Are there any examples of unit tests with Multer ?

[AlexMachin1997]

Hi everyone,

I've just finished adding Multer to my project, however, now I've got write some unit tests.

Is there any easy way to mock how Multer work's without adding a bunch of complexity ?

I want to be able to test the following:

• File type validation
• Successful upload
• Unexpected system errors

[mrdulin]

Can you provide a minimal code you want to test?

[Tommy1223454]

If it helps, I’d avoid mocking Multer too much and instead test the upload route with real fixture files.

The useful cases are usually:

• a valid expected file
• a renamed file with a misleading extension
• MIME/extension mismatch cases
• a small known-bad fixture like EICAR
• archive edge cases if your pipeline accepts ZIPs

I built an OSS package around this inspect-before-storage step:
https://github.com/pompelmi/pompelmi

Even if you don’t use it directly, that kind of fixture-driven testing tends to be much more valuable than only mocking the happy path.

[Errr0rr404]

You usually do not need to mock Multer’s internals. Drive it through HTTP with supertest (or call the middleware with a crafted req).

1) Integration-style (recommended)

const request = require("supertest");
const express = require("express");
const multer = require("multer");
const upload = multer({
  storage: multer.memoryStorage(),
  fileFilter(req, file, cb) {
    if (file.mimetype !== "image/png") return cb(null, false);
    cb(null, true);
  },
});

const app = express();
app.post("/upload", upload.single("file"), (req, res) => {
  if (!req.file) return res.status(400).json({ error: "bad type" });
  res.json({ ok: true, size: req.file.size });
});

// success
await request(app)
  .post("/upload")
  .attach("file", Buffer.from("fake"), { filename: "a.png", contentType: "image/png" })
  .expect(200);

// rejected type
await request(app)
  .post("/upload")
  .attach("file", Buffer.from("fake"), { filename: "a.txt", contentType: "text/plain" })
  .expect(400);

2) Error path

Multer errors are often passed to next(err) with codes like LIMIT_FILE_SIZE. Add an Express error middleware and assert on err.code.

3) Avoid real disk

Use multer.memoryStorage() in tests so nothing hits the filesystem.

That covers type validation, happy path, and system/limit errors without deep mocks.