Spread props in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does {...props} do in a Svelte component?

It takes all properties from the props object and passes them as individual attributes to a component or element.

beginner

How do you use spread props to pass multiple attributes to an HTML element in Svelte?

You create an object with the attributes and then use {...object} inside the element tag to apply all attributes at once.

intermediate

Can you override a spread prop by specifying the same attribute after the spread in Svelte?

Yes. Attributes specified after the spread will override those from the spread object.

beginner

Why use spread props instead of listing each prop individually in Svelte?

It saves time and keeps code clean when passing many props, especially if they come from an object.

intermediate

What happens if you spread an object with undefined or null values in Svelte props?

Undefined or null values are omitted; the corresponding attributes are not rendered.

In Svelte, how do you pass all properties from an object props to a component?

A <Component (props) />

B <Component [props] />

C <Component {...props} />

D <Component props />

Using {...props} spreads all properties as individual attributes.

If you write <button {...attrs} disabled> in Svelte, what happens to the disabled attribute?

A It causes an error

B The button is disabled regardless of <code>attrs</code>

C The button is not disabled

D The disabled attribute from <code>attrs</code> overrides the explicit disabled

Explicit attributes after the spread override the spread props, so disabled is set.

Which of these is a benefit of using spread props in Svelte?

A Passing many props quickly and cleanly

B Making components slower

C Preventing props from being passed

D Automatically validating props

Spread props help pass many props quickly and keep code clean.

What happens if you spread an object with a property set to undefined in Svelte?

A The attribute is ignored

B An error is thrown

C The attribute is passed with no value

D The attribute is set to 'undefined' string

Undefined values are ignored; the attribute is not rendered.

Can you use spread props on native HTML elements in Svelte?

A No, only on components

B Only on custom elements

C Only on <div> elements

D Yes, to pass multiple attributes at once

Spread props work on any element to pass multiple attributes quickly.

Explain how spread props work in Svelte and why they are useful.

Think about how you pass many settings to a component or element at once.

Describe a scenario where spread props can help simplify your Svelte code.

Imagine you have a settings object with many keys to pass to a button or input.

Practice

1. What does the spread props syntax {...props} do in a Svelte component?

easy

A. It passes all properties from the props object as individual props to the component.

B. It creates a new component with the name props.

C. It merges two components into one.

D. It removes all props from the component.

Solution

1. Step 1: Understand spread props syntax

   The syntax {...props} takes all key-value pairs from the props object.

2. Step 2: Apply spread props in component

   These key-value pairs are passed as individual props to the component, making them accessible inside.

3. Final Answer:

   It passes all properties from the props object as individual props to the component. -> Option A

4. Quick Check:

   Spread props = pass all object keys as props [OK]

Hint: Spread props pass all object keys as separate props [OK]

Common Mistakes:

• Thinking spread props create a new component
• Confusing spread props with merging components
• Believing spread props remove props

2. Which of the following is the correct self-closing syntax to pass all properties of an object user as props to a Svelte component <Profile />?

easy

A. <Profile {...user}>

B. <Profile {...user} />

C. <Profile {...user};>

D. <Profile {...user}() />

Solution

1. Step 1: Recall spread props syntax in Svelte

   Spread props use {...object} inside the component tag to pass all object keys as props.

2. Step 2: Check correct self-closing tag syntax

   In Svelte, self-closing tags use /> without extra characters or parentheses.

3. Final Answer:

   <Profile {...user} /> -> Option B

4. Quick Check:

   Correct spread syntax + self-closing tag = <Profile {...user} /> [OK]

Hint: Use {...object} inside component tag without extra symbols [OK]

Common Mistakes:

• Adding semicolons inside spread syntax
• Using parentheses after spread props
• Incorrectly closing the component tag

3. Given the code:

  <script>
    let data = {name: 'Alice', age: 30};
  </script>

  <UserCard {...data} age={25} />

What will be the value of age inside UserCard?

medium

A. 30

B. undefined

C. 25

D. Error: Duplicate props

Solution

1. Step 1: Understand spread props and explicit props order

   The spread {...data} passes name: 'Alice' and age: 30 first.

2. Step 2: Recognize explicit prop override

   The explicit prop age={25} comes after spread and overrides the previous age value.

3. Final Answer:

   25 -> Option C

4. Quick Check:

   Explicit props override spread props [OK]

Hint: Explicit props after spread override earlier values [OK]

Common Mistakes:

• Assuming spread props always override explicit props
• Thinking duplicate props cause errors
• Ignoring prop order in component tag

4. Identify the error in this Svelte component usage:

  <script>
    let props = {title: 'Hello'};
  </script>

  <Card {...props} title />

medium

A. Spread props cannot be used with explicit props.

B. The component tag must not be self-closing.

C. The object props must be declared inside the component.

D. Explicit prop title without a value sets it to true.

Solution

1. Step 1: Check explicit prop syntax

   The explicit prop title without a value is treated as title={true} in Svelte.

2. Step 2: Understand Svelte prop override

   This overrides the spread title='Hello' to true.

3. Final Answer:

   Explicit prop title without value sets it to true. -> Option D

4. Quick Check:

   Explicit props need values unless boolean [OK]

Hint: Explicit props must have values unless boolean attributes [OK]

Common Mistakes:

• Thinking spread and explicit props can't be combined
• Assuming missing value is allowed for any prop
• Believing self-closing tags are invalid here

5. You have a Svelte component <Button /> that accepts props type, disabled, and label. You want to pass an object btnProps with {type: 'submit', disabled: false} but override disabled to true and add label as 'Send'. Which is the correct usage?

hard

A. <Button {...btnProps} disabled={true} label='Send' />

B. <Button disabled={true} label='Send' {...btnProps} />

C. <Button label='Send' disabled={true} {...btnProps} />

D. <Button {...btnProps} label='Send'; disabled={true} />

Solution

1. Step 1: Pass base props with spread first

   Use {...btnProps} to pass type and disabled: false.

2. Step 2: Override disabled and add label explicitly

   Explicit props after spread override previous values, so disabled={true} and label='Send' must come after.

3. Step 3: Check order of props

   Only <Button {...btnProps} disabled={true} label='Send' /> places spread first, then explicit props, ensuring correct override.

4. Final Answer:

   <Button {...btnProps} disabled={true} label='Send' /> -> Option A

5. Quick Check:

   Spread first, explicit override after [OK]

Hint: Put spread props first, explicit props after to override [OK]

Common Mistakes:

• Placing explicit props before spread, losing override effect
• Duplicating props in wrong order
• Forgetting to add new props explicitly