JavaScript Variables

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JavaScript

Variables


JavaScript variables are containers for storing data values.

In this example,

x

,

y

, and

z

, are variables, declared with the

var

keyword:

Example

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

From the example above, you can expect:

  • x stores the value 5
  • y stores the value 6
  • z stores the value 11

Using

let

and

const

(ES6)

Before 2015, using the

var

keyword was the only way to declare a JavaScript variable.

The 2015 version of
JavaScript (ES6)
allows the use of the

const

keyword to define a variable that cannot be reassigned, and
the

let

keyword to define a variable with restricted scope.

Because it is a little complicated to describe the difference between these keywords,
and because they are not supported in older browsers, the first part of this tutorial
will most often use

var

.

Safari 10 and Edge 14 were the first browsers to fully support ES6:

Chrome 58 Edge 14 Firefox 54 Safari 10 Opera 55
Jan 2017 Aug 2016 Mar 2017 Jul 2016 Aug 2018

Much Like Algebra

In this example,

price1

,

price2

, and

total

, are variables:

Example

var price1 = 5;
var price2 = 6;
var total = price1 + price2;

In programming, just like in algebra, we use variables (like price1) to hold values.

In programming, just like in algebra, we use variables in expressions (total = price1 + price2).

From the example above, you can calculate the total to be 11.

JavaScript variables are containers for storing data values.





JavaScript Identifiers

All JavaScript

variables

must be

identified

with

unique names

.

These unique names are called

identifiers

.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

The general rules for constructing names for variables (unique identifiers) are:

  • Names can contain letters, digits, underscores, and dollar signs.
  • Names must begin with a letter
  • Names can also begin with $ and _ (but we will not use it in this tutorial)
  • Names are case sensitive (y and Y are different variables)
  • Reserved words (like JavaScript keywords) cannot be used as names

JavaScript identifiers are case-sensitive.


The Assignment Operator

In JavaScript, the equal sign (

=

) is an “assignment” operator, not an
“equal to” operator.

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

x = x + 5

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

(It calculates the value of x + 5 and puts the result into x. The value of x
is incremented by 5.)

The “equal to” operator is written like

==

in JavaScript.


JavaScript Data Types

JavaScript variables can hold numbers like 100 and text values like “John
Doe”.

In programming, text values are called text strings.

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

Strings are written inside double or single quotes. Numbers are written
without quotes.

If you put a number in quotes, it will be treated as a text string.

Example

var pi = 3.14;
var person = “John Doe”;
var answer = ‘Yes I am!’;


Declaring (Creating) JavaScript Variables

Creating a variable in JavaScript is called “declaring” a variable.

You declare a JavaScript variable with the

var

keyword:

var carName;

After the declaration, the variable has no value (technically it has the
value of

undefined

).

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>

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 person = “John Doe”, carName = “Volvo”, price = 200;

A declaration can span multiple lines:

var person = “John Doe”,
carName = “Volvo”,
price = 200;


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.

A variable declared without a value will have the value

undefined

.

The variable carName will have the value

undefined

after the execution of this statement:

Example

var carName;


Re-Declaring JavaScript Variables

If you re-declare a JavaScript variable, it will not lose its value.

The variable

carName

will still have the value “Volvo” after the execution of these statements:

Example

var
carName = “Volvo”;
var carName;


JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables, using
operators like

=

and

+

:

Example

var x = 5 + 2 + 3;

You can also add strings, but strings will be concatenated:

Example

var x = “John” + ” ” + “Doe”;

Also try this:

Example

var x = “5” + 2 + 3;

If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.

Now try this:

Example

var x = 2 + 3 + “5”;


JavaScript Dollar Sign $

Remember that JavaScript identifiers (names) must begin with:

  • A letter (A-Z or a-z)
  • A dollar sign ($)
  • Or an underscore (_)

Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names:

Example

var $$$ = “Hello World”;
var $ = 2;
var $myMoney = 5;

Using the dollar sign is not very common in JavaScript,
but professional programmers often use it
as an alias for the main function in a JavaScript library.

In the JavaScript library jQuery, for instance, the main function

$

is used to select HTML elements.
In jQuery

$("p");

means “select all p elements”.


JavaScript Underscore (_)

Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names:

Example

var _lastName = “Johnson”;
var _x = 2;
var _100 = 5;

Using the underscore is not very common in JavaScript,

but a convention among professional programmers is to use it as an alias for
“private (hidden)” variables.


Test Yourself With Exercises

Exercise:

Create a variable called

carName

and assign the value

Volvo

to it.

var  = "";

Start the Exercise