Transition parameters (duration, delay) in Svelte - Step-by-Step Execution

Concept Flow - Transition parameters (duration, delay)

Component mounts

↓

Transition starts

↓

Check delay

↓

Wait delay

↓

Animate over duration

↓

Transition ends

↓

Component fully visible

When a component appears, Svelte waits for the delay, then animates the transition over the duration time before finishing.

Execution Sample

Svelte

  <script>
    import { fade } from 'svelte/transition';
  </script>

  <div transition:fade={{ duration: 1000, delay: 500 }}>
    Hello
  </div>

This code fades in a div with a 500ms delay and a 1000ms fade duration.

Execution Table

Step	Action	Delay Timer	Animation Progress	Output
1	Component mounts	0ms	0%	Div hidden
2	Start delay timer	0ms	0%	Waiting...
3	Delay timer at 500ms	500ms	0%	Delay finished, start animation
4	Animation starts	500ms	0%	Div begins fading in
5	Animation at 250ms	500ms	25%	Div partially visible
6	Animation at 500ms	500ms	50%	Div half visible
7	Animation at 750ms	500ms	75%	Div mostly visible
8	Animation at 1000ms	500ms	100%	Div fully visible
9	Transition ends	500ms	100%	Div fully visible, transition complete

💡 Animation completes after duration (1000ms) following delay (500ms).

Variable Tracker

Variable	Start	After Delay	Mid Animation	End Animation
delayTimer	0ms	500ms	500ms	500ms
animationProgress	0%	0%	50%	100%
divOpacity	0	0	0.5	1

Key Moments - 2 Insights

Why does the div stay hidden at the start even though the component is mounted?

Because the delay parameter causes Svelte to wait before starting the animation, as shown in execution_table row 2 and 3.

What happens if duration is set to 0?

The animation jumps instantly to fully visible after the delay, skipping gradual fade steps, as duration controls animation length.

Visual Quiz - 3 Questions

Look at the execution table, what is the animation progress at step 6?

A 50%

B 0%

C 25%

D 75%

💡 Hint

Check the 'Animation Progress' column at step 6 in the execution_table.

At which step does the delay finish and animation start?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for when 'Delay finished, start animation' appears in the 'Output' column.

If delay is set to 0, how would the execution table change?

A Animation would never start

B Delay timer would stay at 0ms and animation starts immediately

C Animation progress would start at 50%

D Delay timer would increase beyond 500ms

💡 Hint

Refer to the 'Delay Timer' and 'Output' columns in the execution_table for delay behavior.

Concept Snapshot

Svelte transitions use parameters like duration and delay.
- delay: wait time before animation starts
- duration: how long animation runs
Example: transition:fade={{ duration: 1000, delay: 500 }}
Delay pauses before fade begins.
Duration controls fade speed.
Useful for smooth, timed animations.

Full Transcript

When a Svelte component with a transition mounts, it first checks the delay parameter. If delay is set, Svelte waits that amount of time before starting the animation. After the delay, the animation runs over the duration time, gradually changing the component's appearance, such as fading in. For example, with a delay of 500 milliseconds and duration of 1000 milliseconds, the component stays hidden for 500ms, then fades in over 1 second. Variables like delayTimer track the waiting time, animationProgress tracks how far the animation has run, and divOpacity changes from 0 to 1 during the fade. This process ensures smooth, timed transitions that can be controlled precisely.