@extend

The Stylus @extend directive is inspired by (and essentially the same as) the SASS Implementation, with few subtle differences. This feature significantly simplifies maintenance of semantic rulesets that inherit from other semantic rulesets.

“Extending” with mixins

Although you can use mixins to achieve a similar effect, this can lead to duplicate CSS. A typical pattern is to define several classes as shown below, then combine them on the element such as "warning message".

While this technique works just fine, it's difficult to maintain.

.message,
.warning {
  padding: 10px;
  border: 1px solid #eee;
}

.warning {
  color: #E2E21E;
}

.message,
.warning {
  padding: 10px;
  border: 1px solid #eee;
}

.warning {
  color: #E2E21E;
}

Using `@extend`

To produce this same output with @extend, simply pass it the desired selector (note that @extend and @extends are equal, one is just an alias of another). Stylus will then append the .warning selector to the existing .message ruleset. The .warning class then inherits properties from .message.

.message {
  padding: 10px;
  border: 1px solid #eee;
}

.warning {
  @extend .message;
  color: #E2E21E;
}

.message {
  padding: 10px;
  border: 1px solid #eee;
}

.warning {
  @extend .message;
  color: #E2E21E;
}

Here's a more complex example, demonstrating how @extend cascades:

red = #E33E1E
yellow = #E2E21E

.message
  padding: 10px
  font: 14px Helvetica
  border: 1px solid #eee

.warning
  @extends .message
  border-color: yellow
  background: yellow + 70%

.error
  @extends .message
  border-color: red
  background: red + 70%

.fatal
  @extends .error
  font-weight: bold
  color: red

red = #E33E1E
yellow = #E2E21E

.message
  padding: 10px
  font: 14px Helvetica
  border: 1px solid #eee

.warning
  @extends .message
  border-color: yellow
  background: yellow + 70%

.error
  @extends .message
  border-color: red
  background: red + 70%

.fatal
  @extends .error
  font-weight: bold
  color: red

Yielding the following CSS:

.message,
.warning,
.error,
.fatal {
  padding: 10px;
  font: 14px Helvetica;
  border: 1px solid #eee;
}
.warning {
  border-color: #e2e21e;
  background: #f6f6bc;
}
.error,
.fatal {
  border-color: #e33e1e;
  background: #f7c5bc;
}
.fatal {
  font-weight: bold;
  color: #e33e1e;
}

.message,
.warning,
.error,
.fatal {
  padding: 10px;
  font: 14px Helvetica;
  border: 1px solid #eee;
}
.warning {
  border-color: #e2e21e;
  background: #f6f6bc;
}
.error,
.fatal {
  border-color: #e33e1e;
  background: #f7c5bc;
}
.fatal {
  font-weight: bold;
  color: #e33e1e;
}

Where Stylus currently differs from SASS is, SASS won't allow @extend nested selectors:

form
  button
    padding: 10px

a.button
  @extend form button
Syntax error: Can't extend form button: can't extend nested selectors
        on line 6 of standard input
  Use --trace for backtrace.

form
  button
    padding: 10px

a.button
  @extend form button
Syntax error: Can't extend form button: can't extend nested selectors
        on line 6 of standard input
  Use --trace for backtrace.

With Stylus, as long as the selectors match, it works!

form
  input[type=text]
    padding: 5px
    border: 1px solid #eee
    color: #ddd

textarea
  @extends form input[type=text]
  padding: 10px

form
  input[type=text]
    padding: 5px
    border: 1px solid #eee
    color: #ddd

textarea
  @extends form input[type=text]
  padding: 10px

Yielding:

form input[type=text],
textarea {
  padding: 5px;
  border: 1px solid #eee;
  color: #ddd;
}
textarea {
  padding: 10px;
}

form input[type=text],
textarea {
  padding: 5px;
  border: 1px solid #eee;
  color: #ddd;
}
textarea {
  padding: 10px;
}

Extending multiple selectors

Stylus allows you to extend multiple selectors at once, just write them with the comma:

.a
  color: red

.b
  width: 100px

.c
  @extend .a, .b
  height: 200px

.a
  color: red

.b
  width: 100px

.c
  @extend .a, .b
  height: 200px

Yielding:

.a,
.c {
  color: #f00;
}
.b,
.c {
  width: 100px;
}
.c {
  height: 200px;
}

.a,
.c {
  color: #f00;
}
.b,
.c {
  width: 100px;
}
.c {
  height: 200px;
}

Extending placeholder selectors

Stylus has a feature similar to the one in Sass — placeholder selectors.

Those selectors should start with a $ symbol (for example, $foo), and are not yielded in the resulting CSS. But you can still extend them:

$foo
  color: #FFF

$foo2
  color: red

.bar
  background: #000
  @extends $foo

.baz
  @extends $foo

$foo
  color: #FFF

$foo2
  color: red

.bar
  background: #000
  @extends $foo

.baz
  @extends $foo

Yielding:

.bar,
.baz {
  color: #fff;
}
.bar {
  background: #000;
}

.bar,
.baz {
  color: #fff;
}
.bar {
  background: #000;
}

Note that if the selector is not extended, it won't be in the resulting CSS, so it's a powerful way to create a library of extendable code. While you can insert code through mixins, they would insert the same code every time you use them, while extending placeholders would give you compact output.

Optional extending

Sometimes it might be usefull to be able to extend something that might or might not exist depending on the context. You can suffix any selector with !optional to achieve this:

$specialDesign
  color: #FFF

.btn
  @extend .design !optional, $specialDesign !optional

$specialDesign
  color: #FFF

.btn
  @extend .design !optional, $specialDesign !optional

Yielding:

.btn {
  color: #fff;
}

.btn {
  color: #fff;
}

@extend #

“Extending” with mixins #

Using @extend #

Extending multiple selectors #

Extending placeholder selectors #

Optional extending #

@extend

“Extending” with mixins

Using `@extend`

Extending multiple selectors

Extending placeholder selectors

Optional extending