Serving from multiple directories in Express - Step-by-Step Execution

Concept Flow - Serving from multiple directories

Start Express App

↓

Set Static Dir 1

↓

Set Static Dir 2

↓

Client Requests File

↓

Check Dir 1 for File

Yes→Serve File

No↓

Check Dir 2 for File

Yes→Serve File

No↓

Send 404 Not Found

Express checks each static directory in order for the requested file and serves it if found; otherwise, it sends a 404 error.

Execution Sample

Express

const express = require('express');
const app = express();
app.use(express.static('public'));
app.use(express.static('assets'));
app.listen(3000);

This code sets up an Express server that serves static files from 'public' first, then 'assets' if not found in 'public'.

Execution Table

Step	Request URL	Check Directory	File Found?	Action Taken
1	/image.png	public	Yes	Serve /public/image.png
2	/style.css	public	No	Check next directory
3	/style.css	assets	Yes	Serve /assets/style.css
4	/script.js	public	No	Check next directory
5	/script.js	assets	No	Send 404 Not Found

💡 Request ends when file is found or all directories are checked without success.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Requested File	None	/image.png	/style.css	/style.css	/script.js	/script.js
Current Directory	None	public	public	assets	public	assets
File Found	False	True	False	True	False	False

Key Moments - 2 Insights

Why does Express check directories in the order they are added?

Express serves the first matching file it finds. The execution_table shows it checks 'public' before 'assets', so files in 'public' take priority.

What happens if the file is not found in any directory?

As shown in step 5 of the execution_table, Express sends a 404 Not Found response when no directories contain the requested file.

Visual Quiz - 3 Questions

Look at the execution_table, what directory is checked for '/style.css' after 'public'?

A root

B public

C assets

D node_modules

💡 Hint

Check rows 2 and 3 in the execution_table under 'Check Directory'

At which step does Express send a 404 Not Found response?

A Step 5

B Step 3

C Step 1

D Step 2

💡 Hint

Look at the 'Action Taken' column in the execution_table

If the order of directories is reversed, which directory will Express check first?

A public

B assets

C node_modules

D views

💡 Hint

The order of app.use(express.static(...)) calls determines the check order

Concept Snapshot

Express serves static files from multiple directories in the order they are added.
Use app.use(express.static('dir')) multiple times.
Requests check each directory in sequence.
First found file is served.
If none found, 404 is sent.

Full Transcript

This example shows how Express serves static files from multiple directories. The server first checks the 'public' directory for the requested file. If the file is found there, it is served immediately. If not, Express checks the next directory, 'assets'. If the file is found in 'assets', it is served. If the file is not found in any directory, Express sends a 404 Not Found response. The order of directories matters because Express stops checking once it finds the file. This behavior allows organizing files in different folders and controlling which files take priority.