sass-site/source/documentation/snippets/_truthiness-and-falsiness.erb

## Truthiness and Falsiness

Anywhere `true` or `false` are allowed, you can use other values as well. The
values `false` and [`null`][] are *falsey*, which means Sass considers them to
indicate falsehood and cause conditions to fail. Every other value is considered
*truthy*, so Sass considers them to work like `true` and cause conditions to
succeed.

[`null`]: /documentation/values/null

For example, if you want to check if a string contains a space, you can just
write `index($string, " ")`. The [`index()` function][] returns `null` if the
string isn't found and a number otherwise.

[`index()` function]: /documentation/functions/string#index

<% heads_up do %>
  Some languages consider more values falsey than just `false` and `null`. Sass
  isn't one of those languages! Empty strings, empty lists, and the number `0`
  are all truthy in Sass.
<% end %>