variable it is and to not accept any other type. This is the reason for the term strictly-typed . A loosely-typed language like JavaScript is more flexible and doesn't require any of that. This makes your code more flexible, but some will argue it will also make it more error-prone. We're not going to get into the pros and cons of strictly- and loosely-typed languages here, but it's good to be aware of the basic differences now as you will most likely encounter them in your programming endeavors.
