In addition to allowing users to define their own functions, there are many useful functions built right into Sass. These functions are called using the normal CSS function syntax, with the addition of special Sass argument syntax.
Sass’s built-in functions are divided into a few categories:
Any function that Sass doesn’t recognize as built-in or user-defined is treated as a plain CSS function.
Selector functions provide access to Sass’s powerful selector engine.
Introspection functions expose the details of Sass’s inner workings.
$condition is truthy, and
This function is special in that it doesn’t even evaluate the argument that isn’t returned, so it’s safe to call even if the unused argument would throw an error.