Default prop values in Svelte - Performance & Optimization

Performance: Default prop values

MEDIUM IMPACT

This affects initial component rendering speed and bundle size by controlling how props are assigned default values.

Assigning default values to component props

Svelte

export let count = 0;

Default value is assigned at declaration, eliminating runtime checks.

📈 Performance GainReduces JavaScript execution time during initial render, improving LCP.

Assigning default values to component props

Svelte

export let count;
if (count === undefined) {
  count = 0;
}

This pattern requires an extra runtime check during component initialization to assign defaults.

📉 Performance CostTriggers extra JavaScript execution during component initialization, slightly delaying LCP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Default value assigned inside render logic	No extra DOM nodes	No extra reflows	Minimal paint	[!] OK
Default value assigned at prop declaration	No extra DOM nodes	No extra reflows	Minimal paint	[OK] Good

Rendering Pipeline

Default prop values assigned at declaration are processed during component initialization, reducing runtime overhead during rendering.

→JavaScript Execution

→Style Calculation

→Layout

⚠️ BottleneckJavaScript Execution

Core Web Vital Affected

LCP

This affects initial component rendering speed and bundle size by controlling how props are assigned default values.

Optimization Tips

1Always assign default prop values directly in the export declaration in Svelte.

2Avoid runtime conditional checks for defaults inside the component body.

3Reducing JavaScript execution during initial render improves LCP.

Performance Quiz - 3 Questions

What is the performance benefit of assigning default prop values directly in Svelte prop declarations?

A It reduces the number of DOM nodes created.

B It avoids runtime checks during rendering, speeding up initial load.

C It decreases CSS selector complexity.

D It prevents layout shifts after paint.

DevTools: Performance

How to check: Record a performance profile while loading the component. Look for JavaScript execution time related to prop initialization.

What to look for: Lower scripting time and fewer function calls during component setup indicate better default prop handling.

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