Grid column generator with loops in SASS - Performance & Optimization

Performance: Grid column generator with loops

MEDIUM IMPACT

This affects the CSS generation size and browser rendering performance by controlling how many grid column classes are created and used.

Creating reusable grid column classes for layout

SASS

@for $i from 1 through 12 {
  .col-#{$i} {
    grid-column: span #{$i};
  }
}

Limits generated classes to common grid columns, reducing CSS size and improving load speed.

📈 Performance GainSaves ~4-8kb CSS, reduces LCP by faster CSS parsing

Creating reusable grid column classes for layout

SASS

@for $i from 1 through 100 {
  .col-#{$i} {
    grid-column: span #{$i};
  }
}

Generates 100 CSS classes increasing CSS file size and parsing time, even if many are unused.

📉 Performance CostAdds ~5-10kb to CSS bundle, increases LCP due to larger CSS parsing

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Generating 100 grid column classes	No extra DOM nodes	No extra reflows	Higher paint cost due to larger CSS	[X] Bad
Generating 12 grid column classes	No extra DOM nodes	No extra reflows	Lower paint cost with smaller CSS	[OK] Good

Rendering Pipeline

The generated CSS classes are parsed during Style Calculation. More classes increase CSS parsing time and memory. During Layout, grid columns affect element sizing and positioning. Excessive classes increase CSS size but do not increase DOM nodes.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckStyle Calculation due to large CSS size

Core Web Vital Affected

LCP

This affects the CSS generation size and browser rendering performance by controlling how many grid column classes are created and used.

Optimization Tips

1Limit loop ranges to only necessary grid columns to reduce CSS size.

2Avoid generating unused CSS classes to improve load speed.

3Smaller CSS files improve Largest Contentful Paint (LCP) by faster style calculation.

Performance Quiz - 3 Questions

What is the main performance cost of generating many grid column classes with a loop in Sass?

A More JavaScript execution time

B More DOM nodes created in the browser

C Increased CSS file size causing slower CSS parsing

D More HTTP requests for each class

DevTools: Performance

How to check: Record page load and look at the 'Style Recalculation' and 'CSS Parsing' times in the flame chart.

What to look for: Long CSS parsing or style recalculation times indicate large CSS impacting LCP.

Practice

1. What does the @for loop in SASS help you do when creating grid columns?

easy

A. Add images to grid columns

B. Create JavaScript functions for grid behavior

C. Write HTML grid elements faster

D. Generate multiple CSS classes for different column widths automatically

Solution

1. Step 1: Understand the purpose of @for in SASS

   The @for loop repeats code blocks a set number of times, useful for generating CSS classes.

2. Step 2: Connect @for with grid columns

   Using @for, you can create many classes like .col-1, .col-2, etc., each with different widths.

3. Final Answer:

   Generate multiple CSS classes for different column widths automatically -> Option D

4. Quick Check:

   @for loop = Generate CSS classes [OK]

Hint: Think of @for as a shortcut to write many classes fast [OK]

Common Mistakes:

• Confusing SASS loops with JavaScript loops
• Thinking @for writes HTML elements
• Assuming it adds images or content

2. Which of the following is the correct SASS syntax to create a loop from 1 to 4 for grid columns?

easy

A. @each $i from 1 through 4 { .col-#{$i} { width: 25% * $i; } }

B. @for $i in 1 to 4 { .col-#{$i} { width: 25% * $i; } }

C. @for $i from 1 through 4 { .col-#{$i} { width: 25% * $i; } }

D. @loop $i from 1 to 4 { .col-#{$i} { width: 25% * $i; } }

Solution

1. Step 1: Recall correct @for syntax in SASS

   The correct syntax uses @for $var from start through end to include the end number.

2. Step 2: Check each option

   @for $i from 1 through 4 { .col-#{$i} { width: 25% * $i; } } uses @for $i from 1 through 4, which is correct. Options A, B, and C use invalid keywords or syntax.

3. Final Answer:

   @for $i from 1 through 4 { .col-#{$i} { width: 25% * $i; } } -> Option C

4. Quick Check:

   Correct @for syntax = @for $i from 1 through 4 { .col-#{$i} { width: 25% * $i; } } [OK]

Hint: Remember: use 'from' and 'through' for inclusive loops in SASS [OK]

Common Mistakes:

• Using 'in' instead of 'from' and 'through'
• Using '@loop' which is not valid SASS
• Confusing '@each' with '@for'

3. Given this SASS code:

@for $i from 1 through 3 {
  .col-#{$i} {
    grid-column: span $i;
  }
}

What CSS will be generated?

medium

A. .col-1 { grid-column: span 3; } .col-2 { grid-column: span 2; } .col-3 { grid-column: span 1; }

B. .col-1 { grid-column: span 1; } .col-2 { grid-column: span 2; } .col-3 { grid-column: span 3; }

C. .col-1 { grid-column: 1; } .col-2 { grid-column: 2; } .col-3 { grid-column: 3; }

D. Syntax error, no CSS generated

Solution

1. Step 1: Understand the loop iterations

   The loop runs for $i = 1, 2, 3, creating classes .col-1, .col-2, .col-3.

2. Step 2: Check the property values

   Each class sets grid-column: span $i;, so .col-1 spans 1 column, .col-2 spans 2, and .col-3 spans 3.

3. Final Answer:

   .col-1 { grid-column: span 1; } .col-2 { grid-column: span 2; } .col-3 { grid-column: span 3; } -> Option B

4. Quick Check:

   Loop variable $i matches span value [OK]

Hint: Match loop variable to property value inside the loop [OK]

Common Mistakes:

• Confusing 'span $i' with just '$i'
• Reversing the span values
• Assuming syntax error due to interpolation

4. Identify the issue in this SASS code for generating grid columns from 1 to 4:

@for $i from 1 to 4 {
  .col-#{$i} {
    width: 100% / $i;
  }
}

medium

A. Using 'to' instead of 'through' in the loop range

B. Missing interpolation in class name

C. Division inside width property is invalid

D. Loop variable $i is not declared

Solution

1. Step 1: Check the loop syntax

   The loop uses @for $i from 1 to 4, which excludes the end number 4. Use through to include it.

2. Step 2: Verify other parts

   Class name uses interpolation correctly. Division in width is valid in SASS. Loop variable is declared.

3. Final Answer:

   Using 'to' instead of 'through' in the loop range -> Option A

4. Quick Check:

   Loop range must use 'through' for inclusive end [OK]

Hint: Use 'through' to include last number in @for loops [OK]

Common Mistakes:

• Using 'to' which excludes the last number
• Forgetting interpolation in class names
• Thinking division in width is invalid

5. You want to create grid column classes from 1 to 6, but only for even numbers, using SASS loops. Which code correctly generates classes .col-2, .col-4, and .col-6 with widths as fractions of 6 columns?

hard

A. @for $i from 2 through 6 {
@if $i % 2 == 0 {
.col-#{$i} {
width: (100% / 6) * $i;
}
}
}

B. @for $i from 1 through 6 {
@if $i / 2 == 0 {
.col-#{$i} {
width: (100% / 6) * $i;
}
}
}

C. @each $i from (2, 4, 6) {
.col-#{$i} {
width: (100% / 6) * $i;
}
}

D. @for $i from 2 to 6 step 2 {
.col-#{$i} {
width: (100% / 6) * $i;
}
}

Solution

1. Step 1: Understand how to loop only even numbers

   SASS @for loops do not support 'step' directly, so use @if $i % 2 == 0 to filter even numbers.

2. Step 2: Check each option

   @for $i from 2 through 6 { @if $i % 2 == 0 { .col-#{$i} { width: (100% / 6) * $i; } } } uses @if $i % 2 == 0 correctly. @for $i from 1 through 6 { @if $i / 2 == 0 { .col-#{$i} { width: (100% / 6) * $i; } } } uses division instead of modulo, which is wrong. @each $i from (2, 4, 6) { .col-#{$i} { width: (100% / 6) * $i; } } uses invalid syntax ('from' instead of 'in' for @each). @for $i from 2 to 6 step 2 { .col-#{$i} { width: (100% / 6) * $i; } } uses invalid 'step' syntax.

3. Final Answer:

   @for $i from 2 through 6 { @if $i % 2 == 0 { .col-#{$i} { width: (100% / 6) * $i; } } } -> Option A

4. Quick Check:

   Use modulo (%) to filter even numbers in loops [OK]

Hint: Use modulo (%) inside @if to pick even numbers in @for loops [OK]

Common Mistakes:

• Using division (/) instead of modulo (%) for condition
• Trying to use 'step' in @for which is invalid
• Confusing @each with @for loops