@warn

When writing mixins and functions, you may want to discourage users from passing certain arguments or certain values. They may be passing legacy arguments that are now deprecated, or they may be calling your API in a way that’s not quite optimal.

The @warn rule is designed just for that. It’s written @warn <expression> and it prints the value of the expression (usually a string) for the user, along with a stack trace indicating how the current mixin or function was called. Unlike the @error rule, though, it doesn’t stop Sass entirely.

SCSS Syntax

$known-prefixes: webkit, moz, ms, o;

@mixin prefix($property, $value, $prefixes) {
  @each $prefix in $prefixes {
    @if not index($known-prefixes, $prefix) {
      @warn "Unknown prefix #{$prefix}.";
    }

    -#{$prefix}-#{$property}: $value;
  }
  #{$property}: $value;
}

.tilt {
  // Oops, we typo'd "webkit" as "wekbit"!
  @include prefix(transform, rotate(15deg), wekbit ms);
}

Sass Syntax

$known-prefixes: webkit, moz, ms, o

@mixin prefix($property, $value, $prefixes)
  @each $prefix in $prefixes
    @if not index($known-prefixes, $prefix)
      @warn "Unknown prefix #{$prefix}."


    -#{$prefix}-#{$property}: $value

  #{$property}: $value


.tilt
  // Oops, we typo'd "webkit" as "wekbit"!
  @include prefix(transform, rotate(15deg), wekbit ms)

CSS Output

.tilt {
  -wekbit-transform: rotate(15deg);
  -ms-transform: rotate(15deg);
  transform: rotate(15deg);
}

The exact format of the warning and stack trace varies from implementation to implementation. This is what it looks like in Dart Sass:

Warning: Unknown prefix wekbit.
    example.scss 6:7   prefix()
    example.scss 16:3  root stylesheet