Multiple file uploads in Express - Step-by-Step Execution

Concept Flow - Multiple file uploads

Client selects multiple files

↓

Client sends POST request with files

↓

Express server receives request

↓

Middleware parses files

↓

Files saved to server storage

↓

Server sends response to client

The client selects multiple files and sends them in a POST request. The Express server uses middleware to parse and save these files, then responds back.

Execution Sample

Express

const express = require('express');
const multer = require('multer');
const app = express();
const upload = multer({ dest: 'uploads/' });
app.post('/upload', upload.array('files', 3), (req, res) => {
  res.send(req.files.length + ' files uploaded');
});

This code sets up an Express route to handle uploading up to 3 files named 'files' and responds with the count of uploaded files.

Execution Table

Step	Action	Files in Request	Files Parsed	Files Saved	Response Sent
1	Client selects 2 files	2 files	0	0	No
2	Client sends POST /upload	2 files	0	0	No
3	Middleware parses files	2 files	2 files	0	No
4	Files saved to 'uploads/'	2 files	2 files	2 files	No
5	Handler sends response	2 files	2 files	2 files	Yes
6	Request complete	2 files	2 files	2 files	Yes

💡 All files parsed and saved; response sent confirming upload count.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
req.files.length	undefined	2	2	2
Files saved count	0	0	2	2

Key Moments - 3 Insights

Why does req.files have multiple entries?

Because upload.array() middleware parses multiple files and stores them in req.files as an array, shown in execution_table step 3.

What happens if more than 3 files are sent?

Only the first 3 files are parsed and saved due to the limit in upload.array('files', 3), so req.files length will be at most 3.

Why do we specify 'files' in upload.array('files', 3)?

It matches the form field name from the client side that holds the files, ensuring middleware parses the correct files, as seen in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, how many files are saved after step 4?

A 0

B 3

C 2

D 1

💡 Hint

Check the 'Files Saved' column at step 4 in the execution_table.

At which step does the server send the response to the client?

A Step 4

B Step 5

C Step 3

D Step 2

💡 Hint

Look at the 'Response Sent' column in the execution_table.

If the client sends 4 files, how would req.files.length change according to the variable_tracker?

A It would be 3

B It would be 0

C It would be 4

D It would be undefined

💡 Hint

Recall the limit set in upload.array('files', 3) and how req.files.length is tracked.

Concept Snapshot

Multiple file uploads in Express:
- Use multer middleware with upload.array(fieldName, maxCount)
- fieldName matches client form input name
- maxCount limits number of files accepted
- Files available in req.files array
- Save files to server folder
- Respond after all files processed

Full Transcript

This visual trace shows how multiple file uploads work in Express using multer middleware. The client selects multiple files and sends them in a POST request. The server receives the request and multer parses the files into req.files array. The files are saved to the server storage folder. Finally, the server sends a response confirming how many files were uploaded. The execution table tracks each step from client selection to response. The variable tracker shows how req.files length and saved files count change. Key moments clarify why req.files is an array, the effect of the max file limit, and the importance of matching the form field name. The quiz tests understanding of file counts and response timing. This helps beginners see the step-by-step flow of multiple file uploads in Express.