BEM class selectors and the Sass ampersand

The Sass ampersand is incredibly useful in build selectors based on the parent selector, but it has a limitation. Take the following nested BEM-style selectors:

Sass

.component {
  // A BEM modifier.
  &--reversed {
    background: white;
    border-color: lightgray;
 
    // Target a descendent only when the parent has the modifier? Not exactly...
    &__child-element {
      background: rebeccapurple;
    }
  }
}

The linked post explains the problem:

Wait, why is this not working? The problem is that the & has a scope of .component--reversed, so &__child-element compiles to .component--reversed__child-element, which doesn’t [exist] in the markup.

The fix is to save the & to a variable in the parent and use that in the child selector:

Sass

.component {
  // Save the current value of & as $self.
  $self: &;
 
  &--reversed {
    background: white;
    border-color: lightgray;
 
    // Print $self in place of & to get the correct scope of the parent above.
    #{$self}__child-element {
      background: rebeccapurple;
    }
  }
}

The compiled CSS for that element is now .component--reversed .component__child-element[.]

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