@for loop (through vs to) in SASS - When to Use Which

The Big Idea

Discover how a tiny change in your loop can save hours of repetitive work!

The Scenario

Imagine you want to create 10 colored boxes on a webpage. You write the CSS for each box one by one, changing colors and sizes manually.

The Problem

If you decide to add more boxes or change the number, you must rewrite or copy-paste many lines. This wastes time and can cause mistakes like missing a box or repeating styles.

The Solution

The @for loop in Sass lets you write one block of code that repeats automatically. Using through or to, you control if the loop includes the last number or stops before it, making your code cleaner and easier to update.

Before vs After

✗ Before

.box1 { color: red; } .box2 { color: orange; } .box3 { color: yellow; }

✓ After

@for $i from 1 through 3 { .box#{$i} { color: nth((red, orange, yellow), $i); } }

What It Enables

You can quickly create many similar styles with fewer lines and change the number of items by editing just one place.

Real Life Example

When designing a navigation menu with multiple items, you can style each item with a loop instead of writing separate styles for each link.

Key Takeaways

@for loops automate repetitive CSS tasks.

through includes the last number; to stops before it.

Loops save time and reduce errors when scaling styles.

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