JavaScript Variables

 JavaScript Variables

Variables are "containers" for storing information:

JavaScript Data Types

var answer1="He is called 'Johnny'";
var answer2='He is called "Johnny"';var pi=3.14;

var x=123;
var y=123e5;
var z=123e-5;
var cars=new Array("Saab","Volvo","BMW");
var person={firstname:"John", lastname:"Doe", id:5566};

---

Like School Algebra

Remember algebra from school?
x=5
y=6
z=x+y
Do you remember that letters (like x) can be used to hold a value (like 5), and that you can use the information above to calculate the value of z to be 11?
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).

	Think of variables as names or labels given to values.

---

JavaScript Variables

As with algebra, JavaScript variables are used to hold values or expressions.
Variable can have a short names, like x and y, or more descriptive names, like age, sum, or, totalvolume.
JavaScript variables can also be used to hold text values, like: name="John Doe".
Here are the rules for JavaScript variable names:

• Variable names are case sensitive (y and Y are two different variables)
• Variable names must begin with a letter, the $ character, or the underscore character

	Both JavaScript statements and JavaScript variables are case-sensitive.

---

Declaring (Creating) JavaScript Variables

Creating a variable in JavaScript is most often referred to as "declaring" a variable.
You declare JavaScript variables with the var keyword:

var carname;

After the declaration, the variable is empty (it has no value).
To assign a value to the variable, use the equal sign:

carname="Volvo"; 

However, you can also assign a value to the variable when you declare it:
 

var carname="Volvo";

In the example below we create a variable called carname, assigns the value "Volvo" to it, and put the value inside the HTML paragraph with id="demo":

Example

<p id="demo"></p>
var carname="Volvo";
document.getElementById("demo").innerHTML=carname;

	It's a good programming practice to declare all the variables you will need, in one place, at the beginning of your code.

---

JavaScript Data Types

There are many types of JavaScript variables, but for now, just think of two types: text and numbers.
When you assign a text value to a variable, put double or single quotes around the value.
When you assign a numeric value to a variable, do not put quotes around the value. If you put quotes around a numeric value, it will be treated as text.

---

One Statement, Many Variables

You can declare many variables in one statement. Just start the statement with var and separate the variables by comma:

var name="Doe", age=30, job="carpenter"; 

 

Your declaration can also span multiple lines:

var name="Doe",
age=30,
job="carpenter";

---

Value = undefined

In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input. Variable declared without a value will have the value undefined.
The variable carname will have the value undefined after the execution of the following statement:

var carname;

---

Re-Declaring JavaScript Variables

If you re-declare a JavaScript variable, it will not lose its value:.
The value of the variable carname will still have the value "Volvo" after the execution of the following two statements:

var carname="Volvo";
var carname;

---

JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables, using operators like = and +:

Example

y=5;
x=y+2;

You will learn more about JavaScript operators in a later chapter of this tutorial.