Understanding the !default flag in Sass

The !default flag in Sass is a quintessential feature that simplifies the process of managing stylesheet variables.

As the correct response to the quiz question suggests, the !default flag in Sass is used to assign a default value to a variable only if it's not already defined. What this means is that if a variable has been given a value previously, the !default flag will refrain from reassigning the value. The !default flag is only used when the variable hasn't been assigned a value or is null.

For example, consider the following code:

$font-size: 16px;
$font-size: 20px !default;
body {
  font-size: $font-size;
}

In the code snippet above, even though we have assigned a default flag to the $font-size variable with a value of 20px, the output of the code won't be 20px. Instead, it takes the previously assigned value of 16px. So, the resulting CSS will look something like this:

body {
  font-size: 16px;
}

However, if the variable $font-size was not defined before or was null, the !default flag would assign it a value of 20px.

Now, where does this come in handy? The !default flag is particularly useful when working with Sass libraries (like Bootstrap or Foundation), where you might want to override default variables. By assigning a !default flag to a variable, you're effectively saying - "If the user hasn't specified a value for this variable, use this default value." This permits you to customize your styles without modifying the source files of your library.

To summarize, the !default flag in Sass provides a cornerstone feature for managing stylesheets and variables, especially when integrating third-party libraries in your project. It's a best practice to utilize this flag for supplying default functionality, providing a more convenient and robust approach to the customizability in styling.