@for

The @for rule, written @for <variable> from <expression> to <expression> { ... } or @for <variable> from <expression> through <expression> { ... }, counts up or down from one number (the result of the first expression) to another (the result of the second) and evaluates a block for each number in between. Each number along the way is assigned to the given variable name. If to is used, the final number is excluded; if through is used, it’s included.

$base-color: #036;

@for $i from 1 through 3 {
  ul:nth-child(3n + #{$i}) {
    background-color: lighten($base-color, $i * 5%);
  }
}
// SASS
$base-color: #036

@for $i from 1 through 3
  ul:nth-child(3n + #{$i})
    background-color: lighten($base-color, $i * 5%)
/* CSS */
ul:nth-child(3n+1) {
  background-color: rgb(0%, 25%, 50%);
}

ul:nth-child(3n+2) {
  background-color: rgb(0%, 30%, 60%);
}

ul:nth-child(3n+3) {
  background-color: rgb(0%, 35%, 70%);
}

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Licensed under the MIT License.
https://sass-lang.com/documentation/at-rules/control/for