CSS Variables
CSS Variables - The
var() Function
CSS Variables
The var() function
is used to insert the value of a CSS variable.
CSS variables have access
to the DOM, which means that you can create variables with local or global
scope, change the variables with JavaScript, and change the variables based on
media queries.
A good way to use CSS
variables is when it comes to the colors of your design. Instead of copy and
paste the same colors over and over again, you can place them in variables.
The Traditional Way
The following example shows
the traditional way of defining some colors in a style sheet (by defining the
colors to use, for each specific element):
Example
body { background-color: #1e90ff; }
h2 { border-bottom: 2px solid
#1e90ff; }
.container {
color: #1e90ff;
background-color: #ffffff;
padding: 15px;
}
button {
background-color: #ffffff;
color: #1e90ff;
border: 1px solid #1e90ff;
padding: 5px;
}
Syntax of the var() Function
The var() function
is used to insert the value of a CSS variable.
The syntax of the var() function
is as follows:
var(--name, value)
|
Value |
Description |
|
name |
Required. The variable
name (must start with two dashes) |
|
value |
Optional. The fallback
value (used if the variable is not found) |
Note: The
variable name must begin with two dashes (--) and it is case sensitive!
How var() Works
First of all: CSS variables
can have a global or local scope.
Global variables can be
accessed/used through the entire document, while local variables can be used
only inside the selector where it is declared.
To create a variable with
global scope, declare it inside the :root selector. The :root selector matches the
document's root element.
To create a variable with
local scope, declare it inside the selector that is going to use it.
The following example is
equal to the example above, but here we use the var() function.
First, we declare two
global variables (--blue and --white). Then, we use the var() function
to insert the value of the variables later in the style sheet:
Example
:root {
--blue: #1e90ff;
--white: #ffffff;
}
body { background-color: var(--blue); }
h2 { border-bottom: 2px solid
var(--blue); }
.container {
color: var(--blue);
background-color: var(--white);
padding: 15px;
}
button {
background-color: var(--white);
color: var(--blue);
border: 1px solid var(--blue);
padding: 5px;
}
Advantages of using var()
are:
- makes
the code easier to read (more understandable)
- makes
it much easier to change the color values
To change the blue and
white color to a softer blue and white, you just need to change the two variable
values:
Example
:root {
--blue: #6495ed;
--white: #faf0e6;
}
body { background-color: var(--blue); }
h2 { border-bottom: 2px solid
var(--blue); }
.container {
color: var(--blue);
background-color: var(--white);
padding: 15px;
}
button {
background-color: var(--white);
color: var(--blue);
border: 1px solid var(--blue);
padding: 5px;
}
Browser Support
The numbers in the table specify
the first browser version that fully supports the var() function.
|
Function |
|||||
|
var() |
49.0 |
15.0 |
31.0 |
9.1 |
36.0 |
CSS var() Function
|
Property |
Description |
|
var() |
Inserts the value of a
CSS variable |
CSS Overriding Variables
Override Global Variable With Local Variable
From the previous page we have learned that global variables can
be accessed/used through the entire document, while local variables can be used
only inside the selector where it is declared.
Look at the example from the previous page:
Example
:root {
--blue: #1e90ff;
--white: #ffffff;
}
body {
background-color: var(--blue);
}
h2 {
border-bottom: 2px solid var(--blue);
}
.container {
color: var(--blue);
background-color: var(--white);
padding: 15px;
}
button {
background-color: var(--white);
color: var(--blue);
border: 1px solid var(--blue);
padding: 5px;
}
Sometimes we want the variables to change only in a specific
section of the page.
Assume we want a different color of blue for button elements.
Then, we can re-declare the --blue variable inside the button selector. When we
use var(--blue) inside this selector, it will use the local --blue variable
value declared here.
We see that the local --blue variable will override the global
--blue variable for the button elements:
Example
:root {
--blue: #1e90ff;
--white: #ffffff;
}
body {
background-color: var(--blue);
}
h2 {
border-bottom: 2px solid var(--blue);
}
.container {
color: var(--blue);
background-color: var(--white);
padding: 15px;
}
button {
--blue: #0000ff; /* local variable will override global */
background-color: var(--white);
color: var(--blue);
border: 1px solid var(--blue);
padding: 5px;
}
Add a New Local Variable
If a variable is to be used at only one single place, we could
also have declared a new local variable, like this:
Example
:root {
--blue: #1e90ff;
--white: #ffffff;
}
body {
background-color: var(--blue);
}
h2 {
border-bottom: 2px solid var(--blue);
}
.container {
color: var(--blue);
background-color: var(--white);
padding: 15px;
}
button {
--button-blue: #0000ff; /* new local variable */
background-color: var(--white);
color: var(--button-blue);
border: 1px solid var(--button-blue);
padding: 5px;
}
Browser Support
The numbers in the table specify the first browser version that
fully supports the var() function.
|
Function |
|||||
|
var() |
49.0 |
15.0 |
31.0 |
9.1 |
36.0 |
CSS var() Function
|
Property |
Description |
|
var() |
Inserts the value of a CSS
variable |
CSS Change Variables With JavaScript
Change Variables With JavaScript
CSS variables have access to the DOM, which means that you can
change them with JavaScript.
Here is an example of how you can create a script to display and
change the --blue variable from the example used in the previous pages.
Example
<script>
// Get the root element
var r = document.querySelector(':root');
// Create a function for getting a variable
value
function myFunction_get() {
//
Get the styles (properties and values) for the root
var rs
= getComputedStyle(r);
//
Alert the value of the --blue variable
alert("The value of --blue is: " + rs.getPropertyValue('--blue'));
}
// Create a function for setting a variable
value
function myFunction_set() {
//
Set the value of variable --blue to another value (in this case
"lightblue")
r.style.setProperty('--blue', 'lightblue');
}
</script>
Browser Support
The numbers in the table specify the first browser version that
fully supports the var() function.
|
Function |
|||||
|
var() |
49.0 |
15.0 |
31.0 |
9.1 |
36.0 |
CSS var() Function
|
Property |
Description |
|
var() |
Inserts the value of a CSS
variable |
CSS Using Variables in Media Queries
Using Variables in Media Queries
Now we want to change a variable value inside a media query.
Tip: Media Queries are about defining
different style rules for different devices (screens, tablets, mobile phones,
etc.). You can learn more Media Queries in our Media Queries Chapter.
Here, we first declare a new local variable named --fontsize for
the .container class. We set its value to
25 pixels. Then we use it in the .container class
further down. Then, we create a @media rule that says
"When the browser's width is 450px or wider, change the --fontsize
variable value of the .container class
to 50px."
Here is the complete example:
Example
/*
Variable declarations */
:root {
--blue: #1e90ff;
--white: #ffffff;
}
.container {
--fontsize: 25px;
}
/* Styles */
body {
background-color: var(--blue);
}
h2 {
border-bottom: 2px solid var(--blue);
}
.container {
color: var(--blue);
background-color: var(--white);
padding: 15px;
font-size: var(--fontsize);
}
@media screen and (min-width: 450px) {
.container {
--fontsize: 50px;
}
}
Here is another example where we also change the value of the
--blue variable in the @media rule:
Example
/*
Variable declarations */
:root {
--blue: #1e90ff;
--white: #ffffff;
}
.container {
--fontsize: 25px;
}
/* Styles */
body {
background-color: var(--blue);
}
h2 {
border-bottom: 2px solid var(--blue);
}
.container {
color: var(--blue);
background-color: var(--white);
padding: 15px;
font-size: var(--fontsize);
}
@media screen and (min-width: 450px) {
.container {
--fontsize: 50px;
}
:root {
--blue: lightblue;
}
}
Browser Support
The numbers in the table specify the first browser version that
fully supports the var() function.
|
Function |
|||||
|
var() |
49.0 |
15.0 |
31.0 |
9.1 |
36.0 |
CSS var() Function
|
Property |
Description |
|
var() |
Inserts the value of a CSS
variable |
