CSS Variable vs Sass Variable: Key Differences and Usage

A CSS variable is a custom property defined in CSS that works at runtime in the browser and can be changed dynamically, while a Sass variable is a preprocessor variable replaced during build time and cannot be changed after compilation. CSS variables support cascading and inheritance, but Sass variables do not.

Quick Comparison

Here is a quick side-by-side comparison of CSS variables and Sass variables based on key factors.

Factor	CSS Variable	Sass Variable
Definition	Custom properties in CSS, runtime in browser	Preprocessor variables replaced at build time
Syntax	--name: value;	$name: value;
Scope	Scoped to CSS selectors, supports inheritance	Scoped to Sass files or blocks, no inheritance
Runtime Change	Can be changed dynamically with JavaScript	Fixed after compilation, cannot change at runtime
Browser Support	Supported in modern browsers	No browser support needed, compiled to CSS
Use Case	Dynamic theming, runtime changes	Reusable values during development

Key Differences

CSS variables are part of the CSS language itself. They are defined with a double dash prefix, like --main-color, and can be used anywhere in CSS. Because they exist in the browser, they can be changed dynamically using JavaScript, allowing for live theme switching or user preferences without reloading the page.

On the other hand, Sass variables are part of the Sass preprocessor. They start with a dollar sign, like $main-color, and are replaced with their values during the build process before the CSS reaches the browser. This means they cannot be changed once the CSS is generated.

Another important difference is scope. CSS variables follow CSS rules, so they can be scoped inside selectors and inherit down the DOM tree. Sass variables have lexical scope limited to the Sass files or blocks where they are defined and do not inherit or cascade.

Code Comparison

Here is how you define and use a color variable in CSS variables:

css

:root {
  --main-color: #3498db;
}

.button {
  background-color: var(--main-color);
  color: white;
  padding: 10px 20px;
  border: none;
  border-radius: 4px;
}

Output

A blue button with white text and rounded corners

Sass Variable Equivalent

Here is the same color variable defined and used in Sass:

scss

$main-color: #3498db;

.button {
  background-color: $main-color;
  color: white;
  padding: 10px 20px;
  border: none;
  border-radius: 4px;
}

Output

A blue button with white text and rounded corners

When to Use Which

Choose CSS variables when you need dynamic styling that can change after the page loads, such as user themes or responsive adjustments with JavaScript. They are also useful when you want to leverage CSS inheritance and cascade.

Choose Sass variables when you want to keep your styles organized during development, reuse values consistently, and do not need runtime changes. Sass variables help write cleaner, maintainable code but produce static CSS.

Key Takeaways

CSS variables work in the browser and can change dynamically at runtime.

Sass variables are replaced during build time and cannot change after compilation.

CSS variables support inheritance and cascading; Sass variables do not.

Use CSS variables for dynamic theming and runtime changes.

Use Sass variables for development-time value reuse and organization.