Variable interpolation with #{} in SASS

Introduction

Variable interpolation lets you insert variable values inside strings or selectors in Sass. It helps create dynamic styles easily.

When you want to combine variables with text in CSS property names or values.

When creating dynamic class or ID names using variables.

When you need to build complex selectors that include variable parts.

When you want to generate CSS rules with variable parts inside strings.

When you want to reuse values inside strings without repeating them.

Syntax

SASS

$variable: value;
.some-class-#{$variable} {
  property: value;
}

Use #{} to insert a variable inside strings or selectors.

Interpolation works inside selectors, property names, and property values.

Examples

This creates a class named .button-red and sets its text color to red.

SASS

$color: red;

.button-#{$color} {
  color: $color;
}

This creates a class .container-large with a font size of 2rem.

SASS

$size: large;

.container-#{$size} {
  font-size: 2rem;
}

This sets margin-left: 10px; inside a class named .margin-left.

SASS

$side: left;

.margin-#{$side} {
  margin-#{$side}: 10px;
}

Sample Program

This Sass code creates two classes: .button-dark and .alert-dark. Both use the $theme variable inside their names. The .button-dark has a blue background and white text. The .alert-dark has a border and lighter text color based on the main color.

SASS

@use 'sass:color';

$theme: dark;
$main-color: #3498db;

.button-#{$theme} {
  background-color: $main-color;
  color: white;
  padding: 1rem 2rem;
  border-radius: 0.5rem;
}

.alert-#{$theme} {
  border: 2px solid $main-color;
  color: color.scale($main-color, $lightness: 50%);
  padding: 1rem;
  margin-top: 1rem;
}

Important Notes

Interpolation only works inside strings or selectors, not directly in variable declarations.

You can interpolate any Sass expression inside #{}, not just variables.

Summary

Use #{} to insert variables inside strings or selectors.

It helps create dynamic class names, property names, and values.

Interpolation makes your styles more flexible and reusable.

Practice

1. What does the #{} syntax do in Sass?

easy

A. It defines a mixin.

B. It comments out a line of code.

C. It creates a new variable.

D. It inserts the value of a variable inside a string or selector.

Solution

1. Step 1: Understand the purpose of #{} in Sass

   The #{} syntax is used to insert or interpolate variables inside strings or selectors, making styles dynamic.

2. Step 2: Compare with other options

   Commenting, variable creation, and mixin definition use different syntax, so they are incorrect here.

3. Final Answer:

   It inserts the value of a variable inside a string or selector. -> Option D

4. Quick Check:

   #{variable} inserts variable value [OK]

Hint: Remember #{ } means insert variable value inside text [OK]

Common Mistakes:

• Thinking #{ } creates variables
• Confusing interpolation with comments
• Using #{ } to define mixins

2. Which of the following is the correct way to use variable interpolation in a selector in Sass?

easy

A. .button-#{$size} { color: blue; }

B. .button-$size { color: blue; }

C. .button-{$size} { color: blue; }

D. .button-#size { color: blue; }

Solution

1. Step 1: Identify correct interpolation syntax

   Variable interpolation requires #{} around the variable name to insert its value inside selectors.

2. Step 2: Check each option

   .button-#{$size} { color: blue; } uses .#{$size} correctly. Options A, C, and D miss the # or curly braces or both, so they are invalid.

3. Final Answer:

   .button-#{$size} { color: blue; } -> Option A

4. Quick Check:

   Use .class-#{$var} for interpolation [OK]

Hint: Use #{} around variables inside selectors [OK]

Common Mistakes:

• Omitting # in interpolation
• Omitting curly braces {}
• Using $ without interpolation

3. Given the Sass code:

$color: red;
$size: large;

.button-#{$size} {
  color: #{$color};
}

What CSS will this generate?

medium

A. .button-large { color: #{$color}; }

B. .button-#{$size} { color: #{$color}; }

C. .button-large { color: red; }

D. .button-$size { color: red; }

Solution

1. Step 1: Replace variables inside interpolation

   The variable $size is 'large', so .button-#{$size} becomes .button-large. The variable $color is 'red', so color: #{$color} becomes color: red.

2. Step 2: Write the final CSS output

   After interpolation, the CSS is .button-large { color: red; }.

3. Final Answer:

   .button-large { color: red; } -> Option C

4. Quick Check:

   Interpolation replaces variables with values [OK]

Hint: Replace #{} with variable values to get CSS output [OK]

Common Mistakes:

• Leaving interpolation syntax in output
• Not replacing variables correctly
• Confusing variable names with values

4. Identify the error in this Sass code:

$name: "header";

.#$name {
  font-weight: bold;
}

medium

A. Missing interpolation syntax #{ } around $name in selector

B. Variable $name is not defined

C. Font-weight property is invalid

D. Selector cannot start with a dot

Solution

1. Step 1: Check variable usage in selector

   The selector uses .$name but misses #{} for interpolation, so Sass treats it as a literal string, causing an error.

2. Step 2: Correct usage

   It should be .#{$name} to insert the variable value inside the selector.

3. Final Answer:

   Missing interpolation syntax #{ } around $name in selector -> Option A

4. Quick Check:

   Use .#{$var} to interpolate variables in selectors [OK]

Hint: Always wrap variables with #{ } inside selectors [OK]

Common Mistakes:

• Using $var without #{ } in selectors
• Assuming variables work without interpolation
• Confusing property errors with selector errors

5. You want to create CSS classes for button sizes dynamically using Sass variables:

$sizes: small, medium, large;

@each $size in $sizes {
  .btn-#{$size} {
    font-size: if($size == small, 1rem, if($size == medium, 1.5rem, 2rem));
  }
}

What will be the font-size for .btn-large in the generated CSS?

hard

A. 1rem

B. 2rem

C. 1.5rem

D. if($size == small, 1rem, if($size == medium, 1.5rem, 2rem))

Solution

1. Step 1: Understand the loop and condition

   The loop iterates over sizes: small, medium, large. For each, it creates a class with font-size based on the condition.

2. Step 2: Evaluate font-size for large

   For large, the condition $size == small is false, $size == medium is false, so it uses the else value 2rem.

3. Final Answer:

   2rem -> Option B

4. Quick Check:

   Conditional font-size for large = 2rem [OK]

Hint: Check nested if conditions carefully for each variable value [OK]

Common Mistakes:

• Assuming large equals small or medium
• Not evaluating nested if correctly
• Confusing variable interpolation with condition logic