Flow Control Rules
Sass provides a number of at-rules that make it possible to control whether styles get emitted, or to emit them multiple times with small variations. They can also be used in mixins and functions to write small algorithms to make writing your Sass easier. Sass supports four flow control rules:
@if controls whether or not a block is evaluated.
@each evaluates a block for each element in a list or each pair in a map.
@for evaluates a block a certain number of times.
@while evaluates a block until a certain condition is met.