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Learn how to use mixins and & selector to create an interactive index card that flips when you hover.

Mixins and the & Selector

Mixins also have the ability to take in a value. 

An _argument_, or parameter, is a value passed to the mixin that will be used inside the mixin, such as `$visibility` in this example:

```scss
@mixin backface-visibility($visibility) {
  backface-visibility: $visibility;
  -webkit-backface-visibility: $visibility;
  -moz-backface-visibility: $visibility;
  -ms-backface-visibility: $visibility;
  -o-backface-visibility: $visibility;
}
```

In fact, you should only ever use a mixin if it takes an argument. We will learn more about this in a later exercise.

The syntax to pass in a value is as follows:

```scss
@include backface-visibility(hidden);
```

In the code above, `hidden` is passed into the `backface-visibility` mixin, where it will be assigned as the value of its argument, `$visibility`.

Mixins: Arguments

In addition to variables and nesting, Sass has multiple constructs that reduce repetition. 

In Sass, a _mixin_ lets you make groups of CSS declarations that you want to reuse throughout your site.

The notation for creating a mixin is as follows:

```scss
@mixin backface-visibility {
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  -moz-backface-visibility: hidden;
  -ms-backface-visibility: hidden;
  -o-backface-visibility: hidden;
}
```

**Note:** Mixin names and all 
other Sass identifiers use hyphens 
and underscores interchangeably. The following code:

```scss
.notecard {
  .front, .back {
    width: 100%;
    height: 100%;
    position: absolute;
    
    @include backface-visibility;
  }
}  
```

is equivalent to the following CSS:

```css
.notecard .front, .notecard .back {
  width: 100%;
  height: 100%;
  position: absolute;
  
   backface-visibility: hidden;
  -webkit-backface-visibility: hidden; 
  -moz-backface-visibility: hidden;
  -ms-backface-visibility: hidden;
  -o-backface-visibility: hidden;
}
```

What is a Mixin?

Mixin arguments can be assigned a _default value_ in the mixin definition by using a special notation.

A default value is assigned to the argument if no value is passed in when the mixin is included. Defining a default value for each argument is optional. 

The notation is as follows:

```scss
@mixin backface-visibility($visibility: hidden) {
   backface-visibility: $visibility;
  -webkit-backface-visibility: $visibility;
  -moz-backface-visibility: $visibility;
  -ms-backface-visibility: $visibility;
  -o-backface-visibility: $visibility;
}
```

In the example above, if no value is passed in when `backface-visibility` is included, `hidden` would be assigned to all properties.


Default Value Arguments

This lesson has introduced you to some of the most powerful concepts Sass has to offer! 

* *Mixins* are a powerful tool that allow you to keep your code DRY. Their ability to take in arguments, assign default values to those arguments, and accept said arguments in whatever format is most readable and convenient for you makes the mixin Sass's most popular directive.

* The `&` selector is a Sass construct that allows for expressive flexibility by referencing the parent selector when working with CSS pseudo elements and classes.

* *String interpolation* is the glue that allows you to insert a string in the middle of another when it is in a variable format. Its applications vary, but the ability to interpolate is especially useful for passing in file names. 

Let's keep up the awesome job in the next lesson, where we will learn about functions, arithmetic, and color operations in Sass!
 

Review

In general, here are 5 important facts about arguments and mixins:
1. Mixins can take multiple arguments.
2. Sass allows you to explicitly define each argument in your `@include` statement.
3. When values are explicitly specified you can send them out of order. 
4. If a mixin definition has a combination of arguments with and without a default value, you should define the ones with no default value first.
5. Mixins can be nested.

Here are some concrete examples of the rules:
```scss
@mixin dashed-border($width, $color: #FFF) {
  border: {
     color: $color;
     width: $width;
     style: dashed;
  }
}

span { //only passes non-default argument
    @include dashed-border(3px);
}

p { //passes both arguments
    @include dashed-border(3px, green);
}

div { //passes out of order but explicitly defined
   @include dashed-border(color: purple, width: 5px); 
}
```

In the example above, the color of the border of `span` elements would be white, the border of `paragraph` elements would be green, while the `div` elements would have a thicker purple border.



Mixin Facts

In Sass, **string interpolation** is the process of placing a variable string in the middle of two other strings. 

In a mixin context, interpolation is handy when you want to make use of variables in selectors or file names. The notation is as follows:

```scss
@mixin photo-content($file) {
  content: url(#{$file}.jpg); //string interpolation
  object-fit: cover;
}

//....

.photo { 
  @include photo-content('titanosaur');
  width: 60%;
  margin: 0px auto; 
}
```

String interpolation would enable the following CSS:

```css
.photo { 
  content: url(titanosaur.jpg);
  width: 60%;
  margin: 0px auto; 
}
```

String Interpolation

Great job! You've accomplished some fancy styling with mixins. Now it's time to tie in how the `&` selector plays into mixins.

Sass allows `&` selector usage inside of mixins. The flow works much like you think it would: 

1. The `&` selector gets assigned the value of the parent at the point where the mixin is included. 

2. If there is no parent selector, then the value is null and Sass will throw an error.
 
```scss
@mixin text-hover($color){
  &:hover {
      color: $color; 
  }
}
```

In the above, the value of the parent selector will be assigned based on the parent at the point where it is invoked.  
 
 ```scss
  .word { //SCSS: 
    display: block; 
    text-align: center;
    position: relative;
    top: 40%;
    @include text-hover(red);
  }
  ```
  The above will compile to the following CSS:
  
  ```css
  .word{ 
    display: block;
    text-align: center;
    position: relative;
    top: 40%;
  }
  .word:hover{
    color: red;
  }
  
```
Notice that the mixin inherited the parent selector `.word` because that was the parent at the point where the mixin was included. 



The & Selector in Mixins

Sass allows you to pass in multiple arguments in a list or a map format.

For example, take the multiple properties needed to create the college-ruled stripes in the back of our notecard. 

```scss
@mixin stripes($direction, $width-percent, $stripe-color, $stripe-background: #FFF) {
  background: repeating-linear-gradient(
    $direction,
    $stripe-background,
    $stripe-background ($width-percent - 1),
    $stripe-color 1%,
    $stripe-background $width-percent
  );
}
```
In this scenario, it makes sense to create a map with these properties in case we ever want to update or reference them.

```scss
$college-ruled-style: ( 
  direction: to bottom,
  width-percent: 15%,
  stripe-color: blue,
  stripe-background: white
);
```

When we include our mixin, we can then pass in these arguments in a map with the following `...` notation:

```scss
.definition {
  width: 100%;
  height: 100%;
  @include stripes($college-ruled-style...);
 }
 ```

**Note:** Remember to always prioritize readability over writing less code. This approach is only useful if you find it adds clarity to your codebase.

List Arguments

In the next set of exercises, you'll use new Sass concepts to fix and add styles to the notecard on the right so that it flips when you hover over it!
 
Recall that, in CSS, a pseudo-element is used to style parts of an element, for example:

- Styling the content `::before` or `::after` the content of an element.

- Using a pseudo class such as `:hover` to set the properties of an element when the user's mouse is touching the area of the element. 

In Sass, the parent selector, `&`, is used to specify exactly where a parent selector should be inserted. It also helps write pseudo-classes in a much less repetitive way.

For example, the following Sass:


```scss
.notecard{ 
  &:hover{
      @include transform (rotatey(-180deg));  
    }
  }
``` 

will compile to the following CSS:

```css
.notecard:hover {
  transform: rotatey(-180deg);
}
```