Context API vs stores in Svelte

Introduction

Context API and stores help share data between components in Svelte without passing props everywhere.

You want to share a theme or user info across many components.

You need a simple way to share data only between parent and child components.

You want a global state that many parts of your app can read and update.

You want to avoid prop drilling through many layers of components.

You want reactive data that updates UI automatically when changed.

Syntax

Svelte

/* Context API in Svelte */
// In parent component
import { setContext } from 'svelte';
setContext('key', value);

// In child component
import { getContext } from 'svelte';
const value = getContext('key');

/* Stores in Svelte */
import { writable } from 'svelte/store';
const count = writable(0);

// In any component
import { count } from './store.js';
$count; // reactive value
count.set(5); // update value

Context API shares data only between parent and its descendants.

Stores can be imported anywhere and provide reactive shared state.

Examples

Parent sets a user object in context. Child reads it directly without props.

Svelte

// Using Context API
// Parent.svelte
<script>
  import { setContext } from 'svelte';
  setContext('user', { name: 'Anna' });
</script>

// Child.svelte
<script>
  import { getContext } from 'svelte';
  const user = getContext('user');
</script>
<p>User: {user.name}</p>

Store holds count value. Any component can import and react to changes.

Svelte

// Using a writable store
// store.js
import { writable } from 'svelte/store';
export const count = writable(0);

// Counter.svelte
<script>
  import { count } from './store.js';
</script>
<button on:click={() => count.update(n => n + 1)}>
  Count: {$count}
</button>

Sample Program

This example shows how Context API passes a color string from parent to child. The store shares a message string that child can update and reactively display.

Svelte

<!-- App.svelte -->
<script>
  import { setContext } from 'svelte';
  import Child from './Child.svelte';
  import { writable } from 'svelte/store';

  // Using Context API
  setContext('color', 'blue');

  // Using Store
  export const message = writable('Hello from store!');
</script>
<h1>Parent Component</h1>
<Child />

<!-- Child.svelte -->
<script>
  import { getContext } from 'svelte';
  import { message } from './App.svelte';

  const color = getContext('color');
</script>
<p style="color: {color}">Context color: {color}</p>
<p>Store message: {$message}</p>
<button on:click={() => message.set('Updated message!')}>Update Store</button>

Important Notes

Context API is good for passing data down a component tree without props.

Stores are better for global or shared reactive state accessible anywhere.

Use stores when many components need to read and write the same data.

Summary

Context API shares data only between parent and children components.

Stores provide reactive global state accessible from any component.

Choose Context API for simple parent-child data sharing, stores for app-wide state.

Practice

1. Which statement best describes the main difference between Svelte's Context API and stores?

easy

A. Context API is used for styling components, stores manage component lifecycle.

B. Stores are only for passing data from parent to child, Context API is for global state.

C. Context API shares data only between parent and child components, while stores provide global reactive state accessible anywhere.

D. Stores and Context API are identical in functionality and usage.

Solution

1. Step 1: Understand Context API scope

   Context API shares data only between parent and its children components, not globally.

2. Step 2: Understand stores scope

   Stores provide reactive global state accessible from any component in the app.

3. Final Answer:

   Context API shares data only between parent and child components, while stores provide global reactive state accessible anywhere. -> Option C

4. Quick Check:

   Context API = parent-child, stores = global state [OK]

Hint: Context API is local parent-child, stores are global [OK]

Common Mistakes:

• Thinking stores only pass data down like props
• Confusing Context API with global state management
• Assuming stores are for styling or lifecycle

2. Which of the following is the correct way to set a value in a Svelte writable store?

easy

A. store.set(newValue);

B. store.value = newValue;

C. store.update(newValue);

D. store.change(newValue);

Solution

1. Step 1: Recall writable store methods

   Svelte writable stores have a set() method to assign a new value.

2. Step 2: Check other options

   store.value is not valid, update() takes a function, change() does not exist.

3. Final Answer:

   store.set(newValue); -> Option A

4. Quick Check:

   Use set() to assign store value [OK]

Hint: Use store.set(value) to update writable stores [OK]

Common Mistakes:

• Trying to assign store.value directly
• Using update() with a value instead of a function
• Using non-existent methods like change()

3. Given this Svelte code using Context API:

import { setContext, getContext } from 'svelte';

// Parent.svelte
setContext('color', 'blue');

// Child.svelte
const color = getContext('color');

What will color be inside Child.svelte?

medium

A. Throws an error

B. undefined

C. null

D. 'blue'

Solution

1. Step 1: Understand setContext and getContext

   setContext('color', 'blue') sets a value accessible to child components via getContext('color').

2. Step 2: Check child access

   Child.svelte calls getContext('color') and receives 'blue' because it is a child of the parent setting the context.

3. Final Answer:

   'blue' -> Option D

4. Quick Check:

   Context API passes 'blue' from parent to child [OK]

Hint: getContext returns parent's setContext value [OK]

Common Mistakes:

• Expecting undefined if not wrapped properly
• Confusing context keys or forgetting to setContext
• Assuming context works globally like stores

4. What is wrong with this Svelte store usage?

import { writable } from 'svelte/store';

const count = writable(0);

function increment() {
  count = count + 1;
}

medium

A. writable stores cannot hold numbers.

B. You cannot reassign the store variable; must use count.update() or count.set().

C. The import statement is incorrect.

D. The function increment should be async.

Solution

1. Step 1: Identify store usage error

   The code tries to do count = count + 1, which reassigns the store variable instead of updating its value.

2. Step 2: Correct way to update store

   Use count.update(n => n + 1) or count.set(newValue) to change the store's value reactively.

3. Final Answer:

   You cannot reassign the store variable; must use count.update() or count.set(). -> Option B

4. Quick Check:

   Update store with update() or set(), not reassignment [OK]

Hint: Use update() or set(), never reassign store variable [OK]

Common Mistakes:

• Trying to assign store variable directly
• Ignoring reactive update methods
• Thinking writable stores can't hold numbers

5. You want to share a theme color across your entire Svelte app and reactively update it from any component. Which approach is best and why?

hard

A. Use a writable store for the theme color because stores provide reactive global state accessible anywhere.

B. Use Context API because it automatically updates all components globally.

C. Pass the theme color as a prop to every component manually.

D. Use a local variable in each component to store the theme color.

Solution

1. Step 1: Identify requirement for global reactive state

   The theme color must be shared app-wide and update reactively from any component.

2. Step 2: Compare Context API and stores

   Context API only shares data between parent and children, not globally. Stores provide reactive global state accessible anywhere.

3. Final Answer:

   Use a writable store for the theme color because stores provide reactive global state accessible anywhere. -> Option A

4. Quick Check:

   Global reactive state needs stores, not Context API [OK]

Hint: Global reactive state? Use stores, not Context API [OK]

Common Mistakes:

• Choosing Context API for global state
• Passing props everywhere manually
• Using local variables that don't sync