Deferred transitions in Svelte - Step-by-Step Execution

Concept Flow - Deferred transitions

Component renders

↓

Trigger transition

↓

Check if transition is deferred

Yes No↓

Wait delay

↓

Start transition

↓

Transition completes

↓

Component updates

This flow shows how a transition can be delayed (deferred) before starting, allowing smoother UI updates.

Execution Sample

Svelte

<script>
  import { fade } from 'svelte/transition';
  let visible = false;
</script>

<button on:click={() => visible = !visible}>Toggle</button>

{#if visible}
  <div transition:fade={{ delay: 500 }}>Hello</div>
{/if}

This Svelte code toggles a div with a fade transition that starts after a 500ms delay (deferred transition).

Execution Table

Step	Action	Transition State	Delay Timer	DOM Change
1	Component renders, visible = false	No transition	No timer	No div shown
2	Click button, visible = true	Transition scheduled	Start 500ms timer	Div not yet visible
3	500ms delay passes	Transition starts	Timer cleared	Div begins fade-in
4	Fade-in completes	Transition ended	No timer	Div fully visible
5	Click button, visible = false	Transition scheduled	Start 500ms timer	Div still visible
6	500ms delay passes	Transition starts	Timer cleared	Div begins fade-out
7	Fade-out completes	Transition ended	No timer	Div removed from DOM

💡 Transitions complete after delay and animation; no further changes until next toggle.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
visible	false	true	true	true	false	false	false
Transition State	No transition	Scheduled (delay)	Running	Ended	Scheduled (delay)	Running	Ended
Delay Timer	None	500ms started	Cleared	None	500ms started	Cleared	None
DOM	No div	No div	Div fading in	Div visible	Div visible	Div fading out	No div

Key Moments - 3 Insights

Why doesn't the div appear immediately after clicking the button?

Because the transition has a delay (500ms), the div waits before starting to fade in, as shown in step 2 and 3 of the execution_table.

What happens if the delay timer is not cleared?

The transition would not start properly, causing the div to stay hidden or stuck in transition. The execution_table shows the timer cleared at step 3 and 6 to start transitions.

Does the div get removed from the DOM immediately when visible becomes false?

No, it waits for the fade-out transition to complete after the delay, as seen in steps 5 to 7 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Transition State at Step 3?

A Transition ended

B Transition starts

C Transition scheduled

D No transition

💡 Hint

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

At which step does the div become fully visible?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'DOM Change' column to see when the div is fully visible.

If the delay was removed, how would Step 2 change?

A Transition starts immediately

B Transition scheduled with 500ms timer

C No transition scheduled

D Div removed from DOM

💡 Hint

Refer to the 'Delay Timer' and 'Transition State' columns in Step 2.

Concept Snapshot

Deferred transitions delay the start of an animation.
Use the delay option in Svelte transitions.
The element waits before fading in or out.
This creates smoother UI changes.
Transition state changes from scheduled to running after delay.
DOM updates after transition completes.

Full Transcript

Deferred transitions in Svelte allow delaying the start of an animation. When a component's state changes to show or hide an element, the transition does not start immediately but waits for a specified delay time. After the delay, the transition runs, fading the element in or out. This process involves scheduling the transition, starting a timer for the delay, then running the animation once the timer finishes. The element remains in the DOM during the transition and is removed only after the fade-out completes. This technique helps create smooth and controlled UI updates.