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JavaScript Variables


JavaScript variables are "containers" for storing information:

Example

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

Try it Yourself »


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.

Note JavaScript variables are containers for storing data.


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 can contain letters, digits, underscores, and dollar signs.

  • 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)
  • Reserved words (like JavaScript keywords) cannot be used as variable names
Note Both JavaScript statements and JavaScript variables are case-sensitive.


The Assignment Operator

In JavaScript, the equal sign (=) is an "assignment" operator, is not an "equal to" operator.

This is different from algebra. The following does not make any sense in algebra:

x = x + 5

In JavaScript, however it makes perfect sense: Assign the value of x + 5 to the variable x.

In reality: Calculate the value of x + 5. Then put the result into the variable x.

Note The "equal to" operator in JavaScript, is written like == or ===. You will see it soon!


JavaScript Data Types

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

In JavaScript texts are called strings or text strings.

JavaScript can handle many types of data, but for now, just think of numbers and strings.

When you assign a string value to a variable, you put double or single quotes around the value.

When you assign a numeric value to a variable, you do not put quotes around the value.

If you put quotes around a numeric value, it will be treated as a text string.

Example

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

Try it Yourself »


Declaring (Creating) JavaScript Variables

Creating a variable in JavaScript is called "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";

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 and assign the value "Volvo" to it.

Then we "output" the value inside an HTML paragraph with id="demo":

Example

<p id="demo"></p>

<script>
var carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
</script>

Try it Yourself »

Note It's a good programming practice to declare all variables at the beginning of a script.


One Statement, Many Variables

You can declare many variables in one statement.

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";

In JavaScript you can always separate statements by semicolon, but then you cannot omit the var keyword.

Wrong:

var lastName = "Doe"; age = 30; job = "carpenter";

Right:

var lastName = "Doe"; var age = 30; var 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

var y = 5;
var x = y + 2;

Try it Yourself »

You can also add strings, but strings will be concatenated (added end-to-end):

Example

var y = "5";
var x = y + 2;

Try it Yourself »

Note that if you add a number to a string, both will be treated as strings.

You will learn a lot more about arithmetic operators later in this tutorial.



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