Custom properties or CSS variables

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Senior Software Engineer

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5 years with GlobalLogic

about 6+ years in Web Development

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Inna Ivashchuk

CSS custom properties

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Using CSS custom properties (variables)

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Custom properties (sometimes referred to as CSS variables or cascading variables) are entities defined by CSS authors that contain specific values to be reused throughout a document. They are set using custom property notation (e.g., --main-color: black;) and are accessed using the var() function (e.g., color: var(--main-color);).

Basic usages

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Declaring a custom property is done using a custom property name that begins with a double hyphen (--), and a property value that can be any valid CSS value. Like any other property, this is written inside a ruleset, like so:

buttom {
  --jll-ds-button-primary-background: #333333;
}

Note: the selector given to the ruleset defines the scope that the custom property can be used in

Best practice

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A common best practice is to define custom properties on the :root pseudo-class, so that it can be applied globally across your HTML document:

:root {
  --jll-ds-button-primary-background: #333333;
  --jll-ds-main-color: violet;
}

Ok, but how to use them?

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How to use CSS variables

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var() function

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To use the custom property value by specifying your custom property name inside the var() function, in place of a regular property value:

element {
  background-color: var(--jll-ds-global-surface-base-reversed);
}

Using CSS variabes

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:root {
  --jll-ds-global-surface-base-reversed: #333;
}

button {
  color: white;
  background-color: var(--jll-ds-global-surface-base-reversed);
  margin: 10px;
  width: 50px;
  height: 50px;
  display: inline-block;
}

“It’s quite possible that there are more functionality.”

 

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What you should know about CSS vars

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Custom properties do inherit. This means that if no value is set for a custom property on a given element, the value of its parent is used. Take this HTML:

Inheritance of custom properties

<div class="two">
    <div class="three"></div>
    <div class="four"></div>
</div>
.two {
  --font-size: 10px;
}

.three {
  --font-size: 2em;
}

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In this case, the results of var(--test) are:

  • For the class="two" element: 10px
  • For the class="three" element: 2em
  • For the class="four" element: 10px (inherited from its parent)

Inheritance of custom properties

<div class="two">
    <div class="three"></div>
    <div class="four"></div>
</div>
.two {
  --font-size: 10px;
}

.three {
  --font-size: 2em;
}

Fallback values

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Using the var() function, you can define multiple fallback values when the given variable is not yet defined; this can be useful when working with Custom Elements and Shadow DOM

.two {
  /* Red if --my-var is not defined */
  color: var(--my-var, red);
}

.three {
  /* pink if --my-var and --my-background are not defined */
  background-color: var(--my-var, var(--my-background, pink));
}

.three {
  /* Invalid: "--my-background, pink" */
  background-color: var(--my-var, --my-background, pink);
}

Custom property fallback values

Note: The syntax of the fallback, like that of custom properties, allows commas. For example, var(--foo, red, blue) defines a fallback of red, blue — anything between the first comma and the end of the function is considered a fallback value.

element {
  background-color: var(--my-var, red, blue);
}

Browsers support

:root {
  --primary: yellow;
}

body {
  background-color: yellow;
  background-color: var(--primary);
}

CSS variables with fallback for older browsers

Thank you!

46

CSS variables

By Inna Ivashchuk

CSS variables

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