When to use SASS vs CSS-in-JS - Performance Comparison

Performance: When to use SASS vs CSS-in-JS

MEDIUM IMPACT

This affects page load speed and runtime rendering performance by influencing CSS bundle size and style recalculations.

Styling a large static website with reusable styles

SASS

$primary-color: blue;
.button {
  background-color: $primary-color;
  padding: 1rem;
}

SASS compiles to static CSS, reducing runtime cost and enabling browser caching.

📈 Performance GainFaster LCP by avoiding runtime style injection and smaller JS bundle

Styling a large static website with reusable styles

SASS

@jsx import React from 'react';
const Button = () => {
  return <button style={{ backgroundColor: 'blue', padding: '10px' }}>Click</button>;
};

Inline styles or CSS-in-JS for static styles cause runtime style recalculations and larger JS bundles.

📉 Performance CostBlocks rendering for extra 50-100ms due to JS parsing and style recalculation

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
SASS static CSS	Minimal - styles precompiled	0 on load	Low - browser optimized	[OK] Good
CSS-in-JS for static styles	Extra JS nodes for styles	Multiple on load	Medium - runtime style injection	[X] Bad
CSS-in-JS for dynamic styles	Scoped style nodes	Few on interaction	Low to medium - conditional styles	[!] OK

Rendering Pipeline

SASS compiles styles ahead of time, so the browser only processes CSS during Style Calculation and Paint. CSS-in-JS injects styles at runtime, adding JavaScript execution and possible style recalculation during interaction.

→JavaScript Execution

→Style Calculation

→Layout

→Paint

⚠️ BottleneckJavaScript Execution and Style Calculation for CSS-in-JS

Core Web Vital Affected

LCP

This affects page load speed and runtime rendering performance by influencing CSS bundle size and style recalculations.

Optimization Tips

1Use SASS for static styles to reduce runtime cost and improve load speed.

2Use CSS-in-JS only for dynamic or conditional styling to minimize style recalculations.

3Avoid mixing heavy CSS-in-JS with static styles to keep bundle size and reflows low.

Performance Quiz - 3 Questions

Which styling approach generally improves Largest Contentful Paint (LCP) for static websites?

A Inline styles in HTML

B Precompiled SASS CSS

C CSS-in-JS with runtime style injection

D Using many CSS classes toggled by JavaScript

DevTools: Performance

How to check: Record page load and interactions; look for long scripting times and style recalculations.

What to look for: High JS execution time and multiple style recalculations indicate heavy CSS-in-JS usage.

Practice

1. Which situation is best suited for using SASS instead of CSS-in-JS?

easy

A. When you want to avoid using any CSS preprocessors or JavaScript.

B. When you want styles tightly coupled with JavaScript components for dynamic styling.

C. When you want to write inline styles directly inside HTML tags.

D. When you want to write styles in separate files with powerful CSS features like variables and nesting.

Solution

1. Step 1: Understand SASS purpose

   SASS is a CSS preprocessor that adds features like variables, nesting, and mixins to CSS, usually in separate style files.

2. Step 2: Compare with CSS-in-JS use case

   CSS-in-JS is best when styles need to be tightly integrated with JavaScript components, often for dynamic styling.

3. Final Answer:

   When you want to write styles in separate files with powerful CSS features like variables and nesting. -> Option D

4. Quick Check:

   SASS for separate powerful CSS files = C [OK]

Hint: SASS = separate style files; CSS-in-JS = styles inside JS [OK]

Common Mistakes:

• Confusing CSS-in-JS as better for all styling needs
• Thinking SASS is only for inline styles
• Assuming CSS-in-JS cannot use variables

2. Which of the following is the correct way to import a SASS file into another SASS file?

easy

A. @import 'styles.css';

B. @import 'variables.scss';

C. import 'variables.scss';

D. require('variables.scss');

Solution

1. Step 1: Recall SASS import syntax

   SASS uses @import 'filename.scss'; to include other SASS files.

2. Step 2: Check options for correct syntax

   @import 'variables.scss'; uses @import 'variables.scss'; which is correct. @import 'styles.css'; imports a CSS file, which is allowed but not typical for SASS partials. Options C and D use JavaScript syntax, which is incorrect in SASS.

3. Final Answer:

   @import 'variables.scss'; -> Option B

4. Quick Check:

   SASS import uses @import 'file.scss' = A [OK]

Hint: SASS imports use @import with quotes and .scss extension [OK]

Common Mistakes:

• Using JavaScript import syntax in SASS
• Importing CSS files instead of SASS partials
• Omitting quotes around file names

3. Given this SASS code:

$primary-color: blue;
.button {
  color: $primary-color;
  &:hover {
    color: darken($primary-color, 20%);
  }
}

What will be the color of the button text on hover in the compiled CSS?

medium

A. a darker shade of blue

B. blue

C. light blue

D. red

Solution

1. Step 1: Understand variable usage

   The variable $primary-color is set to blue and used as the button text color.

2. Step 2: Analyze hover color function

   The darken($primary-color, 20%) function makes the blue color 20% darker on hover.

3. Final Answer:

   a darker shade of blue -> Option A

4. Quick Check:

   darken(blue, 20%) = darker blue [OK]

Hint: darken() makes colors darker by given percent [OK]

Common Mistakes:

• Thinking hover color stays the same
• Confusing darken() with lighten()
• Assuming color changes to red

4. You wrote this CSS-in-JS code inside a React component:

const styles = {
  button: {
    color: 'blue',
    '&:hover': {
      color: 'darkblue'
    }
  }
};

But the hover style is not working. What is the likely problem?

medium

A. CSS-in-JS does not support pseudo-classes like :hover.

B. The color values must be variables, not strings.

C. The syntax for nested selectors in CSS-in-JS is incorrect; it should use a string key like ':hover' instead of '&:hover'.

D. The styles object must be converted to a CSS file manually.

Solution

1. Step 1: Understand CSS-in-JS pseudo-class syntax

   In many CSS-in-JS libraries, nested selectors use the key ':hover' without the ampersand (&).

2. Step 2: Identify syntax error

   The code uses '&:hover' which is valid in SASS but often incorrect in CSS-in-JS, causing hover styles to fail.

3. Final Answer:

   The syntax for nested selectors in CSS-in-JS is incorrect; it should use a string key like ':hover' instead of '&:hover'. -> Option C

4. Quick Check:

   CSS-in-JS pseudo-classes use ':hover' key, not '&:hover' [OK]

Hint: Use ':hover' key in CSS-in-JS, not '&:hover' [OK]

Common Mistakes:

• Using SASS syntax in CSS-in-JS
• Thinking CSS-in-JS can't do hover
• Forgetting to apply styles via className

5. You have a React project where components need dynamic styles based on props, but also want to share common styles across many components in separate files. Which approach best fits this need?

hard

A. Combine SASS for shared styles in separate files and CSS-in-JS for dynamic styles inside components.

B. Use only CSS-in-JS to keep all styles inside components with no external files.

C. Write all styles inline in HTML style attributes.

D. Use only SASS with separate .scss files and import them everywhere.

Solution

1. Step 1: Analyze project needs

   The project needs shared common styles in separate files and dynamic styles based on props inside components.

2. Step 2: Match approaches to needs

   SASS is great for shared styles in separate files. CSS-in-JS excels at dynamic styling inside components.

3. Step 3: Combine approaches

   Using both allows shared styles in SASS files and dynamic styles with CSS-in-JS, fitting both requirements.

4. Final Answer:

   Combine SASS for shared styles in separate files and CSS-in-JS for dynamic styles inside components. -> Option A

5. Quick Check:

   Shared styles + dynamic props = combine SASS + CSS-in-JS [OK]

Hint: Use SASS for shared, CSS-in-JS for dynamic styles [OK]

Common Mistakes:

• Trying to do all styling only with SASS or only CSS-in-JS
• Ignoring benefits of combining both
• Using inline styles for complex shared styles