CSS custom properties vs SASS variables - Quick Revision & Key Differences

Recall & Review

beginner

What is a CSS custom property?

A CSS custom property is a variable defined directly in CSS using the syntax --property-name. It can be accessed and changed in the browser at runtime.

intermediate

How do SASS variables differ from CSS custom properties?

SASS variables are defined in SASS files and replaced during compilation before the CSS is sent to the browser. CSS custom properties exist in the final CSS and can be changed dynamically in the browser.

beginner

Show an example of defining and using a CSS custom property.

Define: :root { --main-color: #3498db; }
Use: color: var(--main-color);

intermediate

Can CSS custom properties be changed with JavaScript after the page loads?

Yes, CSS custom properties can be updated dynamically using JavaScript, allowing live style changes without reloading the page.

intermediate

Why might you choose SASS variables over CSS custom properties?

Use SASS variables when you want to use variables during development for things like calculations or nesting, and when you don't need to change values dynamically in the browser.

Which of these can be changed dynamically in the browser after the page loads?

A CSS custom properties

B SASS variables

C Both CSS custom properties and SASS variables

D Neither CSS custom properties nor SASS variables

CSS custom properties exist in the final CSS and can be changed dynamically with JavaScript. SASS variables are compiled away before the page loads.

Where are SASS variables replaced with actual values?

A In the HTML file

B In the browser at runtime

C Only when JavaScript runs

D During SASS compilation before sending CSS to the browser

SASS variables are replaced during the compilation step before the CSS is sent to the browser.

How do you use a CSS custom property in your CSS code?

A custom-property(--property-name)

B var(--property-name)

C property-name()

D $property-name

CSS custom properties are accessed using the var(--property-name) syntax.

Which variable type supports calculations and functions during development?

A SASS variables

B Both support calculations

C CSS custom properties

D Neither support calculations

SASS variables support calculations and functions during compilation, which CSS custom properties do not.

What is the correct way to define a SASS variable?

A var(main-color): #333;

B --main-color: #333;

C $main-color: #333;

D main-color: #333;

SASS variables start with a dollar sign, like $main-color: #333;.

Explain the main differences between CSS custom properties and SASS variables.

Think about when and where each variable type works.

Describe a situation where you would prefer to use CSS custom properties over SASS variables.

Consider dynamic style changes after page load.

Practice

1. Which of the following is true about CSS custom properties compared to SASS variables?

easy

A. SASS variables start with -- and are dynamic in the browser.

B. SASS variables start with -- and work in the browser at runtime.

C. CSS custom properties start with $ and are replaced before the browser sees CSS.

D. CSS custom properties start with -- and work in the browser at runtime.

Solution

1. Step 1: Identify CSS custom properties syntax

   CSS custom properties always start with -- and are live in the browser, meaning they can be changed dynamically.

2. Step 2: Identify SASS variables syntax and behavior

   SASS variables start with $ and are replaced during the build process, so they don't exist in the browser.

3. Final Answer:

   CSS custom properties start with -- and work in the browser at runtime. -> Option D

4. Quick Check:

   CSS variables = -- prefix and runtime [OK]

Hint: CSS vars use -- and work live; SASS vars use $ and compile away [OK]

Common Mistakes:

• Confusing $ and -- prefixes
• Thinking SASS variables work in the browser
• Assuming CSS variables are replaced before runtime

2. Which is the correct way to declare a SASS variable for a color?

easy

A. primary-color: #3498db;

B. --primary-color: #3498db;

C. $primary-color: #3498db;

D. $primary-color == #3498db;

Solution

1. Step 1: Recall SASS variable syntax

   SASS variables start with $ and use a colon : to assign values.

2. Step 2: Check each option

   $primary-color: #3498db; uses correct syntax. --primary-color: #3498db; uses the CSS custom property prefix. primary-color: #3498db; omits the $ prefix. $primary-color == #3498db; uses double equals which is invalid.

3. Final Answer:

   $primary-color: #3498db; -> Option C

4. Quick Check:

   SASS variable = $name: value; [OK]

Hint: SASS vars start with $ and use colon for assignment [OK]

Common Mistakes:

• Using -- instead of $ for SASS variables
• Using == instead of : for assignment
• Omitting $ prefix

3. Given the following code, what color will the text be in the browser?

:root { --main-color: red; }
.box { color: var(--main-color); }
$main-color: blue;
.box2 { color: $main-color; }

medium

A. Both .box and .box2 text will be red

B. .box text will be red; .box2 text will be blue

C. .box text will be blue; .box2 text will be red

D. Both .box and .box2 text will be blue

Solution

1. Step 1: Understand CSS custom property usage

   The --main-color is set to red in :root. The class .box uses var(--main-color), so it will be red at runtime.

2. Step 2: Understand SASS variable usage

   The SASS variable $main-color is blue and used in .box2. This is replaced during compilation, so .box2 color will be blue in the final CSS.

3. Final Answer:

   .box text will be red; .box2 text will be blue -> Option B

4. Quick Check:

   CSS var = red; SASS var = blue [OK]

Hint: CSS vars live in browser; SASS vars replaced before browser sees CSS [OK]

Common Mistakes:

• Thinking SASS variables work at runtime
• Confusing which variable affects which class
• Assuming CSS variables are replaced before runtime

4. What is wrong with this SASS code snippet?

$color: green;
.text { color: var($color); }

medium

A. The var() function is only for CSS custom properties, not SASS variables.

B. The variable name should start with -- instead of $.

C. SASS variables cannot be used inside var() function.

D. The color value green is invalid.

Solution

1. Step 1: Understand var() function usage

   The var() function is a CSS function used only for CSS custom properties (those starting with --).

2. Step 2: Understand SASS variable usage

   SASS variables start with $ and are used directly without var(). Using var($color) is invalid because var() expects a CSS custom property name.

3. Final Answer:

   The var() function is only for CSS custom properties, not SASS variables. -> Option A

4. Quick Check:

   var() = CSS vars only, not SASS [OK]

Hint: Use $var directly in SASS; var() only for CSS custom properties [OK]

Common Mistakes:

• Using var() with SASS variables
• Confusing $ and -- prefixes
• Thinking var() works for SASS variables

5. You want to create a theme where users can switch colors dynamically in the browser. Which approach is best and why?

$primary-color: #0055ff;
:root { --primary-color: #0055ff; }

hard

A. Use CSS custom property --primary-color because it can change dynamically in the browser.

B. Use SASS variable $primary-color because it compiles faster.

C. Use both SASS and CSS variables together for better performance.

D. Use inline styles instead of variables for dynamic changes.

Solution

1. Step 1: Understand dynamic changes in the browser

   CSS custom properties (starting with --) live in the browser and can be changed with JavaScript or media queries without recompiling CSS.

2. Step 2: Understand SASS variable limitations

   SASS variables are replaced during build time and cannot change after the CSS is loaded, so they are not suitable for dynamic theming.

3. Final Answer:

   Use CSS custom property --primary-color because it can change dynamically in the browser. -> Option A

4. Quick Check:

   Dynamic theme = CSS vars [OK]

Hint: Dynamic changes need CSS vars; SASS vars are static [OK]

Common Mistakes:

• Choosing SASS variables for runtime changes
• Thinking both together improve dynamic behavior
• Ignoring CSS custom properties' runtime flexibility