Use directive syntax in Svelte - Step-by-Step Execution

Concept Flow - Use directive syntax

Start Svelte Component

↓

Parse Template

↓

Identify Directive Syntax

↓

Bind Directive to Behavior

↓

Render DOM with Directive Effects

↓

User Interaction or State Change

↓

Directive Reacts and Updates DOM

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Component Reflects Changes

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End

Svelte reads the template, finds directives, binds them to behaviors, renders DOM, and updates reactively on changes.

Execution Sample

Svelte

<script>
  let count = 0;
</script>
<button on:click={() => count++}>Clicked {count} times</button>

A button uses the 'on:click' directive to increase count and update the displayed text.

Execution Table

Step	Action	Directive Detected	State Before	State After	DOM Update
1	Parse <button>	on:click	count = 0	count = 0	Button rendered with text 'Clicked 0 times'
2	User clicks button	on:click triggered	count = 0	count = 1	Button text updates to 'Clicked 1 time'
3	User clicks button again	on:click triggered	count = 1	count = 2	Button text updates to 'Clicked 2 times'
4	No further clicks	No directive triggered	count = 2	count = 2	DOM remains 'Clicked 2 times'

💡 User stops clicking; no more directive events triggered.

Variable Tracker

Variable	Start	After 1	After 2	Final
count	0	1	2	2

Key Moments - 2 Insights

Why does the button text update automatically when count changes?

Because the 'on:click' directive triggers a state change, and Svelte's reactive system updates the DOM as shown in execution_table rows 2 and 3.

What does the 'on:click' directive do in the code?

It listens for the click event on the button and runs the provided function to increment 'count', as seen in execution_table steps 2 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'count' after the second click?

A 1

B 2

C 0

D 3

💡 Hint

Check the 'State After' column at Step 3 in the execution_table.

At which step does the DOM text update to 'Clicked 1 time'?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'DOM Update' column in execution_table for when the text changes to 'Clicked 1 time'.

If the user never clicks the button, what will the displayed text be?

A 'Clicked 2 times'

B 'Clicked 1 times'

C 'Clicked 0 times'

D No text displayed

💡 Hint

Refer to execution_table Step 1 where initial rendering happens without clicks.

Concept Snapshot

Svelte directives use syntax like on:event={handler}.
They bind DOM events or behaviors directly in markup.
When events happen, state changes trigger reactive updates.
Example: on:click increments a variable and updates UI.
Directives keep code simple and reactive without extra setup.

Full Transcript

This example shows how Svelte uses directive syntax to bind events. The 'on:click' directive listens for clicks on a button. When clicked, it runs a function that increases the 'count' variable. Svelte automatically updates the button text to show the new count. The execution table traces parsing, user clicks, state changes, and DOM updates step-by-step. The variable tracker shows how 'count' changes from 0 to 2 after two clicks. Key moments clarify why the UI updates and what the directive does. The quiz tests understanding of state and DOM changes from the table. Overall, directives in Svelte connect user actions to reactive UI updates simply and clearly.