Passing styles to components (--style-props) in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of passing styles to Svelte components using --style-props?

It allows you to customize the look of a component by sending CSS style values as properties, making components flexible and reusable without changing their internal code.

beginner

How do you define a style property in a Svelte component to accept a passed style value?

You declare a export let variable in the component script and then use it in the style attribute or CSS variables inside the component.

intermediate

Example: How would you pass a background color to a Svelte button component using style props?

In the parent: <code>&lt;Button bgColor="lightblue" /&gt;</code>. In the Button component: <code>export let bgColor = 'white';</code> and use <code>style="background-color: {bgColor};"</code> in the button element.

beginner

Why is passing styles as props better than hardcoding styles inside components?

It keeps components flexible and reusable. You can change their appearance without editing their code, which helps maintain clean and scalable projects.

intermediate

What is a common mistake when passing styles to components and how to avoid it?

A common mistake is passing raw CSS strings that break syntax or forgetting to set default values for style props. Avoid this by validating inputs and providing defaults.

How do you declare a style property in a Svelte component to accept a passed style value?

A const styleProp = 'value';

B let styleProp = export;

C export let styleProp;

D styleProp: string;

In Svelte, you use export let to declare a property that can receive values from outside.

What is the benefit of passing styles as props to components?

A It makes components reusable with different looks.

B It makes components slower.

C It hides the component code.

D It forces all components to have the same style.

Passing styles as props allows you to reuse the same component with different appearances.

Which of these is a correct way to apply a passed style prop inside a Svelte component?

A <button style="background-color: {bgColor};">Click</button>

B <button style="bgColor: background-color;">Click</button>

C <button style="{bgColor}">Click</button>

D <button style="color: bgColor;">Click</button>

You use curly braces to insert the variable value inside the style attribute.

What should you do to avoid errors when passing style props?

A Hardcode all styles inside components.

B Never use style props.

C Use only inline styles without props.

D Provide default values and validate inputs.

Providing defaults and validating helps prevent broken styles and unexpected behavior.

If you want to pass a font size to a component, which is the best way?

A Pass font size as a number without units.

B export let fontSize = '16px'; and use style="font-size: {fontSize};"

C Hardcode font size inside the component.

D Use a global CSS file only.

Passing font size as a prop with units and applying it in style is flexible and clear.

Explain how you can pass and use style properties in a Svelte component to customize its appearance.

Think about how variables and style attributes work together in Svelte.

Describe common mistakes when passing styles to components and how to avoid them.

Consider what can break styles and how to keep components stable.

Practice

1. What is the main benefit of passing styles as props to a Svelte component?

easy

A. It automatically adds animations to the component.

B. It makes the component run faster.

C. It allows customizing the component's appearance without changing its internal code.

D. It disables the component's default styles permanently.

Solution

1. Step 1: Understand the purpose of style props

   Passing styles as props lets you change how a component looks without editing its code.

2. Step 2: Compare options

   Only It allows customizing the component's appearance without changing its internal code. correctly describes this benefit; others mention speed, animations, or disabling styles, which are unrelated.

3. Final Answer:

   It allows customizing the component's appearance without changing its internal code. -> Option C

4. Quick Check:

   Style props = customize without code change [OK]

Hint: Style props let you change look without touching component code [OK]

Common Mistakes:

• Thinking style props improve performance
• Confusing style props with animations
• Assuming style props disable default styles

2. Which of the following is the correct way to accept a style prop in a Svelte component?

easy

A. export let style;

B. let style = props.style;

C. const style = this.props.style;

D. function style() {}

Solution

1. Step 1: Recall Svelte prop syntax

   Svelte uses export let to declare props that components accept.

2. Step 2: Evaluate options

   export let style; uses correct Svelte syntax. Options B and C use React or other framework styles. function style() {} is a function, not a prop declaration.

3. Final Answer:

   export let style; -> Option A

4. Quick Check:

   Svelte props use export let [OK]

Hint: Use 'export let' to declare props in Svelte components [OK]

Common Mistakes:

• Using React or Vue syntax inside Svelte
• Declaring props without export
• Confusing functions with props

3. Given this parent component code:

<ChildComponent style="color: red; font-weight: bold;" />

And this child component code:

  <script>
    export let style;
  </script>
  <div style={style}>Hello</div>

What will be the color and font weight of the text rendered by ChildComponent?

medium

A. Default color and font weight

B. No text will be rendered

C. Blue color and normal font weight

D. Red color and bold font weight

Solution

1. Step 1: Understand style passing

   The parent passes a style string with color red and bold font weight to the child as a prop.

2. Step 2: Check child usage

   The child applies the style prop directly to the div's style attribute, so the styles apply to the text.

3. Final Answer:

   Red color and bold font weight -> Option D

4. Quick Check:

   Passed style string applies directly [OK]

Hint: Style string passed as prop applies directly to element [OK]

Common Mistakes:

• Assuming styles won't apply without CSS files
• Thinking default styles override passed styles
• Believing text won't render without explicit content

4. What is wrong with this child component code that tries to accept a style prop?

<script>
  let style;
</script>
<div style={style}>Content</div>

medium

A. The div tag is missing a closing tag.

B. The style prop is not exported, so it won't receive values from the parent.

C. The style variable should be a function, not a let variable.

D. The style attribute cannot be bound to variables in Svelte.

Solution

1. Step 1: Check prop declaration

   In Svelte, props must be declared with export let to receive values from parents.

2. Step 2: Identify the issue

   The code declares let style; without export, so it stays local and undefined.

3. Final Answer:

   The style prop is not exported, so it won't receive values from the parent. -> Option B

4. Quick Check:

   Props need export let [OK]

Hint: Always export props to receive parent values [OK]

Common Mistakes:

• Forgetting to export props
• Thinking style binding is invalid
• Misreading HTML tag syntax

5. You want to create a reusable Button component that accepts custom styles from its parent. Which approach correctly passes and applies styles while keeping default styles intact?

Choose the best code snippet for the Button.svelte component:

hard

A. <script>
export let style = '';
</script>
<button style={`background: blue; color: white; ${style}`}>Click me</button>

B. <script>
let style;
</script>
<button style={style}>Click me</button>

C. <script>
export let style;
</script>
<button class={style}>Click me</button>

D. <script>
export let style;
</script>
<button>Click me</button>

Solution

1. Step 1: Understand default and custom styles

   The component should have default styles but allow extra styles from the parent.

2. Step 2: Analyze options

   <script>
     export let style = '';
   </script>
   <button style={`background: blue; color: white; ${style}`}>Click me</button>

   sets a default style string and appends the passed style, combining both.

   <script>
     let style;
   </script>
   <button style={style}>Click me</button>

   misses export and default styles.

   <script>
     export let style;
   </script>
   <button class={style}>Click me</button>

   uses class binding incorrectly for styles.

   <script>
     export let style;
   </script>
   <button>Click me</button>

   ignores the style prop.

3. Final Answer:

   export let style = ''; button with combined style string -> Option A

4. Quick Check:

   Combine default + passed styles with export let [OK]

Hint: Combine default and passed styles in template string [OK]

Common Mistakes:

• Not exporting style prop
• Overwriting default styles instead of combining
• Using class instead of style attribute for inline styles