Why stores manage shared state in Svelte

Introduction

Stores help keep data in one place so many parts of your app can use it easily. This makes your app simpler and faster to update.

When multiple components need to see or change the same data

When you want to keep data updated everywhere without extra work

When you want to avoid passing data through many layers of components

When you want a simple way to share user settings or app status

When you want to react to data changes automatically in your UI

Syntax

Svelte

import { writable } from 'svelte/store';

const count = writable(0);

// To update:
count.set(5);
// To subscribe:
const unsubscribe = count.subscribe(value => {
  console.log(value);
});

writable creates a store you can read and write.

Use subscribe to react to changes in the store.

Examples

Create a store holding a name string.

Svelte

import { writable } from 'svelte/store';

const name = writable('Alice');

Change the store value to 'Bob'.

Svelte

name.set('Bob');

Listen for changes and log the current name.

Svelte

const unsubscribe = name.subscribe(value => {
  console.log('Name is', value);
});

Sample Program

This Svelte component uses a store to keep a count number. Clicking the button increases the count. The count is shared through the store and shown in the paragraph.

Svelte

<script>
  import { writable } from 'svelte/store';

  // Create a store to hold a number
  export const count = writable(0);

  // Function to increase count
  function increment() {
    count.update(n => n + 1);
  }
</script>

<button on:click={increment} aria-label="Increase count">Increase</button>
<p>Count: {$count}</p>

Important Notes

Stores automatically update all parts of your app that use them.

Use $storeName syntax in Svelte components to read store values reactively.

Stores help avoid complex data passing between components.

Summary

Stores keep shared data in one place for easy access.

They update all parts of the app automatically when data changes.

Using stores makes your app simpler and easier to maintain.

Practice

1. Why do Svelte stores help manage shared state in an app?

easy

A. They store data only for one component at a time.

B. They make the app run faster by skipping updates.

C. They keep data in one place and update all parts automatically.

D. They prevent any data changes after initialization.

Solution

1. Step 1: Understand the role of stores

   Svelte stores hold shared data centrally so multiple components can access it easily.

2. Step 2: Recognize automatic updates

   When the store data changes, all components using it update automatically, keeping UI in sync.

3. Final Answer:

   They keep data in one place and update all parts automatically. -> Option C

4. Quick Check:

   Shared data + auto update = A [OK]

Hint: Stores centralize data and auto-update components [OK]

Common Mistakes:

• Thinking stores only speed up the app
• Believing stores hold data per component only
• Assuming stores freeze data after setup

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

easy

A. import writable from 'svelte'; const count = writable(0);

B. import { writable } from 'svelte/store'; const count = store(0);

C. import { store } from 'svelte/store'; const count = store(0);

D. import { writable } from 'svelte/store'; const count = writable(0);

Solution

1. Step 1: Check import syntax

   The writable store is imported from 'svelte/store' using curly braces around writable.

2. Step 2: Verify store creation

   Creating a writable store uses writable(initialValue), so writable(0) is correct.

3. Final Answer:

   import { writable } from 'svelte/store'; const count = writable(0); -> Option D

4. Quick Check:

   Correct import + writable() = B [OK]

Hint: Use curly braces to import writable from 'svelte/store' [OK]

Common Mistakes:

• Importing writable without braces
• Using 'store' instead of 'writable'
• Calling store() instead of writable()

3. Given this Svelte store code, what will be logged to the console?

import { writable } from 'svelte/store';
const count = writable(0);
count.subscribe(value => console.log(value));
count.set(5);
count.update(n => n + 1);

medium

A. 0, 5, 6

B. 5, 6

C. 0, 1, 6

D. 0, 5, 5

Solution

1. Step 1: Understand subscription initial call

   Subscribing to a writable store immediately calls the callback with the current value, so 0 is logged first.

2. Step 2: Follow set and update calls

   count.set(5) changes value to 5, logging 5. Then count.update(n => n + 1) adds 1, logging 6.

3. Final Answer:

   0, 5, 6 -> Option A

4. Quick Check:

   Subscribe logs initial + set + update values = A [OK]

Hint: Subscribe logs current value immediately, then on changes [OK]

Common Mistakes:

• Ignoring initial subscription call
• Thinking update replaces value instead of adding
• Missing the last logged value

4. What is wrong with this Svelte store usage?

import { writable } from 'svelte/store';
const user = writable({ name: 'Alice' });

// In component
user.name = 'Bob';

medium

A. The store must be declared with let, not const.

B. Directly changing user.name does not update the store value.

C. Writable stores cannot hold objects as values.

D. The store should be imported from 'svelte' not 'svelte/store'.

Solution

1. Step 1: Identify how to update store values

   Writable stores require using set() or update() to change their value, not direct property assignment.

2. Step 2: Explain why direct property change fails

   Changing user.name directly modifies the object but does not notify the store, so subscribers won't update.

3. Final Answer:

   Directly changing user.name does not update the store value. -> Option B

4. Quick Check:

   Use set() or update() to change store values [OK]

Hint: Always use set() or update() to change store data [OK]

Common Mistakes:

• Trying to mutate store value properties directly
• Importing writable incorrectly
• Thinking const prevents store updates

5. You want to share a counter value across multiple components and update it from any of them. Which approach best uses Svelte stores to achieve this?

hard

A. Create a writable store in a separate file and import it in all components.

B. Pass the counter value as a prop from parent to child components only.

C. Use local variables in each component and sync them manually.

D. Create a store inside each component separately.

Solution

1. Step 1: Understand shared state needs

   To share and update data across components, a single source of truth is needed.

2. Step 2: Evaluate store usage

   Creating a writable store in one file and importing it everywhere ensures all components share and react to the same data.

3. Step 3: Compare other options

   Passing props only works top-down and is cumbersome for many components; local variables are isolated; separate stores cause desync.

4. Final Answer:

   Create a writable store in a separate file and import it in all components. -> Option A

5. Quick Check:

   Single shared store file = C [OK]

Hint: Use one store file imported by all components [OK]

Common Mistakes:

• Trying to sync local variables manually
• Passing props for deeply nested sharing
• Creating multiple stores causing inconsistent state