Combining & with BEM naming in SASS - Performance & Optimization

Performance: Combining & with BEM naming

MEDIUM IMPACT

This affects CSS selector matching speed and rendering performance by how selectors are combined and scoped.

Writing CSS selectors for BEM blocks and elements efficiently

SASS

.block {
  &__element { color: red; }
  &__element--modifier { color: blue; }
}

Using & nests selectors under .block, reducing repetition and keeping selectors scoped and shorter.

📈 Performance GainSaves CSS size and reduces selector matching complexity, improving rendering speed

Writing CSS selectors for BEM blocks and elements efficiently

SASS

.block__element { color: red; } .block__element--modifier { color: blue; }

Repeating full BEM selectors increases CSS size and selector complexity, slowing down matching.

📉 Performance CostAdds extra CSS bytes and increases selector matching time linearly with repeated selectors

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Repeated full BEM selectors	No extra DOM nodes	0	Low but slower style matching	[X] Bad
Nested & with BEM selectors	No extra DOM nodes	0	Low and faster style matching	[OK] Good

Rendering Pipeline

The browser matches CSS selectors during Style Calculation. Complex or repeated selectors increase matching time, delaying Layout and Paint.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckStyle Calculation due to complex or repeated selectors

Core Web Vital Affected

LCP

This affects CSS selector matching speed and rendering performance by how selectors are combined and scoped.

Optimization Tips

1Use & in Sass to nest BEM selectors and avoid repeating full selector names.

2Simpler, scoped selectors reduce CSS matching time and improve rendering speed.

3Avoid repeating long BEM selectors to keep CSS size small and style calculation fast.

Performance Quiz - 3 Questions

Why does using & to nest BEM selectors improve CSS performance?

A It increases DOM nodes for better style application

B It adds more CSS rules but speeds up JavaScript

C It reduces repeated selector text and simplifies matching

D It delays style calculation to after layout

DevTools: Performance

How to check: Record a performance profile while loading the page. Look at the Style Calculation phase duration.

What to look for: Longer Style Calculation times indicate complex or repeated selectors; shorter times show efficient nesting.

Practice

1. What does the & symbol represent when used in Sass with BEM naming?

easy

A. It resets all styles inside the block.

B. It creates a new CSS class unrelated to the parent.

C. It imports external CSS files.

D. It refers to the current parent selector in the nested block.

Solution

1. Step 1: Understand the role of & in Sass

   The & symbol in Sass means "use the current parent selector" inside nested rules.

2. Step 2: Connect with BEM naming

   When combined with BEM, &__element or &--modifier appends to the parent block's class name.

3. Final Answer:

   It refers to the current parent selector in the nested block. -> Option D

4. Quick Check:

   & = current parent selector [OK]

Hint: Remember & means "this block" in nested Sass [OK]

Common Mistakes:

• Thinking & creates a new unrelated class
• Confusing & with a CSS variable
• Using & to import files

2. Which of the following Sass snippets correctly uses & to create a BEM modifier for a block named .card?

easy

A. &__active { background: green; }

B. .card--active { background: green; }

C. &--active { background: green; }

D. &-active { background: green; }

Solution

1. Step 1: Identify BEM modifier syntax

   Modifiers use double dashes: --modifier, so &--active appends --active to the parent block.

2. Step 2: Check other options

   .card--active { background: green; } repeats the full class, not using &. &__active { background: green; } uses __ which is for elements, not modifiers. &-active { background: green; } uses a single dash, which is incorrect for BEM modifiers.

3. Final Answer:

   &--active { background: green; } -> Option C

4. Quick Check:

   Modifier = &--modifier [OK]

Hint: Use &--modifier for modifiers inside blocks [OK]

Common Mistakes:

• Using single dash instead of double dash for modifiers
• Confusing elements (__element) with modifiers (--modifier)
• Writing full class name instead of using &

3. Given the Sass code below, what CSS will it generate?

.button {
  color: blue;
  &__icon {
    margin-right: 0.5rem;
  }
  &--large {
    font-size: 2rem;
  }
}

medium

A. .button { color: blue; } &__icon { margin-right: 0.5rem; } &--large { font-size: 2rem; }

B. .button { color: blue; } .button__icon { margin-right: 0.5rem; } .button--large { font-size: 2rem; }

C. .button { color: blue; } .icon { margin-right: 0.5rem; } .large { font-size: 2rem; }

D. .button { color: blue; } .button_icon { margin-right: 0.5rem; } .button_large { font-size: 2rem; }

Solution

1. Step 1: Understand & usage in nested selectors

   The & appends to the parent selector .button. So &__icon becomes .button__icon and &--large becomes .button--large.

2. Step 2: Write the generated CSS

   The CSS will have three selectors: .button, .button__icon, and .button--large with their respective styles.

3. Final Answer:

   .button { color: blue; } .button__icon { margin-right: 0.5rem; } .button--large { font-size: 2rem; } -> Option B

4. Quick Check:

   & + BEM = combined class names [OK]

Hint: &__element and &--modifier append to block class [OK]

Common Mistakes:

• Using underscore instead of double underscore for elements
• Not combining & with parent selector
• Leaving & in final CSS

4. Identify the error in this Sass code using BEM naming:

.menu {
  &__item {
    color: black;
  }
  &-active {
    font-weight: bold;
  }
}

medium

A. The modifier should use double dashes: &--active instead of &-active.

B. The element selector &__item is invalid and should be &-item.

C. The parent selector .menu is missing a dot.

D. The color property cannot be inside &__item.

Solution

1. Step 1: Check BEM modifier syntax

   Modifiers in BEM use double dashes --, so &-active is incorrect; it should be &--active.

2. Step 2: Verify element syntax

   &__item is correct for elements, so no error there.

3. Final Answer:

   The modifier should use double dashes: &--active instead of &-active. -> Option A

4. Quick Check:

   Modifier syntax = &--modifier [OK]

Hint: Modifiers always use double dashes -- not single [OK]

Common Mistakes:

• Using single dash for modifiers
• Confusing element and modifier syntax
• Thinking &__item is invalid

5. You want to style a BEM block .profile with an element avatar and a modifier highlighted. Which Sass code correctly combines & with BEM naming to style both the element and modifier inside .profile?

hard

A. .profile {
&__avatar {
border-radius: 50%;
}
&--highlighted {
background-color: yellow;
}
}

B. .profile {
&-avatar {
border-radius: 50%;
}
&-highlighted {
background-color: yellow;
}
}

C. .profile {
&__avatar {
border-radius: 50%;
}
&-highlighted {
background-color: yellow;
}
}

D. .profile {
&-avatar {
border-radius: 50%;
}
&--highlighted {
background-color: yellow;
}
}

Solution

1. Step 1: Apply BEM element syntax

   Elements use double underscore __, so &__avatar correctly targets .profile__avatar.

2. Step 2: Apply BEM modifier syntax

   Modifiers use double dash --, so &--highlighted correctly targets .profile--highlighted.

3. Step 3: Check other options

   Options A, C, and D use single dash for either element or modifier, which is incorrect in BEM.

4. Final Answer:

   .profile { &__avatar { border-radius: 50%; } &--highlighted { background-color: yellow; } } -> Option A

5. Quick Check:

   Element = &__element, Modifier = &--modifier [OK]

Hint: Use &__element and &--modifier exactly for BEM [OK]

Common Mistakes:

• Using single dash for elements or modifiers
• Mixing element and modifier syntax
• Not nesting inside the block selector