Operators
Operator Precedence
Below is the operator precedence table, highest to lowest:
.
[]
! ~ + -
is defined
** * / %
+ -
... ..
<= >= < >
in
== is != is not isnt
is a
&& and || or
?:
= := ?= += -= *= /= %=
not
if unless
Unary Operators
The following unary operators are available, !, not, -, +, and ~.
!0
// => true
!!0
// => false
!1
// => false
!!5px
// => true
-5px
// => -5px
--5px
// => 5px
not true
// => false
not not true
// => true
The logical not operator has low precedence, therefore the following example could be replaced with
a = 0
b = 1
!a and !b
// => false
// parsed as: (!a) and (!b)
With:
not a or b
// => false
// parsed as: not (a or b)
Binary Operators
Subscript []
The subscript operator allows us to grab a value inside an expression via index (zero-based). Negative index values starts with the last element in the expression.
list = 1 2 3
list[0]
// => 1
list[-1]
// => 3
Parenthesized expressions may act as tuples (e.g. (15px 5px), (1 2 3)).
Below is an example that uses tuples for error handling (and showcasing the versatility of this construct):
add(a, b)
if a is a 'unit' and b is a 'unit'
a + b
else
(error 'a and b must be units!')
body
padding add(1,'5')
// => padding: error "a and b must be units";
padding add(1,'5')[0]
// => padding: error;
padding add(1,'5')[0] == error
// => padding: true;
padding add(1,'5')[1]
// => padding: "a and b must be units";
Here’s a more complex example. Now we’re invoking the built-in error() function with the return error message, whenever the ident (the first value) equals error.
if (val = add(1,'5'))[0] == error
error(val[1])
Range .. ...
Both the inclusive (..) and exclusive (...) range operators are provided, expanding to expressions:
1..5
// => 1 2 3 4 5
1...5
// => 1 2 3 4
5..1
// => 5 4 3 2 1
Additive: + -
Multiplicative and additive binary operators work as expected. Type conversion is applied within unit type classes, or default to the literal value. For example 5s - 2px results in 3s.
15px - 5px
// => 10px
5 - 2
// => 3
5in - 50mm
// => 3.031in
5s - 1000ms
// => 4s
20mm + 4in
// => 121.6mm
"foo " + "bar"
// => "foo bar"
"num " + 15
// => "num 15"
Multiplicative: / * %
2000ms + (1s * 2)
// => 4000ms
5s / 2
// => 2.5s
4 % 2
// => 0
When using / within a property value, you must wrap with parens. Otherwise the / is taken literally (to support CSS line-height):
font: 14px/1.5;
But the following is evaluated as 14px ÷ 1.5:
font: (14px/1.5);
This is only required for the / operator.
Exponent: **
The Exponent operator:
2 ** 8
// => 256
Equality & Relational: == != >= <= > <
Equality operators can be used to equate units, colors, strings, and even identifiers. This is a powerful concept, as even arbitrary identifiers (such as as wahoo) can be utilized as atoms. A function could return yes or no instead of true or false (although not advised).
5 == 5
// => true
10 > 5
// => true
#fff == #fff
// => true
true == false
// => false
wahoo == yay
// => false
wahoo == wahoo
// => true
"test" == "test"
// => true
true is true
// => true
'hey' is not 'bye'
// => true
'hey' isnt 'bye'
// => true
(foo bar) == (foo bar)
// => true
(1 2 3) == (1 2 3)
// => true
(1 2 3) == (1 1 3)
// => false
Only exact values match. For example, 0 == false and null == false are both false.
Aliases:
== is
!= is not
!= isnt
Truthfulness
Nearly everything within Stylus resolves to true, including units with a suffix. Even 0%, 0px, etc. will resolve to true (because it’s common in Stylus for mixins or functions to accept units as valid).
However, 0 itself is false in terms of arithmetic.
Expressions (or “lists”) with a length greater than 1 are considered truthy.
true examples:
0%
0px
1px
-1
-1px
hey
'hey'
(0 0 0)
('' '')
false examples:
0
null
false
''
Logical Operators: && || and or
Logical operators && and || are aliased and / or which apply the same precedence.
5 && 3
// => 3
0 || 5
// => 5
0 && 5
// => 0
#fff is a 'rgba' and 15 is a 'unit'
// => true
Existence Operator: in
Checks for the existence of the left-hand operand within the right-hand expression.
Simple examples:
nums = 1 2 3
1 in nums
// => true
5 in nums
// => false
Some undefined identifiers:
words = foo bar baz
bar in words
// => true
HEY in words
// => false
Works with tuples too:
vals = (error 'one') (error 'two')
error in vals
// => false
(error 'one') in vals
// => true
(error 'two') in vals
// => true
(error 'something') in vals
// => false
Example usage in mixin:
pad(types = padding, n = 5px)
if padding in types
padding n
if margin in types
margin n
body
pad()
body
pad(margin)
body
pad(padding margin, 10px)
Yielding:
body {
padding: 5px;
}
body {
margin: 5px;
}
body {
padding: 10px;
margin: 10px;
}
Conditional Assignment: ?= :=
The conditional assignment operator ?= (aliased as :=) lets us define variables without clobbering old values (if present). This operator expands to an is defined binary operation within a ternary.
For example, the following are equivalent:
color := white
color ?= white
color = color is defined ? color : white
When using plain =, we simply reassign:
color = white
color = black
color
// => black
But when using ?=, our second attempt fails (since the variable is already defined):
color = white
color ?= black
color
// => white
Instance Check: is a
Stylus provides a binary operator named is a used to type check.
15 is a 'unit'
// => true
#fff is a 'rgba'
// => true
15 is a 'rgba'
// => false
Alternatively, we could use the type() BIF:
type(#fff) == 'rgba'
// => true
Note: color is the only special-case, evaluating to true when the
left-hand operand is an RGBA or HSLA node.
Variable Definition: is defined
This pseudo binary operator does not accept a right-hand operator, and does not evaluate the left. This allows us to check if a variable has a value assigned to it.
foo is defined
// => false
foo = 15px
foo is defined
// => true
#fff is defined
// => 'invalid "is defined" check on non-variable #fff'
Alternatively, one can use the lookup(name) built-in function to do this—or to perform dynamic lookups:
name = 'blue'
lookup('light-' + name)
// => null
light-blue = #80e2e9
lookup('light-' + name)
// => #80e2e9
This operator is essential, as an undefined identifier is still a truthy value. For example:
body
if ohnoes
padding 5px
Will yield the following CSS when undefined:
body {
padding: 5px;
}
However this will be safe:
body
if ohnoes is defined
padding 5px
Ternary
The ternary operator works as we would expect in most languages. It’s the only operator with three operands (the condition expression, the truth expression, and the false expression).
num = 15
num ? unit(num, 'px') : 20px
// => 15px
Casting
As an terse alternative to the unit() built-in function, the syntax (expr) unit may be used to force the suffix.
body
n = 5
foo: (n)em
foo: (n)%
foo: (n + 5)%
foo: (n * 5)px
foo: unit(n + 5, '%')
foo: unit(5 + 180 / 2, deg)
Color Operations
Operations on colors provide a terse, expressive way to alter components. For example, we can operate on each RGB:
#0e0 + #0e0
// => #0f0
Another example is adjust the lightness value by adding or subtracting a percentage. To lighten a color, add; to darken, subtract.
#888 + 50%
// => #c3c3c3
#888 - 50%
// => #444
Adjust the hue is also possible by adding or subtracting with degrees. For example, adding 50deg to this red value results in a yellow:
#f00 + 50deg
// => #ffd500
Values clamp appropriately. For example, we can “spin” the hue 180 degrees, and if the current value is 320deg, it will resolve to 140deg.
We may also tweak several values at once (including the alpha) by using rgb(), rgba(), hsl(), or hsla():
#f00 - rgba(100,0,0,0.5)
// => rgba(155,0,0,0.5)
Sprintf
The string sprintf-like operator % can be used to generate a literal value, internally passing arguments through the s() built-in:
'X::Microsoft::Crap(%s)' % #fc0
// => X::Microsoft::Crap(#fc0)
Multiple values should be parenthesized:
'-webkit-gradient(%s, %s, %s)' % (linear (0 0) (0 100%))
// => -webkit-gradient(linear, 0 0, 0 100%)