Development server and HMR in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you save a Svelte component during development?

You have a Svelte component running on the development server with Hot Module Replacement (HMR) enabled. You change the text inside the component and save the file.

What is the expected behavior in the browser?

Svelte

<script>
  let count = 0;
</script>
<button on:click={() => count++}>
  Clicked {count} times
</button>

A Only the changed component updates in the browser, preserving the count state.

B The browser reloads the entire page, resetting the count to zero.

C The browser shows an error because HMR is not supported in Svelte.

D The component does not update until you manually refresh the browser.

💡 Hint

Think about what Hot Module Replacement means for state and UI updates.

With HMR enabled, Svelte updates only the changed component in the browser without a full page reload. This preserves the component's state like the count variable.

📝 Syntax

intermediate

Which command starts the Svelte development server with HMR enabled?

You want to start a Svelte project development server that supports Hot Module Replacement.

Which command should you run?

A npm run build

B npm start

C svelte-hmr serve

D npm run dev

💡 Hint

Check the default scripts in a Svelte project created with Vite.

In a Svelte project using Vite, the npm run dev command starts the development server with HMR enabled by default.

🔧 Debug

advanced

Why does the Svelte component not update after saving changes during development?

You edit a Svelte component and save it, but the browser does not reflect the changes. The development server is running.

What is the most likely cause?

A The browser cache is preventing updates; you need to clear it manually.

B The development server crashed and needs to be restarted.

C HMR is disabled or misconfigured in the project setup.

D Svelte components require a full page reload to update during development.

💡 Hint

Consider the role of HMR in live updates.

If HMR is disabled or not set up correctly, saving changes won't update the component live. The development server may still run, but no hot updates occur.

❓ state_output

advanced

What happens to component state during HMR in Svelte?

Consider a Svelte component with a counter state. You increment the counter to 5, then save a code change with HMR enabled.

What is the value of the counter after the update?

Svelte

<script>
  let count = 0;
</script>
<button on:click={() => count++}>
  Count: {count}
</button>

A The counter resets to 0 because the component reloads fully.

B The counter remains at 5 because HMR preserves component state.

C The counter value becomes undefined due to state loss.

D The counter increments automatically by 1 after update.

💡 Hint

Think about how HMR handles component state during updates.

Svelte's HMR preserves the component's local state during updates, so the counter stays at 5 after saving changes.

🧠 Conceptual

expert

Which statement best describes the role of the development server in Svelte with HMR?

Choose the statement that correctly explains how the development server and Hot Module Replacement work together in a Svelte project.

A The development server watches files, recompiles changed modules, and HMR injects updates into the running app preserving state.

B The development server disables caching and forces full reloads on every file save to simulate HMR.

C The development server only serves the app; HMR is handled by the browser independently without recompilation.

D The development server compiles Svelte code once and serves static files; HMR reloads the entire page on changes.

💡 Hint

Consider how live updates happen without full reloads.

The development server watches for file changes and recompiles only the changed modules. HMR then injects these updates into the running app, preserving the app state and avoiding full reloads.

Practice

1. What is the main purpose of the development server in a Svelte project?

easy

A. To compile the app into a production build

B. To minify the app's JavaScript files

C. To deploy the app to a web server

D. To show your app live and update it as you code

Solution

1. Step 1: Understand the role of the development server

   The development server runs your app locally and shows live updates as you make changes.

2. Step 2: Differentiate from production tasks

   Compiling for production, deploying, and minifying are separate steps done after development.

3. Final Answer:

   To show your app live and update it as you code -> Option D

4. Quick Check:

   Development server = live updates [OK]

Hint: Development server = live preview while coding [OK]

Common Mistakes:

• Confusing development server with production build
• Thinking it deploys the app
• Assuming it minifies code automatically

2. Which command starts the Svelte development server with Hot Module Replacement (HMR)?

easy

A. npm run build

B. npm run dev

C. npm start

D. npm run serve

Solution

1. Step 1: Identify the standard Svelte dev command

   In Svelte projects, npm run dev starts the development server with HMR enabled.

2. Step 2: Understand other commands

   npm run build creates production files; npm start and npm run serve are not standard for Svelte dev server.

3. Final Answer:

   npm run dev -> Option B

4. Quick Check:

   Start dev server = npm run dev [OK]

Hint: Use npm run dev to start Svelte dev server with HMR [OK]

Common Mistakes:

• Using npm run build instead of dev
• Confusing npm start with npm run dev
• Trying npm run serve which is not default

3. Given this Svelte component running with HMR, what happens when you change the text inside the <p> tag and save the file?

medium

A. Nothing happens until you manually refresh the browser

B. The entire page reloads and the app state resets

C. Only the changed text updates instantly without full reload

D. The app crashes due to hot reload errors

Solution

1. Step 1: Understand Hot Module Replacement (HMR) behavior

   HMR updates only the changed parts of the app instantly without reloading the whole page.

2. Step 2: Apply to text change in <p> tag

   Changing text inside the paragraph triggers HMR to update that text immediately, preserving app state.

3. Final Answer:

   Only the changed text updates instantly without full reload -> Option C

4. Quick Check:

   HMR updates changed parts instantly [OK]

Hint: HMR updates only changed parts, no full reload [OK]

Common Mistakes:

• Thinking full page reload happens on every change
• Believing manual refresh is needed
• Assuming app crashes on text change

4. You run npm run dev but the browser does not update when you change your Svelte files. What is the most likely cause?

medium

A. The development server is not running or crashed

B. You forgot to save the file after editing

C. You used npm run build instead of npm run dev

D. Your browser cache is disabled

Solution

1. Step 1: Check if development server is active

   If the dev server is not running or crashed, no updates or HMR can happen.

2. Step 2: Evaluate other options

   Not saving files prevents changes but usually obvious; using build command doesn't start dev server; browser cache disabled does not stop updates.

3. Final Answer:

   The development server is not running or crashed -> Option A

4. Quick Check:

   Dev server running = updates work [OK]

Hint: Check if dev server is running when no updates appear [OK]

Common Mistakes:

• Assuming browser cache blocks updates
• Not saving files before expecting changes
• Confusing build command with dev server

5. You want to keep your app state while updating styles during development. Which feature of the Svelte development server helps you achieve this?

hard

A. Hot Module Replacement (HMR)

B. Full page reload on every change

C. Production build optimization

D. Manual browser refresh

Solution

1. Step 1: Understand state preservation during updates

   HMR updates only changed modules without reloading the whole page, preserving app state like variables and UI state.

2. Step 2: Contrast with other options

   Full reload resets state; production build is unrelated to live updates; manual refresh loses state.

3. Final Answer:

   Hot Module Replacement (HMR) -> Option A

4. Quick Check:

   Keep state during updates = HMR [OK]

Hint: HMR preserves app state during live updates [OK]

Common Mistakes:

• Thinking full reload keeps state
• Confusing production build with dev features
• Relying on manual refresh to keep state