Sass - Importing Files and Partials
So far, we've been exploring the ways that Sass makes it possible to write CSS more efficiently. In this chapter and the next, we'll look at how Sass lets you keep your CSS code DRY (Don't Repeat Yourself).
Anything you can do in CSS you can do in Sass, and that includes using the @import directive. But as you've probably come to expect, Sass extends @import in some useful ways. To start with, it supports inclusion of .sass and .scss files. The basic syntax is the same:
@import <file name>
@import "colors" will include colors.scss or colors.sass in the CSS ouput, if exists in the current directory. Notice that this is slightly different from the CSS @import directive: The file is included in the CSS; no extra http call will be required at runtime. This avoids one of the big performance problems with the standard CSS @import.
You can override the default behavior in several ways. Each of the following, for example, will result in a standard CSS @import directive being output in the CSS:
@import "colors.css"; //the .css extension is specified @import http://test.com/colors.css; //the http:// prefix is used @import "colors" screen; //the import statement includes a media query @import url(colors); //the url() function is used
The resulting CSS would include the following statements:
@import "colors.css"; @import http://test.com/colors.css; @import "colors" screen: @import url(colors);
You'll normally use the @import directive at the top of a file where its contents will have global scope. But with a few exceptions, you can also nest @import statements. You can't use nested imports within mixins (discussed later in next chapter) or inside control statements, but otherwise you can use @import wherever you need it.
One of the best uses of the Sass @import directive is to create separate files containing all the standard definitions for your site-colors, font sizes and families and so forth-and then include them where you need them. This is, of course, a classic example of DRY programming.
Since the sole purpose of these files is to be included in other Sass files, there's no point in transpiling them directly. In fact, many of them won't actually produce any CSS. An example of this is a file that includes only variable definitions: It would result in an empty CSS file.
$red: #ff0000; $green: #00ff00; $blue: #0000ff;
If the Sass transpiler is watching a directory (either through the command window or via an editor extension), you'll want to exclude changes to these files from transpilation, and Sass makes it easy to do so. Just prepend an underscore to their names. Instead of colors.scss, for example, name the file _colors.scss. A file that The transpiler will ignore it, but you can still include it in other Sass files.
Files like these are known as "partials", and you can include them in another .scss file in the normal way. By convention, the leading underscore is omitted, but you can include it if you wish. As we'll see:
@import "colors"; //by convention, omit the underscore @import "_colors"; //but this works, too
Structuring your CSS in this way doesn't have any performance penalties- you'll get clean, stand-alone CSS at the end of the transpilation process and it makes your code less repetitive and easier to maintain. In fact, if you examine CSS frameworks like Bootstrap 4 or MaterializeCSS you'll often find a single SCSS file that contains nothing but @import statements. All the other files are partials.
Learn about the mixin in the next chapter.