Component composition in Svelte - Step-by-Step Execution

Concept Flow - Component composition

Parent Component

↓

Import Child Component

↓

Use Child in Parent's markup

↓

Render Parent

↓

Child renders inside Parent

↓

User sees combined UI

Component composition means putting smaller components inside bigger ones to build UI step-by-step.

Execution Sample

Svelte

/* Child.svelte */
<script>
  export let message = "";
</script>
<p>{message}</p>

/* Parent.svelte */
<script>
  import Child from './Child.svelte';
</script>
<Child message="Hello from Parent!" />

Parent component imports Child and uses it, passing a message prop to show inside Child.

Execution Table

Step	Action	Component	State Before	State After	Rendered Output
1	Parent starts rendering	Parent	No state	Imports Child	<Child message="Hello from Parent!" />
2	Child receives prop	Child	message=""	message="Hello from Parent!"	<p>Hello from Parent!</p>
3	Child renders inside Parent	Parent + Child	Parent markup with <Child>	Child rendered inside Parent	<p>Hello from Parent!</p> inside Parent
4	Parent finishes rendering	Parent	Child rendered	Full UI ready	User sees: Hello from Parent!

💡 Rendering completes when Parent and all children finish rendering.

Variable Tracker

Variable	Start	After Step 2	Final
message	""	"Hello from Parent!"	"Hello from Parent!"

Key Moments - 2 Insights

Why does the Child component show the message from Parent?

Because Parent passes the message as a prop to Child (see execution_table step 2), Child uses it to render its content.

What happens if Parent does not import Child?

Parent cannot use Child in its markup, so Child won't render (see execution_table step 1). Import is required to compose components.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'message' in Child after step 2?

A undefined

B "Hello from Parent!"

C ""

D "Child message"

💡 Hint

Check the 'State After' column for step 2 in execution_table.

At which step does the Child component finish rendering inside Parent?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for when Child is rendered inside Parent in execution_table.

If Parent does not pass the 'message' prop, what will Child display?

A Empty paragraph

B An error

C "Hello from Parent!"

D Nothing renders

💡 Hint

Refer to variable_tracker and how 'message' changes; default is empty string.

Concept Snapshot

Component composition in Svelte:
- Import child with: import Child from './Child.svelte';
- Use child in parent markup: <Child prop="value" />
- Pass data via props (export let propName)
- Child renders inside Parent's UI
- Builds UI by nesting components

Full Transcript

Component composition in Svelte means building UI by putting smaller components inside bigger ones. The Parent component imports the Child component and uses it in its markup. The Parent passes data to Child using props. Child receives the prop and renders content based on it. The rendering process starts with Parent, then Child renders inside Parent, and finally the full UI is shown to the user. Variables like props change as data flows from Parent to Child. Importing Child is necessary to use it. If no prop is passed, Child shows default or empty content.