CastError ObjectId 🟩 Node.js

CastError: Cast to ObjectId failed for value "x" (type string) at path "_id"

A value that isn’t a valid 24-hex-character ObjectId was used in an _id query.

Seen on: Node.js

Meaning

Routes like /users/:id receive anything; strings like "undefined", "me" or route collisions (/users/new hitting /users/:id) fail casting.

Common causes

  • Invalid id from the URL
  • Route order: /users/new matched by /users/:id
  • "undefined" string from the frontend

⚡ Quick fix

  1. Validate with mongoose.isValidObjectId(id) and return 400/404
  2. Register specific routes before parameter routes

Detailed fix by platform

Node.js

  1. if (!mongoose.isValidObjectId(req.params.id)) return res.status(400).json({ error: 'invalid id' });

How to diagnose

  1. Value — Which value?
  2. Route — Which route matched?

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