Less.js extends CSS with dynamic behavior such as variables, mixins, operations, and built-in functions.
Built-in Functions
Unit Operations & Formatting
unit(30px / 5px) // => 6 (strips unit)
unit(5, px) // => 5px (attaches unit)
e("ms:stuff()") // => ms:stuff() (unquote expression)
%("count: %d", 1+2) // => "count: 3" (string formatting)
Type Checking Functions
iscolor(@x) // Returns true if value is a color
isstring(@x) // Returns true if value is a string
isnumber(@x) // Returns true if value is a number
iskeyword(@x) // Returns true if value is a keyword
isurl(url(...)) // Returns true if value is a url
ispixel(@x) // Returns true if unit is px
isem(@x) // Returns true if unit is em
ispercentage(@x) // Returns true if unit is %
isunit(@x, rem) // Returns true if value matches specified unit
Color Channel Operations
hue(@color) // Returns the hue channel (0-360)
saturation(@color) // Returns the saturation percentage
lightness(@color) // Returns the lightness percentage
luma(@color) // Returns perceptual luma
luminance(@color) // Returns relative luminance
Color Blending & Adjustments
fade(@color, 50%) // Set opacity to 50%
fadein(@color, 10%) // Increase opacity by 10%
fadeout(@color, 10%) // Decrease opacity by 10%
spin(@color, 30) // Rotate hue angle by 30 degrees
mix(@color1, @color2, 50%) // Blend two colors evenly
Mixin Guards & Conditionals
Less uses the when keyword to apply conditional logic to mixins:
// Lightness guard
.theme-box when (luma(@color) > 50%) {
color: #000;
background-color: @color;
}
// Negation guard
.theme-box when (not (luma(@color) > 50%)) {
color: #fff;
background-color: @color;
}
// Default fallback guard
.theme-box when (default()) {
border: 1px solid #ccc;
}
// Equality comparison guard
.shape when (e(@shape) = 'circle') {
border-radius: 50%;
}