Default prop values in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a default prop value in Svelte?

A default prop value is a value assigned to a component's prop that is used when no value is passed from the parent component.

beginner

How do you set a default prop value in a Svelte component?

You assign a value to the prop variable when declaring it with <code>export let</code>. For example: <code>export let name = 'Guest';</code>

beginner

What happens if a parent component passes a value to a prop that has a default value in Svelte?

The passed value overrides the default value, so the component uses the parent's value instead of the default.

intermediate

Can default prop values in Svelte be any data type?

Yes, default prop values can be strings, numbers, booleans, objects, arrays, or any valid JavaScript value.

beginner

Why are default prop values useful in Svelte components?

They make components more flexible and easier to use by providing fallback values, so the parent doesn't have to pass every prop every time.

How do you declare a prop with a default value in Svelte?

A export let propName = defaultValue;

B let propName: defaultValue;

C propName = export default defaultValue;

D default propName = defaultValue;

In Svelte, you use export let propName = defaultValue; to declare a prop with a default value.

If a parent component passes a value to a prop, what happens to the default value?

A The default value is ignored and the passed value is used.

B The default value and passed value are merged.

C The default value overrides the passed value.

D An error occurs.

The passed value overrides the default value in Svelte props.

Which of these is a valid default prop value in Svelte?

A default let count = 0;

B export let count;

C export let count = 0;

D let count = 0;

Only export let count = 0; correctly declares a prop with a default value.

Can default prop values be objects in Svelte?

A Only strings are allowed.

B No, only primitive types are allowed.

C Only arrays are allowed, not objects.

D Yes, for example: export let settings = { theme: 'dark' };

Svelte allows any JavaScript value as a default prop, including objects.

Why might you use default prop values in your Svelte components?

A To make props read-only.

B To provide fallback values and simplify component usage.

C To prevent props from being passed.

D To automatically update props from the server.

Default prop values provide fallback values, making components easier to use.

Explain how to set and use default prop values in a Svelte component.

Think about how you declare props and assign values in Svelte.

Describe why default prop values improve component flexibility in Svelte.

Consider what happens if a parent forgets to pass a prop.

Practice

1. What is the correct way to set a default value for a prop named color in a Svelte component?

easy

A. export let color = 'blue';

B. let color = 'blue';

C. prop color = 'blue';

D. export color = 'blue';

Solution

1. Step 1: Understand Svelte prop declaration

   In Svelte, props are declared with export let to make them accessible from outside.

2. Step 2: Assign default value

   Assigning = 'blue' after export let color sets the default value if no prop is passed.

3. Final Answer:

   export let color = 'blue'; -> Option A

4. Quick Check:

   Default prop syntax = export let prop = value [OK]

Hint: Use 'export let' with '=' to set default prop values [OK]

Common Mistakes:

• Forgetting 'export' keyword
• Using 'let' without export
• Using 'prop' keyword which doesn't exist
• Assigning default without 'let'

2. Which of the following is the correct syntax to declare a prop size with a default value of 10 in Svelte?

easy

A. export let size: 10;

B. export size = 10;

C. let export size = 10;

D. export let size = 10;

Solution

1. Step 1: Recognize correct export syntax

   The correct way to declare a prop with default is export let prop = value;.

2. Step 2: Identify correct option

   export let size = 10; matches this syntax exactly with size and default 10.

3. Final Answer:

   export let size = 10; -> Option D

4. Quick Check:

   Default prop syntax uses '=' not ':' or misplaced keywords [OK]

Hint: Remember: 'export let prop = value;' sets default props [OK]

Common Mistakes:

• Using ':' instead of '='
• Misplacing 'export' keyword
• Omitting 'let' keyword
• Assigning default without 'let'

3. Given this Svelte component code:

  <script>
    export let message = 'Hello';
  </script>

  <p>{message}</p>

What will be displayed if the component is used without passing any message prop?

medium

A. An empty paragraph

B. Hello

C. Undefined

D. Error: message not defined

Solution

1. Step 1: Check default prop value

   The prop message has a default value of 'Hello'.

2. Step 2: Understand usage without prop

   If no prop is passed, Svelte uses the default value automatically.

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   Default prop used when no value passed = Hello [OK]

Hint: No prop passed? Default value shows [OK]

Common Mistakes:

• Assuming empty string instead of default
• Expecting undefined or error
• Confusing default with required prop

4. Identify the error in this Svelte component code:

<script>
  export let count;
  if (count === undefined) {
    count = 5;
  }
</script>

<p>Count is {count}</p>

medium

A. The count prop must be a string, not a number.

B. The if condition should check for null, not undefined.

C. You cannot assign to a prop after export; default should be set in declaration.

D. The count variable should not be exported.

Solution

1. Step 1: Understand prop default assignment rules

   In Svelte, default prop values must be set during declaration, not by reassigning later.

2. Step 2: Identify why reassignment is wrong

   Reassigning an exported prop inside script causes runtime errors or unexpected behavior.

3. Final Answer:

   You cannot assign to a prop after export; default should be set in declaration. -> Option C

4. Quick Check:

   Default props set at declaration, not by reassignment [OK]

Hint: Set default with export let, not by reassigning later [OK]

Common Mistakes:

• Trying to assign default after export
• Checking wrong condition for default
• Confusing prop type requirements
• Not exporting the prop

5. You have a Svelte component that accepts a prop items which should default to an empty array if not provided. Which is the best way to declare this prop to avoid errors when iterating over items in the template?

hard

A. export let items = [];

B. export let items; if (!items) items = [];

C. let items = []; export { items };

D. export let items = null;

Solution

1. Step 1: Understand default prop for arrays

   Setting export let items = []; ensures items is always an array, avoiding errors in iteration.

2. Step 2: Evaluate other options

   export let items; if (!items) items = []; tries to assign after export which is incorrect. let items = []; export { items }; exports a local variable incorrectly. export let items = null; sets null which can cause errors when iterating.

3. Final Answer:

   export let items = []; -> Option A

4. Quick Check:

   Default array props set with export let prop = [] [OK]

Hint: Default arrays with export let prop = [] to avoid iteration errors [OK]

Common Mistakes:

• Assigning default after export
• Exporting local variables incorrectly
• Using null as default for arrays
• Not setting default causing runtime errors