sass-site/source/documentation/at-rules/extend.html.md.erb

---
title: "@extend"
table_of_contents: true
---

There are often cases when designing a page when one class should have all the
styles of another class, as well as its own specific styles. For example, the
[BEM methodology][] encourages modifier classes that go on the same elements as
block or element classes. But this can create cluttered HTML, it's prone to
errors from forgetting to include both classes, and it can bring non-semantic
style concerns into your markup.

[BEM methodology]: http://getbem.com/naming/

<%# TODO(jina): I think these code blocks should be side-by-side %>

```html
<div class="error error--serious">
  Oh no! You've been hacked!
</div>
```

```css
.error {
  border: 1px #f00;
  background-color: #fdd;
}

.error--serious {
  border-width: 3px;
}
```

Sass's `@extend` rule solves this. It's written `@extend <selector>`, and it
tells Sass that one selector should inherit the styles of another.

<% example do %>
  .error {
    border: 1px #f00;
    background-color: #fdd;

    &--serious {
      @extend .error;
      border-width: 3px;
    }
  }
  ===
  .error
    border: 1px #f00
    background-color: #fdd

    &--serious
      @extend .error
      border-width: 3px
<% end %>

When one class extends another, Sass styles all elements that match the extender
as though they also match the class being extended. When one class selector
extends another, it works exactly as though you added the extended class to
every element in your HTML that already had the extending class. You can just
write `class="error--serious"`, and Sass will make sure it's styled as though it
had `class="error"` as well.

Of course, selectors aren't just used on their own in style rules. Sass knows to
extend *everywhere* the selector is used. This ensures that your elements are
styled exactly as if they matched the extended selector.

<% example do %>
  .error:hover {
    background-color: #fee;
  }

  .error--serious {
    @extend .error;
    border-width: 3px;
  }
  ===
  .error:hover
    background-color: #fee


  .error--serious
    @extend .error
    border-width: 3px
<% end %>

<% heads_up do %>
  Extends are resolved after the rest of your stylesheet is compiled. In
  particular, it happens after [parent selectors][] are resolved. This means that
  if you `@extend .error`, it won't affect the inner selector in
  `.error { &__icon { ... } }`. It also means that [parent selectors in
  SassScript][] can't see the results of extend.

  [parent selectors]: ../style-rules/parent-selector
  [parent selectors in SassScript]: ../style-rules/parent-selector#in-sassscript
<% end %>

## How It Works

Unlike [mixins][], which copy styles into the current style rule, `@extend`
updates style rules that contain the extended selector so that they contain the
extending selector as well. When extending selectors, Sass does *intelligent
unification*:

[mixins]: mixin

* It never generates selectors like `#main#footer` that can't possibly match
  any elements.

* It ensures that complex selectors are interleaved so that they work no matter
  which order the HTML elements are nested.

* It trims redundant selectors as much as possible, while still ensuring that
  the specificity is greater than or equal to that of the extender.

* It knows when one selector matches everything another does, and can combine
  them together.

* It intelligently handles [combinators][], [universal selectors][], and
  [pseudo-classes that contain selectors][].

[combinators]: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Selectors#Combinators
[pseudo-classes that contain selectors]: https://developer.mozilla.org/en-US/docs/Web/CSS/:not
[universal selectors]: https://developer.mozilla.org/en-US/docs/Web/CSS/Universal_selectors

<% example do %>
  .content nav.sidebar {
    @extend .info;
  }

  // This won't be extended, because `p` is incompatible with `nav`.
  p.info {
    background-color: #dee9fc;
  }

  // There's no way to know whether `<div class="guide">` will be inside or
  // outside `<div class="content">`, so Sass generates both to be safe.
  .guide .info {
    border: 1px solid rgba(#000, 0.8);
    border-radius: 2px;
  }

  // Sass knows that every element matching "main.content" also matches ".content"
  // and avoids generating unnecessary interleaved selectors.
  main.content .info {
    font-size: 0.8em;
  }
  ===
  .content nav.sidebar
    @extend .info


  // This won't be extended, because `p` is incompatible with `nav`.
  p.info
    background-color: #dee9fc


  // There's no way to know whether `<div class="guide">` will be inside or
  // outside `<div class="content">`, so Sass generates both to be safe.
  .guide .info
    border: 1px solid rgba(#000, 0.8)
    border-radius: 2px


  // Sass knows that every element matching "main.content" also matches ".content"
  // and avoids generating unnecessary interleaved selectors.
  main.content .info
    font-size: 0.8em
<% end %>

<% fun_fact do %>
  You can directly access Sass's intelligent unification using [selector
  functions][]! The [`selector-unify()` function][] returns a selector that
  matches the intersection of two selectors, while the [`selector-extend()`
  function][] works just like `@extend`, but on a single selector.

  [selector functions]: ../functions/selector
  [`selector-unify()` function]: ../functions/selector#selector-unify
  [`selector-extend()` function]: ../functions/selector#selector-extend
<% end %>

<% heads_up do %>
  Because `@extend` updates style rules that contain the extended selector, their
  styles have precedence in [the cascade][] based on where the extended selector's
  style rules appear, *not* based on where the `@extend` appears. This can be
  confusing, but just remember: this is the same precedence those rules would have
  if you added the extended class to your HTML!

  [the cascade]: https://developer.mozilla.org/en-US/docs/Web/CSS/Cascade
<% end %>

## Placeholder Selectors

Sometimes you want to write a style rule that's *only* intended to be extended.
In that case, you can use [placeholder selectors][], which look like class
selectors that start with `%` instead of `.`. Any selectors that include
placeholders aren't included in the CSS output, but selectors that extend them
are.

[placeholder selectors]: ../style-rules/placeholder-selectors

<%= partial "code-snippets/example-placeholder" %>

## Mandatory and Optional Extends

Normally, if an `@extend` doesn't match any selectors in the stylesheet, Sass
will produce an error. This helps protect from typos or from renaming a selector
without renaming the selectors that inherit from it. Extends that require that
the extended selector exists are *mandatory*.

This may not always be what you want, though. If you want the `@extend` to do
nothing if the extended selector doesn't exist, just add `!optional` to the end.

## Extends or Mixins?

Extends and [mixins][] are both ways of encapsulating and re-using styles in
Sass, which naturally raises the question of when to use which one. Mixins are
obviously necessary when you need to configure the styles using [arguments][],
but what if they're just a chunk of styles?

[arguments]: mixin#arguments

As a rule of thumb, extends are the best option when you're expressing a
relationship between semantic classes (or other semantic selectors). Because an
element with class `.error--serious` *is an* error, it makes sense for it to
extend `.error`. But for non-semantic collections of styles, writing a mixin can
avoid cascade headaches and make it easier to configure down the line.

<% fun_fact do %>
  Most web servers compress the CSS they serve using an algorithm that's very good
  at handling repeated chunks of identical text. This means that, although mixins
  may produce more CSS than extends, they probably won't substantially increase
  the amount your users need to download. So choose the feature that makes the
  most sense for your use-case, not the one that generates the least CSS!

  [gzip]: https://en.wikipedia.org/wiki/Gzip
<% end %>

## Limitations

### Disallowed Selectors

<% impl_status dart: true, libsass: false, ruby: false do %>
  LibSass and Ruby Sass currently allow complex selectors like `.message.info` to
  be extended. However, this behavior didn't match the definition of `@extend`:
  instead of styling elements that matched the extending selector as though it had
  `class="message info"`, which would be affected by style rules that included
  either `.message` *or* `.info`, it only styled them with rules that included
  both `.message` *and* `info`.

  In order to keep the definition of `@extend` straightforward and understandable,
  and to keep the implementation clean and efficient, that behavior is now
  deprecated and will be removed from future versions.
<% end %>

Only *simple selectors*—individual selectors like `.info` or `a`—can be
extended. If `.message.info` could be extended, the definition of `@extend` says
that elements matching the extender would be styled as though they matched
`.message.info`. That's just the same as matching both `.message` and `.info`,
so there wouldn't be any benefit in writing that instead of
`@extend .message, .info`.

Similarly, if `.main .info` could be extended, it would do (almost) the same
thing as extending `.info` on its own. The subtle differences aren't worth the
confusion of looking like it's doing something substantially different, so this
isn't allowed either.

<% example(autogen_css: false) do %>
  .alert {
    @extend .message.info;
    //      ^^^^^^^^^^^^^
    // Error: Write @extend .message, .info instead.

    @extend .main .info;
    //      ^^^^^^^^^^^
    // Error: write @extend .info instead.
  }
  ===
  .alert
    @extend .message.info
    //      ^^^^^^^^^^^^^
    // Error: Write @extend .message, .info instead.

    @extend .main .info
    //      ^^^^^^^^^^^
    // Error: write @extend .info instead.
<% end %>

### HTML Heuristics

When `@extend` [interleaves complex selectors][], it doesn't generate all
possible combinations of ancestor selectors. Many of the selectors it could
generate are unlikely to actually match real HTML, and generating them all would
make stylesheets way too big for very little real value. Instead, it uses
a [heuristic][]: it assumes that each selector's ancestors will be
self-contained, without being interleaved with any other selector's ancestors.

[interleaves complex selectors]: #how-it-works
[heuristic]: https://en.wikipedia.org/wiki/Heuristic

<% example do %>
  header .warning li {
    font-weight: bold;
  }

  aside .notice dd {
    // Sass doesn't generate CSS to match the <dd> in
    //
    // <header>
    //   <aside>
    //     <div class="warning">
    //       <div class="notice">
    //         <dd>...</dd>
    //       </div>
    //     </div>
    //   </aside>
    // </header>
    //
    // because matching all elements like that would require us to generate nine
    // new selectors instead of just two.
    @extend li;
  }
  ===
  header .warning li
    font-weight: bold


  aside .notice dd
    // Sass doesn't generate CSS to match the <dd> in
    //
    // <header>
    //   <aside>
    //     <div class="warning">
    //       <div class="notice">
    //         <dd>...</dd>
    //       </div>
    //     </div>
    //   </aside>
    // </header>
    //
    // because matching all elements like that would require us to generate nine
    // new selectors instead of just two.
    @extend li
<% end %>

### Extend in `@media`

While `@extend` is allowed within [`@media` and other CSS at-rules][], it's not
allowed to extend selectors that appear outside its at-rule. This is because the
extending selector only applies within the given media context, and there's no
way to make sure that restriction is preserved in the generated selector without
duplicating the entire style rule.

[`@media` and other CSS at-rules]: css

<% example(autogen_css: false) do %>
  @media screen and (max-width: 600px) {
    .error--serious {
      @extend .error;
      //      ^^^^^^
      // Error: ".error" was extended in @media, but used outside it.
    }
  }

  .error {
    border: 1px #f00;
    background-color: #fdd;
  }
  ===
  @media screen and (max-width: 600px)
    .error--serious
      @extend .error
      //      ^^^^^^
      // Error: ".error" was extended in @media, but used outside it.



  .error
    border: 1px #f00
    background-color: #fdd
<% end %>