Context API vs stores in Svelte - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

When to use Context API vs Stores in Svelte?

In Svelte, both Context API and stores help share data. Which situation best fits using the Context API instead of stores?

A Replacing all local component state with a single global store

B Sharing reactive data globally across many unrelated components

C Managing global state that updates frequently and triggers UI changes everywhere

D Sharing data deeply between nested components without needing reactivity outside those components

💡 Hint

Think about when you want to avoid global reactivity and only pass data down the component tree.

The Context API is best for passing data deeply down the component tree without making it reactive globally. Stores are better for global reactive state.

❓ component_behavior

intermediate

What happens when a Svelte store updates?

Given a Svelte writable store used in multiple components, what happens when the store's value changes?

Svelte

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

// Component A and Component B both subscribe to count

count.set(5);

A Only the component that called set() re-renders

B No components re-render until manually refreshed

C All components subscribed to the store re-render with the new value

D Only components that read the store inside onMount re-render

💡 Hint

Stores in Svelte are reactive and notify all subscribers on change.

When a store updates, all components subscribed to it automatically update their UI to reflect the new value.

🔧 Debug

advanced

Why does this Context API example fail to share data?

Consider this Svelte code using Context API. Why does the child component not receive the context value?

Svelte

<script>
  import { setContext, getContext } from 'svelte';

  // Parent.svelte
  setContext('user', { name: 'Alice' });
</script>

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

A Context API only works with stores, not plain objects

B setContext must be called inside a component script, not at module top-level

C getContext key must be a Symbol, not a string

D Child component must import setContext instead of getContext

💡 Hint

Check where setContext is called in the component lifecycle.

setContext must be called inside the component's script block (not at module top-level) so it runs during component initialization and sets context properly.

📝 Syntax

advanced

Which Svelte store syntax correctly creates a readable store with a cleanup function?

Choose the correct syntax to create a readable store that starts a timer and cleans up on unsubscribe.

A

import { readable } from 'svelte/store';
const time = readable(new Date(), set => {
  const interval = setInterval(() => set(new Date()), 1000);
  return () => clearInterval(interval);
});

B

import { writable } from 'svelte/store';
const time = writable(new Date(), set => {
  const interval = setInterval(() => set(new Date()), 1000);
  return () => clearInterval(interval);
});

C

import { readable } from 'svelte/store';
const time = readable(new Date(), () => {
  const interval = setInterval(() => set(new Date()), 1000);
  return () => clearInterval(interval);
});

D

import { readable } from 'svelte/store';
const time = readable(new Date(), set => {
  setInterval(() => set(new Date()), 1000);
});

💡 Hint

Remember the readable store callback receives a set function and must return a cleanup function.

Option A correctly uses readable with a set callback and returns a cleanup function to clear the interval.

❓ state_output

expert

What is the output after updating a nested store value in Svelte?

Given this Svelte writable store holding an object, what is the output after updating a nested property incorrectly?

Svelte

import { writable } from 'svelte/store';

const user = writable({ name: 'Bob', age: 30 });

user.update(u => {
  u.age = 31;
  return u;
});

let currentAge;
user.subscribe(value => currentAge = value.age)();

console.log(currentAge);

A 31

B 30

C undefined

D Throws a TypeError

💡 Hint

Consider how Svelte detects changes in store values and if mutating nested objects works.

Even though the object is mutated, returning the same object triggers subscribers. So currentAge becomes 31.