extend.md

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@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;
  }

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;
  }

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

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;
  }

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.

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

Yielding:

    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

Yielding:

.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

Yielding:

.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 exists or not depending on the context. You can suffix any selector by !optional to achieve this:

$specialDesign
  color: #FFF

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

Yielding:

.btn {
  color: #fff;
}