Recursive mixins in SASS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a recursive mixin that decreases the value of $n by 1 each call.

SASS

@mixin countdown($n) {
  @if $n > 0 {
    @debug $n;
    @include countdown([1]);
  }
}

Drag options to blanks, or click blank then click option'

A$n - 1

B$n + 1

C$n / 1

D$n * 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of subtraction causes infinite recursion.

Multiplying or dividing by 1 does not change the value.

The mixin calls itself with $n decreased by 1 to count down recursively.

2fill in blank

medium

Complete the code to stop recursion when $n reaches zero.

SASS

@mixin countdown($n) {
  @if [1] {
    @debug $n;
    @include countdown($n - 1);
  }
}

Drag options to blanks, or click blank then click option'

A$n >= 0

B$n < 0

C$n > 0

D$n == 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using $n == 0 causes no recursion at all.

Using $n < 0 causes infinite recursion.

The recursion continues only while $n is greater than zero.

3fill in blank

hard

Fix the error in the recursive mixin call to correctly pass the decremented value.

SASS

@mixin countdown($n) {
  @if $n > 0 {
    @debug $n;
    @include countdown([1]);
  }
}

Drag options to blanks, or click blank then click option'

A$n - 1

B$n + 1

C$n

Dn - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the $ sign causes an undefined variable error.

Passing $n without decrement causes infinite recursion.

The mixin must call itself with $n decreased by 1 to progress the recursion.

4fill in blank

hard

Fill both blanks to create a recursive mixin that prints numbers from $start down to 1.

SASS

@mixin print-down($start) {
  @if [1] {
    @debug $start;
    @include print-down([2]);
  }
}

Drag options to blanks, or click blank then click option'

A$start > 0

B$start - 1

C$start >= 1

D$start + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using $start + 1 causes infinite recursion.

Using $start >= 1 is correct but less common than > 0.

The mixin continues while $start is greater than zero and calls itself with $start minus 1.

5fill in blank

hard

Fill all three blanks to create a recursive mixin that builds nested selectors up to $level.

SASS

@mixin nested($level, $max) {
  @if [1] {
    .level-#[2] {
      @include nested([3], $max);
    }
  }
}

Drag options to blanks, or click blank then click option'

A$level <= $max

B$level + 1

C$level

D$level < $max

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= instead of < changes the number of nested levels.

Using $level - 1 causes infinite recursion or wrong nesting.

The mixin recurses while $level is less than $max, uses $level in the selector, and calls itself with $level + 1.

Practice

1. What is the main purpose of a recursive mixin in sass?

easy

A. To call itself repeatedly to apply styles multiple times

B. To import external CSS files

C. To define variables for colors

D. To create animations with keyframes

Solution

1. Step 1: Understand what recursion means in programming

   Recursion means a function or mixin calls itself to repeat an action.

2. Step 2: Apply this to sass mixins

   A recursive mixin calls itself to repeat styles multiple times, often with changes each time.

3. Final Answer:

   To call itself repeatedly to apply styles multiple times -> Option A

4. Quick Check:

   Recursive mixin = repeated self-call [OK]

Hint: Recursive mixins repeat styles by calling themselves [OK]

Common Mistakes:

• Confusing recursion with importing files
• Thinking mixins only define variables
• Mixing up animations with recursion

2. Which of the following is the correct syntax to define a recursive mixin in sass?

easy

A. @mixin repeat($n) { color: red; @include repeat($n + 1); }

B. @mixin repeat($n) { @if $n > 0 { color: red; @include repeat($n - 1); } }

C. @mixin repeat { @include repeat; }

D. @mixin repeat($n) { @if $n < 0 { @include repeat($n - 1); } }

Solution

1. Step 1: Check for stop condition in mixin

   @mixin repeat($n) { @if $n > 0 { color: red; @include repeat($n - 1); } } uses @if $n > 0 to stop recursion when $n reaches 0.

2. Step 2: Verify recursive call decreases $n

   @mixin repeat($n) { @if $n > 0 { color: red; @include repeat($n - 1); } } calls itself with $n - 1, moving towards stop condition.

3. Final Answer:

   @mixin repeat($n) { @if $n > 0 { color: red; @include repeat($n - 1); } } -> Option B

4. Quick Check:

   Stop condition + decrement = correct recursion [OK]

Hint: Look for stop condition and decrement in recursive mixin [OK]

Common Mistakes:

• Missing stop condition causing infinite loop
• Incrementing instead of decrementing parameter
• Calling mixin without parameters

3. Given the following recursive mixin, what will be the color of the text inside .box after compilation?

@mixin colorLayers($n) {
  @if $n > 0 {
    color: lighten(blue, $n * 10%);
    @include colorLayers($n - 1);
  }
}

.box {
  @include colorLayers(2);
}

medium

A. The text color will be pure blue

B. There will be a syntax error and no color applied

C. The text color will be dark blue

D. The text color will be a light blue shade (lightened twice)

Solution

1. Step 1: Understand the recursion steps

   The mixin calls itself twice: first with $n=2, then $n=1, then stops at 0.

2. Step 2: Analyze color changes

   Each call applies color: lighten(blue, $n * 10%). So first lighten by 20%, then by 10%.

3. Final Answer:

   The text color will be a light blue shade (lightened twice) -> Option D

4. Quick Check:

   Recursive lighten steps = light blue [OK]

Hint: Trace recursive calls and their style effects stepwise [OK]

Common Mistakes:

• Assuming only one color applied
• Ignoring recursive layering of styles
• Thinking recursion causes error here

4. Identify the error in this recursive mixin and choose the fix:

@mixin borderLayers($count) {
  border: 1px solid black;
  @include borderLayers($count - 1);
}

medium

A. Missing stop condition; add @if $count > 0 before recursive call

B. Wrong parameter name; change $count to $n

C. Use @extend instead of @include

D. No error; code is correct

Solution

1. Step 1: Check for stop condition

   The mixin calls itself without any condition, causing infinite recursion.

2. Step 2: Fix by adding stop condition

   Adding @if $count > 0 before recursive call stops recursion when count reaches 0.

3. Final Answer:

   Missing stop condition; add @if $count > 0 before recursive call -> Option A

4. Quick Check:

   Stop condition prevents infinite recursion [OK]

Hint: Always add stop condition to recursive mixins [OK]

Common Mistakes:

• Forgetting stop condition
• Confusing @include with @extend
• Changing parameter names unnecessarily

5. You want to create a recursive mixin that adds nested box shadows with increasing blur. Which of these mixins correctly applies 3 layers of shadows with blur increasing by 2px each time?

hard

A. @mixin shadowLayers($n, $blur: 2) { @if $n < 0 { box-shadow: 0 0 #{$blur}px rgba(0,0,0,0.3); @include shadowLayers($n + 1, $blur + 2); } }

B. @mixin shadowLayers($n) { box-shadow: 0 0 2px rgba(0,0,0,0.3); @include shadowLayers($n - 1); }

C. @mixin shadowLayers($n, $blur: 2) { @if $n > 0 { box-shadow: 0 0 #{$blur}px rgba(0,0,0,0.3); @include shadowLayers($n - 1, $blur + 2); } }

D. @mixin shadowLayers($n, $blur: 2) { box-shadow: 0 0 #{$blur}px rgba(0,0,0,0.3); }

Solution

1. Step 1: Check for stop condition and parameter updates

   @mixin shadowLayers($n, $blur: 2) { @if $n > 0 { box-shadow: 0 0 #{$blur}px rgba(0,0,0,0.3); @include shadowLayers($n - 1, $blur + 2); } } has @if $n > 0 to stop recursion and increments blur by 2 each call.

2. Step 2: Verify recursive call and shadow layering

   @mixin shadowLayers($n, $blur: 2) { @if $n > 0 { box-shadow: 0 0 #{$blur}px rgba(0,0,0,0.3); @include shadowLayers($n - 1, $blur + 2); } } calls itself with $n - 1 and increasing blur, layering shadows correctly 3 times.

3. Final Answer:

   @mixin shadowLayers($n, $blur: 2) { @if $n > 0 { box-shadow: 0 0 #{$blur}px rgba(0,0,0,0.3); @include shadowLayers($n - 1, $blur + 2); } } -> Option C

4. Quick Check:

   Stop condition + parameter increment = correct recursive layering [OK]

Hint: Use stop condition and increment parameters in recursion [OK]

Common Mistakes:

• No stop condition causing infinite recursion
• Wrong comparison operator in stop condition
• Not incrementing blur value each recursion