Getting started with CSS variables

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If you have used SASS or other CSS preprocessors, you might have already used Variables. CSS variables (also referred as custom properties) are the native implementation of them in the browser.

CSS variables need to be declared with -- prefix to the name.

:root {
    --primary-color: #eaefed;
    --text-color: #001100;
}

When we want to access them, we just need to use the var function. The first argument of the var function is the name of the variable. If the variable is invalid, we can pass the fallback value as the second argument.

.link {
    border: 1px solid var(--primary-color);
    color: var(--text-color, #111000); 
    /* if --text-color is invalid,fallback value #111000 is used */
}

Scope of the Variable

The scope of the CSS variable is the scope of the selector it is defined. For global scope, we can either declare them in :root or body selector.

For example, let's declare a variable named --btn-danger-color in a class btn . If you try to access the variable --btn-danger-color in another class out of its scope, it will be invalid. Because the scope of --btn-danger-color is only in btn class

.btn {
    --btn-danger-color: #FF4136;
    background-color: var(--btn-danger-color);
}
.link {
    background-color: var(--btn-danger-color, green); 
    /* Background color is green because --btn-danger is invalid */
}

Accessing and Modifying CSS Variables with JavaScript

To access or modify CSS variables via JavaScript, we can use two methods

• getPropertyValue
• setProperty

Getting the CSS Variable Value

To get the value of the CSS variable, we can use getPropertyValue method

const btn = document.querySelector('.btn');

let btnDangerBg = getComputedStyle(element).getPropertyValue("--btn-danger-color"); 

console.log(btnDangerBg); // #FF4136 - value of the css variable defined in .btn class in CSS

To get the CSS variables declared in :root, you can use document.querySelector(':root');

Changing the CSS Variable Value

To set the value of the CSS variable from JavaScript we can use setProperty method.

  element.style.setProperty("--variable-name", value)

Let's write a small function to change the Button background colour to purple by using setProperty method.

function changeColorToPurple(){
   //Set the CSS variable value
  btn.style.setProperty("--btn-danger-color", "purple")
}

Don't forget to link it with HTML.

<button class="btn" onclick="changeColorToPurple()">Change Color</button>

Demo

I'm adding an embedded repl for the above code So that you can play with it.

Thank You.

Reference

CSS custom properties - MDN

Every thing you need to know about CSS variables