Context vs stores decision in Svelte

Introduction

Context and stores help share data in Svelte apps without passing props everywhere. Choosing between them makes your app simpler and easier to manage.

You want to share data only between a few related components deep in the tree.

You need global data accessible by many components across your app.

You want reactive data that updates UI automatically when changed.

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

You want to keep some data private to a part of your app.

Syntax

Svelte

/* Context API */
import { setContext, getContext } from 'svelte';

// In parent component
setContext('key', value);

// In child component
const value = getContext('key');

/* Store API */
import { writable } from 'svelte/store';

const count = writable(0);

// In any component
count.subscribe(value => { /* react to value */ });
count.set(5);

Context is for passing data down the component tree without props.

Stores hold reactive data accessible anywhere by importing them.

Examples

Context shares the user object only with children components.

Svelte

// Using context
// Parent.svelte
<script>
import { setContext } from 'svelte';

setContext('user', { name: 'Anna' });
</script>

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

const user = getContext('user');
console.log(user.name);
</script>

Store shares reactive count value globally across components.

Svelte

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

// AnyComponent.svelte
<script>
import { count } from './store.js';

count.subscribe(value => console.log(value));
count.set(10);
</script>

Sample Program

This example shows how context passes the user object from Parent to Child without props.

Svelte

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

  const user = { name: 'Sam' };
  setContext('user', user);
</script>

<h1>Parent Component</h1>
<Child />

<!-- Child.svelte -->
<script>
  import { getContext } from 'svelte';
  const user = getContext('user');
</script>

<p>Hello, {user.name}!</p>

Important Notes

Use context when data is only needed by child components in a specific part of the tree.

Use stores for app-wide reactive data shared by many components.

Stores can be updated and subscribed to anywhere by importing them.

Summary

Context shares data down the component tree without props.

Stores hold reactive data accessible globally by importing.

Choose context for local sharing, stores for global reactive data.

Practice

1. In Svelte, when should you use context instead of stores?

easy

A. To manage asynchronous data fetching

B. To share data only between a parent and its child components locally

C. To share reactive data globally across unrelated components

D. To create CSS styles scoped to a component

Solution

1. Step 1: Understand context usage

   Context is designed to share data locally down the component tree without passing props explicitly.

2. Step 2: Compare with stores

   Stores are for global reactive data accessible anywhere by importing, not limited to parent-child.

3. Final Answer:

   To share data only between a parent and its child components locally -> Option B

4. Quick Check:

   Context = local sharing [OK]

Hint: Context is for local, stores for global reactive data [OK]

Common Mistakes:

• Using stores for local-only data sharing
• Confusing context with global state management
• Thinking context is reactive globally
• Using context for styling or async tasks

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

easy

A. import { createContext } from 'svelte'; createContext('key', value);

B. import { useContext } from 'svelte'; useContext('key', value);

C. import { setContext } from 'svelte'; setContext('key', value);

D. import { setStore } from 'svelte'; setStore('key', value);

Solution

1. Step 1: Recall Svelte context API

   Svelte uses setContext to provide a value to child components.

2. Step 2: Check import and usage

   The correct import is from 'svelte' and the function is setContext with key and value.

3. Final Answer:

   import { setContext } from 'svelte'; setContext('key', value); -> Option C

4. Quick Check:

   setContext = provide context [OK]

Hint: Use setContext to provide data down the tree [OK]

Common Mistakes:

• Using createContext or useContext which are React terms
• Trying to set stores with setContext
• Wrong import paths or function names
• Confusing setContext with reactive stores

3. Given this Svelte 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, 5]

D. [1, 5, 6]

Solution

1. Step 1: Understand initial subscription call

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

2. Step 2: Trace set and update calls

   Then count.set(5) triggers callback with 5, and count.update(n => n + 1) triggers callback with 6.

3. Final Answer:

   [0, 5, 6] -> Option A

4. Quick Check:

   Subscription calls on current and changes [OK]

Hint: Subscription logs current value immediately [OK]

Common Mistakes:

• Ignoring initial subscription call
• Mixing order of set and update
• Assuming update starts from 0 after set
• Confusing subscribe behavior with events

4. What is wrong with this Svelte code snippet that tries to share data using context?

import { getContext } from 'svelte';

const value = getContext('user');

console.log(value);

medium

A. It tries to get context outside a component setup, causing undefined

B. It uses getContext instead of setContext to provide data

C. The key 'user' must be an object, not a string

D. getContext requires two arguments, key and default value

Solution

1. Step 1: Check context usage location

   getContext must be called inside a component's script during initialization, not in a module scope.

2. Step 2: Understand consequence

   Calling it outside component setup returns undefined because no context is set yet.

3. Final Answer:

   It tries to get context outside a component setup, causing undefined -> Option A

4. Quick Check:

   getContext only works inside component setup [OK]

Hint: Use getContext only inside component scripts [OK]

Common Mistakes:

• Calling getContext in module scope
• Confusing getContext with setContext usage
• Assuming string keys must be objects
• Expecting getContext to have default value argument

5. You want to share a reactive theme color across many unrelated components in a Svelte app. Which approach is best and why?

hard

A. Use a global variable for shared data

B. Use context API for local tree sharing

C. Pass props through all components manually

D. Use a writable store for global reactive data

Solution

1. Step 1: Identify sharing scope

   The theme color must be shared globally across unrelated components, not just parent-child.

2. Step 2: Choose reactive global sharing

   Writable stores provide reactive global state accessible by importing anywhere.

3. Step 3: Evaluate other options

   Context is local only, props are tedious for many components, global variables lack reactivity.

4. Final Answer:

   Use a writable store for global reactive data -> Option D

5. Quick Check:

   Stores = global reactive sharing [OK]

Hint: Use stores for global reactive data, context for local only [OK]

Common Mistakes:

• Using context for global sharing
• Passing props through many unrelated components
• Using global variables without reactivity
• Ignoring reactivity needs for theme changes