JavaScript Variables

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JavaScript variables are "containers" for storing information:

Example

var x=5;
var y=6;
var z=x+y;

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Much Like Algebra

x=5
y=6
z=x+y

In algebra we use letters (like x) to hold values (like 5).

From the expression z=x+y above, we can calculate the value of z to be 11.

In JavaScript these letters are called variables.

	Think of variables as containers for storing data.

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JavaScript Variables

As with algebra, JavaScript variables can be used to hold values (x=5) or expressions (z=x+y).

Variable can have short names (like x and y) or more descriptive names (age, sum, totalvolume).

• Variable names must begin with a letter
• Variable names can also begin with $ and _ (but we will not use it)
• Variable names are case sensitive (y and Y are different variables)

	Both JavaScript statements and JavaScript variables are case-sensitive.

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JavaScript Data Types

JavaScript variables can also hold other types of data, like text values (person="John Doe").

In JavaScript a text like "John Doe" is called a string.

There are many types of JavaScript variables, but for now, just think of numbers and strings.

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.

Example

var pi=3.14;
var person="John Doe";
var answer='Yes I am!';

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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.

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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 lastname="Doe", age=30, job="carpenter";

Your declaration can also span multiple lines:

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

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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 valueundefined.

The variable carname will have the value undefined after the execution of the following statement:

var carname;

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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;

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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.