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How would you stream a large file upload directly to processing (like virus scanning or transcoding) without first saving the entire file to disk? Why should CPU-intensive media processing like image resizing or video transcoding typically not run directly inside an Express request handler? How would you validate an uploaded image's actual dimensions and reject ones that don't meet minimum or maximum size requirements? How would you implement a progress indicator for a large file upload from the client's perspective, given the Node.js server side? How would you process and resize an uploaded image in Node.js using the sharp library, and why is sharp generally preferred over pure-JavaScript alternatives? How would you implement direct-to-S3 file uploads from a client, avoiding routing large file uploads through your Node.js server entirely? How would you generate a video thumbnail in Node.js using ffmpeg via a library like fluent-ffmpeg? What is the security risk of trusting a client-supplied filename for an uploaded file, and how do you mitigate it? How would you scan uploaded files for malware before accepting them into permanent storage in a Node.js application? How would you implement resumable/chunked file uploads in Node.js to handle very large files over unreliable network connections?
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What is the difference between upload.single(), upload.array(), and upload.fields() in multer?

Intermediate

Answer

upload.single(fieldName) handles exactly one uploaded file under a specific form field name, populating req.file. upload.array(fieldName, maxCount) handles multiple files uploaded under the same field name (like several photos), populating req.files as an array. upload.fields([{ name, maxCount }, ...]) handles multiple distinct named fields, each potentially with its own file(s), useful when a form has several different types of file inputs (like a resume and a cover letter uploaded together).
app.post('/single', upload.single('avatar'), handler); // req.file
app.post('/multiple', upload.array('photos', 10), handler); // req.files (array)
app.post('/mixed', upload.fields([
  { name: 'resume', maxCount: 1 },
  { name: 'coverLetter', maxCount: 1 },
]), handler); // req.files.resume, req.files.coverLetter
Real-world example A job-application form uses upload.fields() to accept both a resume and a cover letter as two distinctly named file inputs in a single form submission, letting the handler access each one separately via req.files.resume and req.files.coverLetter.

Common follow-ups: What happens if a client attempts to upload more files than the maxCount limit specified for upload.array()?;How would you handle a form with a mix of both regular text fields and file uploads together?

Express & Middleware;HTTP & Web Servers
All 15 File Uploads & Media Processing questions