Sass 101 - A newb's guide: Mixins w/global default arguments

You can assign to variables if they aren’t already assigned by adding the !default flag to the end of the value. This means that if the variable has already been assigned to, it won’t be re-assigned, but if it doesn’t have a value yet, it will be given one.

$border-radius: 3px !default;
$border-style: 1px solid red !default;

@mixin border($radius: $border-radius, $style: $border-style) {
  -moz-border-radius: $radius;
  -webkit-border-radius: $radius;
  border-radius: $radius;
  border: $style;

.my-new-class {
  @include border;

.my-other-class {
  @include border(3px 3px 0 0, 1px solid orange);

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