Custom store logic in Svelte - Performance & Optimization

Performance: Custom store logic

MEDIUM IMPACT

Custom store logic affects how reactive data updates propagate and how many DOM updates happen, impacting interaction responsiveness and rendering speed.

Creating a custom store that updates UI reactively

Svelte

import { writable } from 'svelte/store';

function createGoodStore() {
  const { subscribe, set, update } = writable(0);
  return {
    subscribe,
    increment: () => {
      // defer heavy computation outside update
      setTimeout(() => {
        update(n => n + 1);
      }, 0);
    }
  };
}

export const goodStore = createGoodStore();

Deferring heavy work avoids blocking the main thread during update, allowing smoother UI and faster interaction response.

📈 Performance GainNon-blocking updates reduce input delay, improving INP significantly.

Creating a custom store that updates UI reactively

Svelte

import { writable } from 'svelte/store';

function createBadStore() {
  const { subscribe, set, update } = writable(0);
  return {
    subscribe,
    increment: () => {
      update(n => {
        // heavy computation inside update
        for (let i = 0; i < 1000000; i++) {}
        return n + 1;
      });
    }
  };
}

export const badStore = createBadStore();

Heavy computation inside the update blocks the main thread and triggers multiple synchronous updates, causing slow UI responsiveness.

📉 Performance CostBlocks rendering for 50-100ms per update, causing input lag and poor INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Heavy synchronous logic in store update	Multiple reactive DOM updates	Triggers multiple reflows	High paint cost due to blocking	[X] Bad
Lightweight async store update	Minimal reactive DOM updates	Single reflow per update	Low paint cost, smooth rendering	[OK] Good

Rendering Pipeline

Custom store updates trigger reactive subscriptions that cause DOM updates. Heavy synchronous logic in store updates delays Style Calculation and Layout, blocking Paint and Composite stages.

→JavaScript Execution

→Style Calculation

→Layout

→Paint

⚠️ BottleneckJavaScript Execution blocking Layout and Paint

Core Web Vital Affected

INP

Custom store logic affects how reactive data updates propagate and how many DOM updates happen, impacting interaction responsiveness and rendering speed.

Optimization Tips

1Avoid heavy synchronous work inside store update functions.

2Defer expensive computations outside reactive updates to keep UI smooth.

3Minimize the number of reactive subscriptions triggered per update.

Performance Quiz - 3 Questions

What is the main performance risk of heavy synchronous logic inside a Svelte custom store update?

A It blocks the main thread causing slow UI responsiveness

B It increases bundle size significantly

C It causes layout shifts (CLS)

D It delays initial page load (LCP)

DevTools: Performance

How to check: Record a performance profile while interacting with the UI that triggers store updates. Look for long tasks or scripting blocking main thread.

What to look for: Long scripting tasks causing delayed frames indicate blocking store logic. Short, quick tasks show efficient updates.

Practice

1. What is the main purpose of creating a custom store in Svelte?

easy

A. To disable reactivity in the app

B. To add custom methods and control how data updates and shares

C. To make the store work only on the server side

D. To replace the built-in writable store completely

Solution

1. Step 1: Understand what a custom store does

   A custom store extends the basic writable store by adding your own methods to control data updates and sharing.

2. Step 2: Compare options with this understanding

   Only "To add custom methods and control how data updates and shares" correctly describes this purpose. The other options misunderstand the role of custom stores: disabling reactivity, replacing the built-in writable store completely, or server-side only.

3. Final Answer:

   To add custom methods and control how data updates and shares -> Option B

4. Quick Check:

   Custom store purpose = Add custom methods [OK]

Hint: Custom stores add your own methods to writable stores [OK]

Common Mistakes:

• Thinking custom stores replace writable stores entirely
• Believing custom stores disable reactivity
• Assuming custom stores only run on the server

2. Which of the following is the correct way to create a custom store in Svelte?

easy

A. import { writable } from 'svelte/store'; function createStore() { const { subscribe, update } = writable(0); return { subscribe, increment: () => update(n => n + 1) }; }

B. import { readable } from 'svelte/store'; function createStore() { const { subscribe } = readable(0); return { subscribe, increment: () => {} }; }

C. import { writable } from 'svelte/store'; function createStore() { const { subscribe, set } = writable(0); return { subscribe, increment: () => set(n => n + 1) }; }

D. import { store } from 'svelte/store'; function createStore() { return { subscribe: () => {}, increment: () => {} }; }

Solution

1. Step 1: Check import and store creation

   import { writable } from 'svelte/store'; function createStore() { const { subscribe, update } = writable(0); return { subscribe, increment: () => update(n => n + 1) }; } correctly imports writable and uses update to change the value.

2. Step 2: Verify method usage

   import { writable } from 'svelte/store'; function createStore() { const { subscribe, set } = writable(0); return { subscribe, increment: () => set(n => n + 1) }; } incorrectly uses set with a function argument, which is invalid. import { readable } from 'svelte/store'; function createStore() { const { subscribe } = readable(0); return { subscribe, increment: () => {} }; } uses readable which is read-only. import { store } from 'svelte/store'; function createStore() { return { subscribe: () => {}, increment: () => {} }; } imports a non-existent store.

3. Final Answer:

   import { writable } from 'svelte/store'; function createStore() { const { subscribe, update } = writable(0); return { subscribe, increment: () => update(n => n + 1) }; } -> Option A

4. Quick Check:

   Correct syntax uses writable and update() [OK]

Hint: Use writable and update() for custom store methods [OK]

Common Mistakes:

• Using set() with a function argument
• Using readable store when write access is needed
• Importing non-existent store types

3. Given this custom store code, what will be logged after calling counter.increment() twice?

import { writable } from 'svelte/store';
function createCounter() {
  const { subscribe, update } = writable(0);
  return {
    subscribe,
    increment: () => update(n => n + 1)
  };
}
const counter = createCounter();
counter.subscribe(value => console.log(value));
counter.increment();
counter.increment();

medium

A. 0, 1, 2

B. 1, 2

C. 0, 2

D. 2

Solution

1. Step 1: Understand subscription behavior

   Subscribing logs the current value immediately, so 0 is logged first.

2. Step 2: Trace increments and logs

   Each increment increases the value by 1 and triggers the subscriber, logging 1 then 2.

3. Final Answer:

   0, 1, 2 -> Option A

4. Quick Check:

   Subscription logs initial and updates [OK]

Hint: Subscription logs initial value immediately [OK]

Common Mistakes:

• Ignoring initial subscription log
• Assuming increments log only final value
• Confusing update with set behavior

4. Identify the error in this custom store code:

import { writable } from 'svelte/store';
function createTimer() {
  const { subscribe, set } = writable(0);
  let interval = null;
  return {
    subscribe,
    start: () => {
      interval = setInterval(() => set(n + 1), 1000);
    },
    stop: () => clearInterval(interval)
  };
}

medium

A. Not clearing interval on stop

B. Missing update method for incrementing

C. Using set(n + 1) without defining n

D. Incorrect import of writable store

Solution

1. Step 1: Check the setInterval callback

   The callback uses set(n + 1) but n is not defined anywhere in scope.

2. Step 2: Understand correct usage

   To increment the value, update should be used or the current value tracked. Using set with undefined n causes an error.

3. Final Answer:

   Using set(n + 1) without defining n -> Option C

4. Quick Check:

   Undefined variable in set() causes error [OK]

Hint: Use update() or track value before set() [OK]

Common Mistakes:

• Using set() with undefined variable
• Forgetting to clear intervals
• Confusing set and update methods

5. You want to create a custom store that holds a list of items and provides a method to add unique items only (no duplicates). Which implementation correctly achieves this?

hard

A. import { writable } from 'svelte/store'; function createList() { const { subscribe, update } = writable([]); return { subscribe, add: (item) => update(list => [...list, item]) }; }

B. import { writable } from 'svelte/store'; function createList() { const { subscribe, set } = writable([]); return { subscribe, add: (item) => set(list => list.includes(item) ? list : [...list, item]) }; }

C. import { readable } from 'svelte/store'; function createList() { const { subscribe } = readable([]); return { subscribe, add: (item) => {} }; }

D. import { writable } from 'svelte/store'; function createList() { const { subscribe, update } = writable([]); return { subscribe, add: (item) => update(list => list.includes(item) ? list : [...list, item]) }; }

Solution

1. Step 1: Check method to add unique items

   import { writable } from 'svelte/store'; function createList() { const { subscribe, update } = writable([]); return { subscribe, add: (item) => update(list => list.includes(item) ? list : [...list, item]) }; } uses update and checks if the item exists before adding, preventing duplicates.

2. Step 2: Verify other options

   import { writable } from 'svelte/store'; function createList() { const { subscribe, set } = writable([]); return { subscribe, add: (item) => set(list => list.includes(item) ? list : [...list, item]) }; } incorrectly uses set with a function argument, which is invalid. import { readable } from 'svelte/store'; function createList() { const { subscribe } = readable([]); return { subscribe, add: (item) => {} }; } uses readable which is read-only. import { writable } from 'svelte/store'; function createList() { const { subscribe, update } = writable([]); return { subscribe, add: (item) => update(list => [...list, item]) }; } adds items without checking duplicates.

3. Final Answer:

   Custom store with update() and duplicate check -> Option D

4. Quick Check:

   Use update() and check includes() for uniqueness [OK]

Hint: Use update() with includes() check to avoid duplicates [OK]

Common Mistakes:

• Using set() with function argument
• Using readable store when write access needed
• Not checking for duplicates before adding