Writable stores in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ state_output

intermediate

What is the output of this writable store update?

Consider this Svelte writable store code. What will be the final value of count after the update?

Svelte

import { writable } from 'svelte/store';

const count = writable(0);

count.update(n => n + 5);
count.update(n => n * 2);

let finalValue;
count.subscribe(value => finalValue = value);

A 10

B 5

C 0

D 15

💡 Hint

Remember that each update uses the current store value as input.

The store starts at 0. First update adds 5, so value is 5. Second update multiplies by 2, so final value is 10.

❓ component_behavior

intermediate

How does this Svelte component react to writable store changes?

Given this Svelte component using a writable store, what will be displayed after clicking the button twice?

Svelte

<script>
  import { writable } from 'svelte/store';
  const clicks = writable(0);
  function increment() {
    clicks.update(n => n + 1);
  }
</script>

<button on:click={increment}>Click me</button>
<p>Clicks: {$clicks}</p>

A Clicks: 1

B Clicks: 0

C Clicks: 2

D Clicks: undefined

💡 Hint

The $ prefix subscribes to the store and updates the UI automatically.

Each click calls increment which adds 1 to the store. After two clicks, the value is 2 and displayed.

📝 Syntax

advanced

Which option correctly creates a writable store with a custom set method?

You want to create a writable store that logs every time its value changes. Which code snippet is correct?

A

import { writable } from 'svelte/store';

function loggerStore(initial) {
  const { subscribe, set, update } = writable(initial);
  return {
    subscribe,
    set(value) {
      console.log('Setting:', value);
      update(value);
    },
    update
  };
}

B

import { writable } from 'svelte/store';

function loggerStore(initial) {
  const { subscribe, set, update } = writable(initial);
  return {
    subscribe,
    set(value) {
      console.log('Setting:', value);
      return set(value);
    },
    update
  };
}

C

import { writable } from 'svelte/store';

function loggerStore(initial) {
  const { subscribe, set, update } = writable(initial);
  return {
    subscribe,
    set(value) {
      console.log('Setting:', value);
      this.set(value);
    },
    update
  };
}

D

import { writable } from 'svelte/store';

function loggerStore(initial) {
  const { subscribe, set, update } = writable(initial);
  return {
    subscribe,
    set: (value) => {
      console.log('Setting:', value);
      set(value);
    },
    update
  };
}

💡 Hint

The custom set method must call the original set function to update the store.

Option D correctly overrides set as an arrow function that logs and calls the original set. Option D uses a method shorthand but calls set recursively causing a stack overflow. Option D returns set's result which is undefined but not an error. Option D calls update with a value instead of a function, causing a runtime error.

🔧 Debug

advanced

Why does this writable store subscription not update the UI?

This Svelte component subscribes to a writable store but the UI does not update. What is the cause?

Svelte

<script>
  import { writable } from 'svelte/store';
  const data = writable(0);

  let value;
  data.subscribe(val => {
    value = val;
  });

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

<button on:click={increment}>Increment</button>
<p>Value: {value}</p>

A The store is not updated correctly because update function is missing a return statement.

B The subscription does not trigger reactive updates because 'value' is not a reactive variable.

C The subscribe callback is never called because the store is not initialized properly.

D The button click handler is not connected to the increment function.

💡 Hint

In Svelte, variables assigned inside subscribe callbacks do not trigger UI updates unless reactive.

The variable 'value' is assigned inside subscribe but is not reactive. The UI does not update because Svelte does not track changes to plain variables outside reactive declarations or $store syntax.

🧠 Conceptual

expert

What happens when multiple components share and update the same writable store?

Two Svelte components import the same writable store and update it independently. What is the expected behavior?

A Both components see the same store value and updates from one component reflect immediately in the other.

B Each component has its own copy of the store, so updates do not affect the other component.

C Updates in one component cause errors in the other due to conflicting subscriptions.

D The store resets to its initial value whenever a component updates it.

💡 Hint

Writable stores are singletons when imported from the same module.

Writable stores are shared instances. When multiple components import the same store, they share the same state. Updates in one component update the store and notify all subscribers, so other components see the change immediately.