What's Mongoose error Cast to ObjectId failed for value XXX at path "_id"?

Mongoose's findById method casts the id parameter to the type of the model's _id field so that it can properly query for the matching doc. This is an ObjectId but "foo" is not a valid ObjectId so the cast fails.

This doesn't happen with 41224d776a326fb40f000001 because that string is a valid ObjectId.

One way to resolve this is to add a check prior to your findById call to see if id is a valid ObjectId or not like so:

if (id.match(/^[0-9a-fA-F]{24}$/)) {
  // Yes, it's a valid ObjectId, proceed with `findById` call.
}