Context with stores in Svelte

Introduction

Context with stores lets you share data easily between components without passing props everywhere.

You want to share a theme color across many components.

You need a user login status accessible in different parts of your app.

You want to manage a shopping cart state shared by multiple components.

You want to avoid prop drilling when many nested components need the same data.

Syntax

Svelte

import { writable } from 'svelte/store';
import { setContext, getContext } from 'svelte';

// Create a store
const count = writable(0);

// Provide context in a parent component
setContext('key', count);

// Access context in a child component
const count = getContext('key');

setContext shares data down the component tree.

getContext retrieves that data in child components.

Examples

Set a theme store in context to share the current theme.

Svelte

import { writable } from 'svelte/store';
import { setContext } from 'svelte';

const theme = writable('light');
setContext('theme', theme);

Get the theme store from context and reactively log its value.

Svelte

import { getContext } from 'svelte';

const theme = getContext('theme');

$: console.log($theme);

Share a user store from parent to child using context.

Svelte

import { writable } from 'svelte/store';
import { setContext, getContext } from 'svelte';

// Parent.svelte
const user = writable({ name: 'Anna' });
setContext('user', user);

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

$: console.log($user.name);

Sample Program

The parent component creates a count store and shares it via context. It also has a button to increase the count. The child component gets the count store from context and shows its current value. When you click the button, the count updates and the child shows the new value.

Svelte

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

  const count = writable(0);
  setContext('count', count);

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

<button on:click={increment}>Increment</button>
<slot />


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

  const count = getContext('count');
</script>

<p>Count value: {$count}</p>

Important Notes

Context only works between parent and child components, not across unrelated components.

Stores in context stay reactive, so changes update all components using them.

Use unique keys for context to avoid conflicts.

Summary

Context with stores shares reactive data easily between components.

Use setContext in a parent and getContext in children.

This avoids passing props through many layers.

Practice

1. In Svelte, what is the main purpose of using setContext and getContext with stores?

easy

A. To create a new store that is only used inside one component

B. To handle HTTP requests inside components

C. To style components globally using CSS variables

D. To share reactive data easily between components without passing props through many layers

Solution

1. Step 1: Understand the role of setContext and getContext

   These functions allow sharing data between components without manually passing props down the tree.

2. Step 2: Recognize the benefit of using stores with context

   Stores provide reactive data, and context lets multiple components access the same store easily.

3. Final Answer:

   To share reactive data easily between components without passing props through many layers -> Option D

4. Quick Check:

   Context with stores = share reactive data [OK]

Hint: Context shares stores to avoid prop drilling [OK]

Common Mistakes:

• Thinking context creates new stores instead of sharing
• Confusing context with styling or HTTP handling
• Assuming context replaces all props

2. Which of the following is the correct way to provide a store via context in a Svelte parent component?

easy

A. import { getContext } from 'svelte'; getContext('key', store);

B. import { setContext } from 'svelte'; setContext('key', store);

C. import { setContext } from 'svelte'; setContext(store);

D. import { createContext } from 'svelte'; createContext('key', store);

Solution

1. Step 1: Recall the correct import and usage for setting context

   Use setContext from 'svelte' with a key and a value (store).

2. Step 2: Identify the correct syntax

   The correct call is setContext('key', store); to provide the store under a key.

3. Final Answer:

   import { setContext } from 'svelte'; setContext('key', store); -> Option B

4. Quick Check:

   setContext with key and store = import { setContext } from 'svelte'; setContext('key', store); [OK]

Hint: Use setContext with key and store in parent [OK]

Common Mistakes:

• Using getContext instead of setContext to provide
• Omitting the key argument in setContext
• Using non-existent createContext function

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

/* Parent.svelte */

medium

A. 5

B. undefined

C. Error: countStore not found

D. 0

Solution

1. Step 1: Understand the store creation and context setting

   The parent creates a writable store with initial value 5 and sets it in context under 'countStore'.

2. Step 2: Child retrieves the store and subscribes

   The child gets the store from context and subscribes to log its current value, which is 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Store initial value logged = 5 [OK]

Hint: Store initial value logs on subscribe [OK]

Common Mistakes:

• Expecting undefined if context key is wrong
• Confusing setContext and getContext usage
• Assuming subscription logs nothing initially

4. What is the error in this Svelte child component code that tries to get a store from context?

medium

A. Cannot subscribe to context value

B. Using getContext instead of setContext

C. Missing key argument in getContext call

D. Import statement is incorrect

Solution

1. Step 1: Check getContext usage

   getContext requires a key argument to retrieve the correct context value.

2. Step 2: Identify the missing key

   The code calls getContext() without any argument, causing an error.

3. Final Answer:

   Missing key argument in getContext call -> Option C

4. Quick Check:

   getContext needs key argument [OK]

Hint: Always pass key to getContext [OK]

Common Mistakes:

• Calling getContext without key
• Confusing getContext with setContext
• Assuming context value is not subscribable

5. You want to share a writable store with a default value of 0 across multiple nested components using context. Which approach correctly sets and uses this store so that updates in any child reflect everywhere?

hard

A. In parent: setContext('count', writable(0)); In children: const count = getContext('count'); Then use count.update() or count.set()

B. In parent: const count = writable(0); In children: import the store directly and use it

C. In parent: setContext('count', 0); In children: const count = getContext('count'); Then update count directly

D. In parent: setContext('count', writable(0)); In children: const count = getContext('count'); But children cannot update the store

Solution

1. Step 1: Provide a writable store via context in the parent

   Use setContext('count', writable(0)); to share a reactive store.

2. Step 2: Retrieve the store in children and update it

   Children get the store with getContext('count'); and can call update() or set() to change the value, reflecting everywhere.

3. Final Answer:

   In parent: setContext('count', writable(0)); In children: const count = getContext('count'); Then use count.update() or count.set() -> Option A

4. Quick Check:

   Writable store in context updates everywhere [OK]

Hint: Set writable store in context, children get and update it [OK]

Common Mistakes:

• Passing raw value instead of store in context
• Importing store directly instead of using context
• Assuming children cannot update shared store