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Stylus features powerful in-language function definitions. Function definitions appear identical to mixins; however, functions may return a value.

Return Values

Let's try a trivial example: creating a function that adds two numbers.

add(a, b)
  a + b

We can then use this function in conditions, in property values, etc.

   padding add(10px, 5)


 body {
   padding: 15px;

Argument Defaults

Optional arguments may default to a given expression. With Stylus we may even default arguments to earlier arguments!

For example:

 add(a, b = a)
   a + b

 add(10, 5)
 // => 15

 // => 20

Note: Since argument defaults are assignments, we can also use function calls for defaults:

 add(a, b = unit(a, px))
   a + b

Function Bodies

We can take our simple add() function further. Let's casting all units passed as px via the unit() built-in. It reassigns each argument, and provides a unit-type string (or identifier), which ignores unit conversion.

 add(a, b = a)
   a = unit(a, px)
   b = unit(b, px)
   a + b

 add(15%, 10deg)
 // => 25

Multiple Return Values

Stylus functions can return several values—just as you can assign several values to a variable.

For example, the following is a valid assignment:

   sizes = 15px 10px

   // => 15px 

Similarly, we may return several values:

     15px 10px

   // => 15px

One slight exception is when return values are identifiers. For example, the following looks like a property assignment to Stylus (since no operators are present):

 swap(a, b)
   b a

To disambiguate, we can either wrap with parentheses, or use the return keyword:

  swap(a, b)
    (b a)

  swap(a, b)
    return b a


Let's say we want to create a function named stringish() to determine whether the argument can be transformed to a string. We check if val is a string, or an ident (which is string-like). Because undefined identifiers yield themselves as the value, we may compare them to themselves as shown below (where yes and no are used in place of true and false):

   if val is a 'string' or val is a 'ident'


 stringish('yay') == yes
 // => true

 stringish(yay) == yes
 // => true

 stringish(0) == no
 // => true

note: yes and no are not boolean literals. They are simply undefined identifiers in this case.

Another example:

compare(a, b)
  if a > b
  else if a < b


compare(5, 2)
// => higher

compare(1, 5)
// => lower

compare(10, 10)
// => equal


To alias a function, simply assign a function's name to a new identifier. For example, our add() function could be aliased as plus(), like so:

  plus = add

  plus(1, 2)
  // => 3

Variable Functions

In the same way that we can "alias" a function, we can pass a function as well. Here, our invoke() function accepts a function, so we can pass it add() or sub().

invoke(a, b, fn)
  fn(a, b)

add(a, b)
  a + b

  padding invoke(5, 10, add)
  padding invoke(5, 10, sub)


body {
  padding: 15;
  padding: -5;


The arguments local is available to all function bodies, and contains all the arguments passed.

For example:

   n = 0
   for num in arguments
     n = n + num

 // => 15

Hash Example

Below we define the get(hash, key) function, which returns the value of key (or null). We iterate each pair in hash, returning the pair's second node when the first (the key) matches.

  get(hash, key)
    return pair[1] if pair[0] == key for pair in hash

As demonstrated below, in-language functions—paired with robust Stylus expressions—can provide great flexibility:

  hash = (one 1) (two 2) (three 3)

  get(hash, two)
  // => 2

  get(hash, three)
  // => 3

  get(hash, something)
  // => null
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