Development server and HMR in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a development server in Svelte?

A development server runs your Svelte app locally, letting you see changes instantly in the browser as you code. It helps you test and develop without building the full app every time.

beginner

What does HMR stand for and what does it do?

HMR means Hot Module Replacement. It updates only the changed parts of your Svelte app in the browser without a full page reload, keeping your app state intact and speeding up development.

intermediate

How does HMR improve the developer experience?

HMR saves time by instantly showing changes without reloading the whole page. This keeps your app's current state, so you don’t lose data or have to navigate back to where you were.

beginner

Which command starts the Svelte development server with HMR enabled?

Running npm run dev or pnpm run dev starts the Svelte development server with HMR enabled by default.

intermediate

Why is a full page reload slower than HMR during development?

A full page reload restarts the entire app and reloads all resources, losing app state and taking more time. HMR updates only changed modules, making updates faster and smoother.

What happens when you save a file while the Svelte development server with HMR is running?

A Only the changed part updates in the browser without full reload

B The entire browser page reloads

C Nothing happens until you refresh manually

D The app crashes

HMR updates only the changed modules instantly without reloading the whole page.

Which command typically starts the Svelte development server?

A npm test

B npm start

C npm build

D npm run dev

The command npm run dev starts the development server with HMR enabled.

Why is HMR useful during development?

A It disables the development server

B It reloads the whole page faster

C It updates only changed code without losing app state

D It compiles code to production

HMR updates only the changed code and keeps the app state, speeding up development.

What does the development server provide?

A A local environment to test your app with live updates

B A way to deploy your app to the internet

C A tool to minify your code

D A database for your app

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

If HMR is not working, what might happen when you save changes?

A The app updates instantly without reload

B The app reloads the entire page

C The app ignores changes

D The app crashes

Without HMR, saving changes causes a full page reload to show updates.

Explain what a development server does in Svelte and why it is helpful.

Think about how you see your app change as you write code.

Describe Hot Module Replacement (HMR) and how it improves your workflow.

Imagine changing a small part of your app without losing your place.

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