Default parameter values in SASS - Browser Rendering Trace

Render Flow - Default parameter values

[Read @mixin with parameters] -> [Check if argument passed] -> [If no argument, use default value] -> [Compile mixin body with parameters] -> [Output CSS]

When a mixin is called, the Sass compiler checks if arguments are provided. If not, it uses the default values defined for parameters, then compiles the CSS accordingly.

Render Steps - 3 Steps

Code Added:@mixin box-shadow($shadow: 2px 2px 5px gray) { box-shadow: $shadow; }

Before

[No CSS for box-shadow]

→

After

[Mixin box-shadow defined with default shadow: 2px 2px 5px gray]

The mixin is defined with a default parameter value for the shadow property.

🔧 Browser Action:Sass compiler stores mixin with default parameter

Code Sample

This Sass code defines a mixin with a default parameter value for box-shadow. The .box class uses the default shadow, while .box-custom uses a custom shadow.

SASS

@mixin box-shadow($shadow: 2px 2px 5px gray) {
  box-shadow: $shadow;
}

.box {
  @include box-shadow;
}

.box-custom {
  @include box-shadow(5px 5px 10px black);
}

Render Quiz - 3 Questions

After step 2, what box-shadow does the .box class have?

A 2px 2px 5px gray

B 5px 5px 10px black

C No box-shadow

D Default browser shadow

Common Confusions - 2 Topics

Why does my mixin always use the default value even when I pass an argument?

Make sure you pass the argument correctly when including the mixin. If the argument is missing or misspelled, Sass uses the default value.

💡 Passing an argument overrides the default; missing argument uses default.

Can I have multiple parameters with default values?

Yes, you can define multiple parameters with defaults. If you omit some arguments, Sass uses defaults for those omitted.

💡 Defaults fill in only missing arguments, others use passed values.

Property Reference

Parameter	Default Value	Used When	Effect
$shadow	2px 2px 5px gray	No argument passed	Applies default box-shadow style
$shadow	Custom value	Argument passed	Applies custom box-shadow style

Concept Snapshot

Sass mixins can have default parameter values. If no argument is passed, the default is used. Passing an argument overrides the default. Defaults provide flexibility and reduce repetition. Use @mixin with ($param: default) syntax.

Practice

1. What is the main purpose of default parameter values in Sass mixins or functions?

easy

A. To force users to always pass arguments

B. To make the code run faster

C. To provide fallback values if no argument is given

D. To create new CSS properties automatically

Solution

1. Step 1: Understand default parameters

   Default parameters give a value when no argument is passed to a mixin or function.

2. Step 2: Identify the purpose

   This helps avoid errors and makes code flexible by using fallback values.

3. Final Answer:

   To provide fallback values if no argument is given -> Option C

4. Quick Check:

   Default parameters = fallback values [OK]

Hint: Defaults fill in missing arguments automatically [OK]

Common Mistakes:

• Thinking defaults speed up code
• Believing defaults force argument passing
• Confusing defaults with CSS property creation

2. Which of the following is the correct syntax to set a default parameter value in a Sass mixin?

easy

A. @mixin example($color = blue) { color: $color; }

B. @mixin example($color: blue) { color: $color; }

C. @mixin example($color ? blue) { color: $color; }

D. @mixin example($color <- blue) { color: $color; }

Solution

1. Step 1: Recall Sass default parameter syntax

   In Sass, default values use a colon ':' after the parameter name.

2. Step 2: Match syntax to options

   @mixin example($color: blue) { color: $color; } uses '$color: blue' which is correct syntax for default values.

3. Final Answer:

   @mixin example($color: blue) { color: $color; } -> Option B

4. Quick Check:

   Default value syntax uses ':' not '=' [OK]

Hint: Use colon ':' to assign default values in parameters [OK]

Common Mistakes:

• Using '=' instead of ':' for defaults
• Using '?' or '<-' which are invalid
• Confusing default syntax with other languages

3. Given the Sass mixin:

@mixin box($width: 100px, $height: 50px) { width: $width; height: $height; }

What CSS is generated by:

.small { @include box($height: 30px); }

medium

A. .small { width: 100px; height: 30px; }

B. .small { width: 30px; height: 30px; }

C. .small { width: 100px; height: 50px; }

D. .small { width: 30px; height: 50px; }

Solution

1. Step 1: Identify passed and default parameters

   The mixin call passes only $height: 30px, so $width uses default 100px.

2. Step 2: Write resulting CSS

   Width is 100px (default), height is 30px (passed value).

3. Final Answer:

   .small { width: 100px; height: 30px; } -> Option A

4. Quick Check:

   Passed overrides height, width uses default [OK]

Hint: Passed args override defaults, others stay default [OK]

Common Mistakes:

• Assuming all defaults are overridden
• Mixing up which parameter was passed
• Forgetting default applies if argument missing

4. What is wrong with this Sass mixin?

@mixin card($bg-color blue) { background-color: $bg-color; }

medium

A. Missing colon ':' between parameter and default value

B. Using '=' instead of ':' for default value

C. Parameter name cannot start with '$'

D. Mixin name cannot be 'card'

Solution

1. Step 1: Check parameter default syntax

   The parameter $bg-color is followed by 'blue' without a colon ':' separator.

2. Step 2: Identify correct syntax

   Default values require a colon ':' like '$bg-color: blue'.

3. Final Answer:

   Missing colon ':' between parameter and default value -> Option A

4. Quick Check:

   Default parameter syntax needs ':' [OK]

Hint: Default values need ':' after parameter name [OK]

Common Mistakes:

• Omitting colon ':' between parameter and value
• Using '=' instead of ':'
• Thinking parameter names can't start with '$'

5. You want a Sass function that returns a color, defaulting to red if no argument is given. Which function correctly uses default parameter values and returns the color?

hard

A. @function get-color($color = red) { @return $color; }

B. @function get-color($color: red) { return $color; }

C. @function get-color($color) { @return $color or red; }

D. @function get-color($color: red) { @return $color; }

Solution

1. Step 1: Check default parameter syntax in functions

   Default values use colon ':' like '$color: red'.

2. Step 2: Verify return statement syntax

   Sass uses '@return' to return values, not 'return'.

3. Step 3: Compare options

   @function get-color($color: red) { @return $color; } uses correct default and '@return'. @function get-color($color: red) { return $color; } uses 'return' which is invalid in Sass.

4. Final Answer:

   @function get-color($color: red) { @return $color; } -> Option D

5. Quick Check:

   Default param ':' and '@return' needed [OK]

Hint: Use ':' for defaults and '@return' to output value [OK]

Common Mistakes:

• Using '=' instead of ':' for defaults
• Using 'return' instead of '@return'
• Trying to use 'or' inside return statement