@for loop (through vs to) in SASS - Performance Comparison

Performance: @for loop (through vs to)

MEDIUM IMPACT

This affects CSS preprocessing time and the size of the generated CSS file, impacting page load speed.

Generating repeated CSS classes with a loop

SASS

@for $i from 1 to 10 {
  .item-#{$i} {
    width: 10px * $i;
  }
}

The 'to' keyword excludes the last number, generating fewer CSS rules and reducing file size.

📈 Performance GainSaves 1 CSS rule, reducing CSS size and improving LCP marginally.

Generating repeated CSS classes with a loop

SASS

@for $i from 1 through 10 {
  .item-#{$i} {
    width: 10px * $i;
  }
}

The 'through' keyword includes the last number, causing one extra iteration which may generate unnecessary CSS rules.

📉 Performance CostAdds 1 extra CSS rule, increasing CSS file size and slightly delaying LCP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
@for ... through (1 through 10)	0 (CSS only)	0	Low	[!] OK
@for ... to (1 to 10)	0 (CSS only)	0	Lower	[OK] Good

Rendering Pipeline

Sass loops run during CSS preprocessing, affecting the size of the CSS file sent to the browser. Larger CSS files increase download and parse time, impacting the Critical Rendering Path.

→Network

→CSS Parsing

→Style Calculation

⚠️ BottleneckNetwork and CSS Parsing due to larger CSS file size

Core Web Vital Affected

LCP

This affects CSS preprocessing time and the size of the generated CSS file, impacting page load speed.

Optimization Tips

1Use '@for ... to' to avoid generating unnecessary CSS rules.

2Smaller CSS files improve Largest Contentful Paint (LCP).

3Avoid extra iterations in loops to keep CSS size minimal.

Performance Quiz - 3 Questions

What is the main difference between '@for ... through' and '@for ... to' in Sass loops?

A Both generate the same number of iterations

B 'to' includes the last number, 'through' excludes it

C 'through' includes the last number, 'to' excludes it

D 'through' is faster to compile than 'to'

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by CSS file, and compare file sizes generated by 'through' vs 'to' loops.

What to look for: Look for smaller CSS file size and faster download time indicating better performance.

Practice

1. What is the main difference between @for $i from 1 through 5 and @for $i from 1 to 5 in Sass?

easy

A. through includes 5, to stops before 5

B. to includes 5, through stops before 5

C. Both include 5

D. Both stop before 5

Solution

1. Step 1: Understand through keyword

   The through keyword means the loop will include the last number in the range.

2. Step 2: Understand to keyword

   The to keyword means the loop stops before the last number, excluding it.

3. Final Answer:

   through includes 5, to stops before 5 -> Option A

4. Quick Check:

   through includes last number, to excludes last number [OK]

Hint: Remember: through = include last, to = exclude last [OK]

Common Mistakes:

• Confusing which keyword includes the last number
• Thinking both keywords behave the same
• Assuming to includes the last number

2. Which of the following is the correct syntax to loop from 1 up to but not including 5 in Sass?

easy

A. @for $i from 1 through 5 { }

B. @for $i in 1 to 5 { }

C. @for $i from 1 to 5 { }

D. @for $i from 1 until 5 { }

Solution

1. Step 1: Identify correct @for syntax

   The correct syntax uses @for $i from start to end or through.

2. Step 2: Choose syntax for excluding last number

   To exclude the last number, use to, so @for $i from 1 to 5 { } is correct.

3. Final Answer:

   @for $i from 1 to 5 { } -> Option C

4. Quick Check:

   Use to to exclude last number [OK]

Hint: Use 'to' to exclude last number in @for loop [OK]

Common Mistakes:

• Using 'through' when wanting to exclude last number
• Using 'in' instead of 'from'
• Using 'until' which is invalid in Sass

3. What CSS classes will be generated by this Sass code?

@for $i from 1 through 3 {
  .item-#{$i} {
    width: 10px * $i;
  }
}

medium

A. .item-1 { width: 10px; } .item-3 { width: 30px; }

B. .item-1 { width: 10px; } .item-2 { width: 20px; }

C. .item-1 { width: 10px; } .item-2 { width: 20px; } .item-3 { width: 10px; }

D. .item-1 { width: 10px; } .item-2 { width: 20px; } .item-3 { width: 30px; }

Solution

1. Step 1: Understand loop range with through

   The loop runs from 1 to 3 including 3, so $i = 1, 2, 3.

2. Step 2: Calculate width for each iteration

   Widths are 10px * 1 = 10px, 10px * 2 = 20px, 10px * 3 = 30px.

3. Final Answer:

   .item-1 { width: 10px; } .item-2 { width: 20px; } .item-3 { width: 30px; } -> Option D

4. Quick Check:

   Loop includes 3, widths multiply by $i [OK]

Hint: Remember 'through' includes last number, so 3 iterations [OK]

Common Mistakes:

• Thinking loop stops before 3
• Multiplying width incorrectly
• Missing last class .item-3

4. Identify the error in this Sass code:

@for $i from 1 to 4 {
  .box-#{$i} {
    height: 5px * $i;
  }
}

@for $j from 1 through 4
  .circle-#{$j} {
    width: 5px * $j;
  }

medium

A. Missing curly braces in second loop

B. Using through instead of to in second loop

C. Multiplication syntax is wrong

D. Variable names must be the same in both loops

Solution

1. Step 1: Check syntax of first loop

   The first loop uses correct syntax with curly braces around the block.

2. Step 2: Check syntax of second loop

   The second loop is missing curly braces after the loop declaration, which is required.

3. Final Answer:

   Missing curly braces in second loop -> Option A

4. Quick Check:

   Loops need curly braces for their blocks [OK]

Hint: Always use curly braces {} after @for loop declaration [OK]

Common Mistakes:

• Omitting curly braces after @for
• Confusing 'to' and 'through' keywords
• Incorrect multiplication syntax

5. You want to create 4 buttons with classes .btn-1 to .btn-4 using a Sass @for loop. Which loop should you use to include the last button class .btn-4?

hard

A. @for $i from 1 to 4 { ... }

B. @for $i from 1 through 4 { ... }

C. @for $i from 1 until 4 { ... }

D. @for $i from 1 to 5 { ... }

Solution

1. Step 1: Understand loop range needed

   You want classes from 1 to 4 inclusive, so the loop must include 4.

2. Step 2: Choose correct keyword to include last number

   Using through includes the last number, so @for $i from 1 through 4 is correct.

3. Final Answer:

   @for $i from 1 through 4 { ... } -> Option B

4. Quick Check:

   Use through to include last number [OK]

Hint: Use 'through' to include last number in loop [OK]

Common Mistakes:

• Using 'to' which excludes last number
• Using invalid keyword 'until'
• Using range that goes beyond needed number