Declaring props with export let in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does export let do in a Svelte component?

It declares a property (prop) that the component can receive from its parent. This lets the parent pass data into the child component.

beginner

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

You assign a value when declaring the prop, like <code>export let name = 'Guest';</code>. If the parent doesn't pass a value, the default is used.

intermediate

Can you change the value of a prop inside a Svelte component after it is set?

Yes, you can reassign the prop inside the component, but it won't affect the parent. Props are one-way: parent to child.

beginner

Why use export let instead of just let in Svelte?

export let makes the variable a prop that the parent can set. Plain let is private to the component and can't receive values from outside.

intermediate

What happens if a parent component does not pass a prop declared with export let and no default is set?

The prop will be undefined inside the child component, which might cause errors if not handled.

In Svelte, how do you declare a prop named title?

A export let title;

B let export title;

C prop title;

D export prop title;

The correct syntax to declare a prop is export let title;.

What is the default value of a prop if you write export let count = 10; and the parent does not pass count?

A 0

B null

C undefined

D 10

The default value 10 is used if the parent does not provide a value.

If a parent passes a prop to a child component, can the child update the parent's value directly?

A No, props are one-way from parent to child

B Only if you use a store

C Only if you use <code>export let</code>

D Yes, props are two-way bound by default

Props flow one-way from parent to child. The child cannot directly change the parent's value.

What happens if you declare let name; without export in a Svelte component?

A It becomes a prop

B It is private to the component and cannot receive values from outside

C It causes an error

D It is automatically exported

Without export, the variable is private and cannot receive values from the parent.

How can you provide a fallback value for a prop in Svelte?

A Use <code>prop: defaultValue;</code>

B Use <code>let prop = defaultValue;</code>

C Use <code>export let prop = defaultValue;</code>

D Use <code>default prop = defaultValue;</code>

Assigning a value during declaration with export let sets a default fallback.

Explain how to declare and use props in a Svelte component using export let.

Think about how a parent sends data to a child component.

Describe what happens if a prop declared with export let is not passed by the parent and no default value is set.

Consider what the child component sees when no value is given.

Practice

1. What does export let do inside a Svelte component's <script> block?

easy

A. It creates a local variable that cannot be changed.

B. It imports a module from another file.

C. It declares a prop that can be set from outside the component.

D. It defines a function inside the component.

Solution

1. Step 1: Understand the role of export let

   In Svelte, export let declares a property (prop) that the parent component can pass in.

2. Step 2: Differentiate from other uses

   Unlike local variables or imports, export let specifically exposes variables as props for customization.

3. Final Answer:

   It declares a prop that can be set from outside the component. -> Option C

4. Quick Check:

   Props = export let [OK]

Hint: Remember: export let means external prop [OK]

Common Mistakes:

• Confusing export let with import statements
• Thinking export let creates constants
• Assuming export let defines functions

2. Which of the following is the correct syntax to declare a prop named title with a default value of "Hello" in Svelte?

easy

A. export let title = "Hello";

B. let export title = "Hello";

C. export const title = "Hello";

D. let title = export "Hello";

Solution

1. Step 1: Recall correct syntax for props

   In Svelte, props are declared with export let followed by the variable name and optional default value.

2. Step 2: Check each option

   export let title = "Hello"; matches the correct syntax: export let title = "Hello";. Others have wrong order or keywords.

3. Final Answer:

   export let title = "Hello"; -> Option A

4. Quick Check:

   export let + name + = default [OK]

Hint: Use 'export let' before variable name for props [OK]

Common Mistakes:

• Swapping 'let' and 'export' keywords
• Using 'const' instead of 'let' for props
• Placing export after variable name

3. Given this Svelte component code:

<script>
  export let count = 5;
</script>

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

What will be displayed if the component is used as <MyComponent count={10} />?

medium

A. Count is 5

B. Count is 10

C. Count is undefined

D. Syntax error

Solution

1. Step 1: Understand default and passed props

   The component sets a default count of 5, but the parent passes 10, which overrides the default.

2. Step 2: Determine rendered output

   The displayed text uses the passed value 10, so it shows "Count is 10".

3. Final Answer:

   Count is 10 -> Option B

4. Quick Check:

   Passed prop overrides default [OK]

Hint: Passed props override defaults [OK]

Common Mistakes:

• Assuming default always shows ignoring passed value
• Thinking undefined appears if prop is passed
• Confusing syntax error with valid code

4. Identify the error in this Svelte component code:

<script>
  export let name;
  name = "John";
</script>

<p>Hello {name}</p>

medium

A. You cannot assign a value to a prop inside the component after declaration.

B. Missing default value for the prop name.

C. The export keyword should be after let.

D. The <p> tag cannot use curly braces.

Solution

1. Step 1: Understand prop assignment rules

   Props declared with export let should not be reassigned inside the component; they come from outside.

2. Step 2: Check the code

   The code tries to assign name = "John" after declaration, which is not allowed and causes an error.

3. Final Answer:

   You cannot assign a value to a prop inside the component after declaration. -> Option A

4. Quick Check:

   Props are read-only inside component [OK]

Hint: Don't reassign props inside component [OK]

Common Mistakes:

• Trying to set default by assignment after export let
• Confusing export let order
• Misunderstanding template syntax

5. You want to create a Svelte component that accepts a prop items which is an array, but if no prop is passed, it should default to an empty array. Which code correctly declares this prop?

hard

A. export const items = [];

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

C. let items = [];

D. export let items = [];

Solution

1. Step 1: Set default value in prop declaration

   In Svelte, you can assign a default value directly when declaring a prop with export let.

2. Step 2: Evaluate options

   export let items = []; correctly declares items with default empty array. export let items;
if (!items) items = []; tries to assign after declaration, which is not allowed. let items = []; is local variable, not prop. export const items = []; uses const which is incorrect for props.

3. Final Answer:

   export let items = []; -> Option D

4. Quick Check:

   Default values go in export let declaration [OK]

Hint: Assign default value in export let line [OK]

Common Mistakes:

• Assigning default after declaration
• Using const instead of let for props
• Not exporting the variable