Dev server and hot module replacement in Remix - Step-by-Step Execution

Concept Flow - Dev server and hot module replacement

Start Dev Server

↓

Watch Files for Changes

↓

File Change Detected?

No→Wait

Yes↓

Apply Hot Module Replacement

↓

Update Browser Without Full Reload

↓

Continue Watching

The dev server starts and watches your files. When a file changes, it updates only that part in the browser without reloading the whole page.

Execution Sample

Remix

import { useState } from 'react';

export default function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

A simple counter component that updates state and UI. When saved, the dev server updates this component instantly using hot module replacement.

Execution Table

Step	Action	File Change Detected	HMR Applied	Browser Update
1	Start dev server	No	No	Initial page load
2	User edits Counter component	Yes	Yes	Update Counter button text without full reload
3	User clicks button	No	No	Count increments and UI updates
4	User edits Counter component again	Yes	Yes	Update Counter button text instantly
5	No further changes	No	No	Idle, waiting for changes

💡 Dev server runs continuously, waiting for file changes to apply HMR.

Variable Tracker

Variable	Start	After Step 3	After Step 5
count	0	1	1

Key Moments - 2 Insights

Why doesn't the whole page reload when I save changes?

Because the dev server detects the file change and applies Hot Module Replacement (HMR), updating only the changed module (see execution_table steps 2 and 4). This keeps the app state and UI updated instantly without a full reload.

What happens if I change a file that cannot be hot replaced?

The dev server will perform a full page reload to apply changes. This is not shown in this trace but is the fallback when HMR is not possible.

Visual Quiz - 3 Questions

Look at the execution table, what happens at step 2 when the Counter component is edited?

A The dev server reloads the entire page

B Hot Module Replacement updates the Counter button text without full reload

C Nothing happens because no file change is detected

D The browser crashes

💡 Hint

Check the 'HMR Applied' and 'Browser Update' columns at step 2 in the execution_table

According to the variable tracker, what is the value of 'count' after step 3?

A 0

B 2

C 1

D Undefined

💡 Hint

Look at the 'count' row in variable_tracker after step 3

If the user edits the Counter component multiple times without clicking the button, what stays the same according to the tables?

A The 'count' variable value

B The dev server stops watching files

C The browser reloads fully each time

D The button stops working

💡 Hint

See variable_tracker and execution_table steps 2 and 4 for file edits without clicks

Concept Snapshot

Dev server watches your files for changes.
When a change happens, Hot Module Replacement (HMR) updates only the changed parts.
This updates the browser instantly without a full reload.
Keeps app state intact and speeds up development.
If HMR can't apply, a full reload happens as fallback.

Full Transcript

The dev server starts and watches your project files. When you save a change, it detects the file update. Instead of reloading the whole page, it uses Hot Module Replacement to update only the changed module in the browser. This means your app updates instantly and keeps its current state. For example, editing a Counter component updates the button text without losing the count. Clicking the button changes the count state and updates the UI normally. The dev server keeps running, waiting for more changes. If a file change cannot be hot replaced, the server reloads the full page as a fallback. This process helps developers see changes quickly and keeps their work smooth.