CSS Triangle Mixin

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There is a fairly popular CSS hack using transparent borders on a 0-width / 0-height element to mimic triangles. There is a CSS snippet here on CSS-Tricks that depicts it.

If, like me, you never quite remember how it works, be sure we can use Sass to help us.

/// Triangle helper mixin
/// @param {Direction} $direction - Triangle direction, either `top`, `right`, `bottom` or `left`
/// @param {Color} $color [currentcolor] - Triangle color 
/// @param {Length} $size [1em] - Triangle size
@mixin triangle($direction, $color: currentcolor, $size: 1em) {
  @if not index(top right bottom left, $direction) {
    @error "Direction must be either `top`, `right`, `bottom` or `left`.";
  }

  width: 0;
  height: 0;
  content: '';
  z-index: 2;
  border-#{opposite-position($direction)}: ($size * 1.5) solid $color;
  
  $perpendicular-borders: $size solid transparent;
  
  @if $direction == top or $direction == bottom {
    border-left:   $perpendicular-borders;
    border-right:  $perpendicular-borders;
  } @else if $direction == right or $direction == left {
    border-bottom: $perpendicular-borders;
    border-top:    $perpendicular-borders;
  }
}

Additional notes:

* The opposite-position function comes from Compass; if you do not use Compass, you might need to have your own;
* The mixin does not deal with positioning the triangle; it is perfectly fine to combine it with a positioning mixin though;
* The content directive is meant to allow it to be used on pseudo-elements, which actually ends up being most cases.

#Usage

.foo::before {
  @include triangle(bottom);
  position: absolute;
  left: 50%;
  bottom: 100%;
}
.foo::before {
  width: 0;
  height: 0;
  content: '';
  z-index: 2;
  border-top: 1.5em solid currentColor;
  border-left: 1em solid transparent;
  border-right: 1em solid transparent;
  position: absolute;
  left: 50%;
  bottom: 100%;
}
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