JavaScript Variables
4 Ways to Declare a JavaScript Variable:
- Using
var
- Using
let
- Using
const
- Using nothing
What are Variables?
Variables are containers for storing data (storing data values).
In this example, x
, y
, and z
,
are variables, declared with the var
keyword:
In this example, x
, y
, and z
,
are variables, declared with the let
keyword:
In this example, x
, y
, and z
,
are undeclared variables:
From all the examples above, you can guess:
- x stores the value 5
- y stores the value 6
- z stores the value 11
When to Use JavaScript var?
Always declare JavaScript variables with var
,let
,
orconst
.
The var
keyword is used in all JavaScript code from 1995 to 2015.
The let
and const
keywords were added to JavaScript in 2015.
If you want your code to run in older browsers, you must use var
.
When to Use JavaScript const?
If you want a general rule: always declare variables with const
.
If you think the value of the variable can change, use let
.
In this example, price1
, price2
, and total
, are variables:
The two variables price1
and price2
are declared with the const
keyword.
These are constant values and cannot be changed.
The variable total
is declared with the let
keyword.
This is a value that can be changed.
Just Like Algebra
Just like in algebra, variables hold values:
let x = 5;
let y = 6;
Just like in algebra, variables are used in expressions:
let z = x + y;
From the example above, you can guess that the total is calculated to be 11.
Note
Variables are containers for storing 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.
Note
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.)
Note
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.
Declaring a JavaScript Variable
Creating a variable in JavaScript is called "declaring" a variable.
You declare a JavaScript variable with the var
or the let
keyword:
var carName;
or:
let carName;
After the declaration, the variable has no value (technically it is 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:
let 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>
let 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 let
and separate the variables by comma:
A declaration can span multiple lines:
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:
Re-Declaring JavaScript Variables
If you re-declare a JavaScript variable declared with var
, it will not lose its value.
The variable carName
will still have the value "Volvo" after the execution of these statements:
Note
You cannot re-declare a variable declared with let
or const
.
This will not work:
let carName = "Volvo";
let carName;
JavaScript Arithmetic
As with algebra, you can do arithmetic with JavaScript variables, using
operators like =
and +
:
You can also add strings, but strings will be concatenated:
Also try this:
Note
If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.
Now try this:
JavaScript Dollar Sign $
Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names:
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:
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.