Difference Between @extend and Mixins in Sass

Sass (Syntactically Awesome Style Sheets) provides two powerful features for reusing styles: @extend and mixins. While both serve the purpose of sharing styles, they have distinct functionalities, use cases, and implications for your CSS output. Understanding the differences between @extend and mixins can help you choose the right approach for your styling needs.

1. What is @extend?

The @extend directive allows one selector to inherit the styles of another selector. This means that when you use @extend, Sass merges the styles of the extended selector into the extending selector, creating a new combined selector in the compiled CSS.

.button {
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    color: white;
}

.primary-button {
    @extend .button; /* Inherits styles from .button */
    background-color: #3498db; /* Specific style for primary button */
}

.secondary-button {
    @extend .button; /* Inherits styles from .button */
    background-color: #2ecc71; /* Specific style for secondary button */
}
    

In this example, both .primary-button and .secondary-button inherit the styles from .button, reducing code duplication.

2. What are Mixins?

Mixins are a way to define reusable styles that can be included in multiple selectors. Unlike @extend, mixins do not create new selectors; instead, they allow you to include a set of styles directly into the selector where the mixin is called. Mixins can also accept parameters, making them highly flexible.

@mixin button-styles($bg-color) {
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    color: white;
    background-color: $bg-color; /* Use parameter for background color */
}

.primary-button {
    @include button-styles(#3498db); /* Include mixin with specific color */
}

.secondary-button {
    @include button-styles(#2ecc71); /* Include mixin with specific color */
}
    

In this example, both .primary-button and .secondary-button use the button-styles mixin, passing different background colors as parameters.

3. Key Differences

3.1. Inheritance vs. Inclusion

@extend creates a relationship between selectors, allowing one to inherit styles from another. In contrast, mixins simply include styles directly into the selector where they are called, without creating any relationship between selectors.

3.2. Output CSS

The output CSS generated by @extend can lead to fewer selectors in the final CSS, as it merges styles. However, this can also create complex and less predictable CSS structures. Mixins, on the other hand, generate separate styles for each selector that includes the mixin, which can lead to more CSS but provides clearer output.

/* Compiled CSS for @extend */
.button, .primary-button, .secondary-button {
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    color: white;
}

.primary-button {
    background-color: #3498db;
}

.secondary-button {
    background-color: #2ecc71;
}

/* Compiled CSS for mixins */
.primary-button {
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    color: white;
    background-color: #3498db;
}

.secondary-button {
    padding: 10px 20px;
    border: none;
    border-radius: 5px;
    color: white;
    background-color: #2ecc71;
}
    

3.3. Flexibility

Mixins are more flexible than @extend because they can accept parameters, allowing you to customize styles on a per-use basis. This makes mixins ideal for creating variations of styles without duplicating code.

3.4. Specificity

Using @extend can lead to specificity issues, as it merges selectors and can create unexpected results in the cascade. Mixins do not have this problem since they generate separate styles for each use, maintaining the specificity of the calling selector.

4. When to Use Each

Choosing between @extend and mixins depends on your specific use case:

• Use @extend when you want to share styles between selectors and reduce the amount of CSS generated, especially when the styles are identical or very similar.
• Use mixins when you need more flexibility, such as passing parameters or when the styles differ significantly between uses.

Conclusion

Both @extend and mixins are valuable tools in Sass for reusing styles, but they serve different purposes. Understanding their differences will help you make informed decisions about which to use in your projects, leading to cleaner, more maintainable CSS.