Context key patterns in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a context value in a Svelte component.

Svelte

<script>
  import { setContext } from 'svelte';
  const key = 'user';
  const user = { name: 'Alice' };
  setContext([1], user);
</script>

Drag options to blanks, or click blank then click option'

AcontextKey

B'user'

Ckey

DUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string literal instead of the variable key.

Using an undefined variable as the key.

You use the variable key as the context key when calling setContext.

2fill in blank

medium

Complete the code to get a context value in a Svelte component.

Svelte

<script>
  import { getContext } from 'svelte';
  const key = 'theme';
  const theme = getContext([1]);
</script>

Drag options to blanks, or click blank then click option'

Akey

BcontextKey

CTheme

D'theme'

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string literal instead of the variable.

Using an undefined variable.

You retrieve the context value using the variable key which holds the context key.

3fill in blank

hard

Fix the error in the code to correctly share context between components.

Svelte

<script>
  import { setContext, getContext } from 'svelte';
  const key = Symbol('settings');
  setContext(key, { darkMode: true });
  const settings = getContext([1]);
</script>

Drag options to blanks, or click blank then click option'

Akey

B'settings'

CSymbol('settings')

DsettingsKey

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of the Symbol variable.

Creating a new Symbol in getContext.

The same key variable holding the Symbol must be used in both setContext and getContext.

4fill in blank

hard

Fill both blanks to create and use a unique context key with Symbol.

Svelte

<script>
  import { setContext, getContext } from 'svelte';
  const [1] = [2]('auth');
  setContext(authKey, { loggedIn: false });
  const auth = getContext(authKey);
</script>

Drag options to blanks, or click blank then click option'

AauthKey

BSymbol

Ckey

DcreateSymbol

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of Symbol.

Using an undefined function like createSymbol.

Use authKey as the variable name and Symbol to create a unique key.

5fill in blank

hard

Fill all three blanks to set and get context using a constant key string.

Svelte

<script>
  import { setContext, getContext } from 'svelte';
  const [1] = '[2]';
  setContext([3], { language: 'en' });
  const lang = getContext(langKey);
</script>

Drag options to blanks, or click blank then click option'

AlangKey

Blanguage

Dlocale

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for key.

Using inconsistent strings for the key.

Define langKey as the constant key variable, assign it the string language, and use langKey in setContext.

Practice

1. What is the main reason to use Symbol as a context key in Svelte?

easy

A. To ensure the key is unique and avoid conflicts between components

B. To make the key a string for easier debugging

C. To allow the key to be reused as a CSS class name

D. To automatically serialize the key for storage

Solution

1. Step 1: Understand context keys purpose

   Context keys identify data shared between components without props.

2. Step 2: Why use Symbol?

   Symbols are unique values, preventing accidental key collisions.

3. Final Answer:

   To ensure the key is unique and avoid conflicts between components -> Option A

4. Quick Check:

   Unique keys = Symbol usage [OK]

Hint: Symbols guarantee unique keys, preventing conflicts [OK]

Common Mistakes:

• Using strings that can clash with other keys
• Thinking keys must be strings for context
• Reusing keys across unrelated components

2. Which of the following is the correct way to create and export a context key in Svelte?

easy

A. export let key = new Symbol('myKey');

B. export const key = 'myKey';

C. const key = Symbol('myKey'); export default key;

D. export const key = Symbol('myKey');

Solution

1. Step 1: Check correct Symbol creation

   Symbols are created with Symbol('description'), not with new.

2. Step 2: Check export syntax

   Exporting a constant with export const key = Symbol('myKey'); is correct and common.

3. Final Answer:

   export const key = Symbol('myKey'); -> Option D

4. Quick Check:

   Correct Symbol export = export const key = Symbol('myKey'); [OK]

Hint: Use export const with Symbol() without new keyword [OK]

Common Mistakes:

• Using string instead of Symbol for key
• Using new Symbol() which is invalid
• Exporting default instead of named export

3. Given this code snippet, what will be logged in the Child.svelte component?

/* key.js */
export const userKey = Symbol('user');

/* Parent.svelte */
import { setContext } from 'svelte';
import { userKey } from './key.js';

setContext(userKey, { name: 'Alice' });

/* Child.svelte */
import { getContext } from 'svelte';
import { userKey } from './key.js';

const user = getContext(userKey);
console.log(user.name);

medium

A. Alice

B. undefined

C. Error: userKey is not defined

D. null

Solution

1. Step 1: Understand setContext and getContext usage

   Parent sets context with key userKey and value { name: 'Alice' }.

2. Step 2: Child retrieves context using same key

   Child imports userKey and calls getContext(userKey), getting the object.

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   Matching keys return value = Alice [OK]

Hint: Matching Symbol keys share data correctly [OK]

Common Mistakes:

• Using different keys causing undefined
• Forgetting to import the shared key
• Expecting string keys instead of Symbol keys

4. Identify the error in this Svelte code using context keys:

/* key.js */
export const themeKey = Symbol('theme');

/* Parent.svelte */
import { setContext } from 'svelte';
import { themeKey } from './key.js';

setContext('themeKey', 'dark');

/* Child.svelte */
import { getContext } from 'svelte';
import { themeKey } from './key.js';

const theme = getContext(themeKey);
console.log(theme);

medium

A. Forgetting to import setContext in Parent

B. Using string 'themeKey' instead of Symbol key in setContext

C. Using getContext before setContext is called

D. Not exporting themeKey from key.js

Solution

1. Step 1: Check setContext key usage

   Parent uses string 'themeKey' instead of the Symbol themeKey imported from key.js.

2. Step 2: Understand key mismatch effect

   Child uses Symbol key, so getContext returns undefined because keys differ.

3. Final Answer:

   Using string 'themeKey' instead of Symbol key in setContext -> Option B

4. Quick Check:

   Key mismatch causes undefined context [OK]

Hint: Always use the same Symbol key in setContext and getContext [OK]

Common Mistakes:

• Mixing string and Symbol keys
• Assuming string keys work like Symbols
• Not importing the shared key properly

5. You want to share a theme object between multiple nested components using context keys. Which pattern is best to avoid key conflicts and ensure consistent usage?

hard

A. Create a new Symbol key in each component separately

B. Use a plain string key like 'theme' directly in setContext and getContext calls

C. Define and export a unique Symbol key in a separate file, then import it in all components

D. Use a global variable without context to share the theme

Solution

1. Step 1: Understand key uniqueness and sharing

   Using a shared Symbol key exported from one file ensures all components use the exact same key.

2. Step 2: Avoid conflicts and inconsistencies

   Using strings or separate Symbols risks conflicts or mismatches causing bugs.

3. Final Answer:

   Define and export a unique Symbol key in a separate file, then import it in all components -> Option C

4. Quick Check:

   Shared Symbol key file = consistent context [OK]

Hint: Keep keys in one file, import everywhere to avoid conflicts [OK]

Common Mistakes:

• Using different keys in different components
• Relying on strings that may clash
• Avoiding context and using globals instead