What is the difference between PUT and PATCH in a REST API?

Imagine a student's record card. With PUT you hand the office a completely new card and they throw the old one away. If you forgot to write the phone number on the new card, the phone number is gone. With PATCH you hand over a sticky note saying "change the phone number to this" and everything else stays as it was.

Existing record: { "name": "Anjali", "city": "Pune", "phone": "98xxxxxx10" }

PUT /students/7
{ "name": "Anjali", "city": "Jamshedpur" }
=> record becomes { "name": "Anjali", "city": "Jamshedpur" }   (phone removed)

PATCH /students/7
{ "city": "Jamshedpur" }
=> record becomes { "name": "Anjali", "city": "Jamshedpur", "phone": "98xxxxxx10" }

app.patch("/students/:id", async (req, res) => {
  const updated = await Student.findByIdAndUpdate(
    req.params.id,
    { $set: req.body },       // only the sent fields
    { new: true }
  );
  if (!updated) return res.status(404).json({ error: "Not found" });
  res.json(updated);
});

Idempotent means repeating the same request gives the same final state. Sending the same PUT twice leaves the same record. A PATCH like "increase marks by 5" is not idempotent, though "set city to Pune" is.

• Using PUT with a partial body and wiping other fields.
• Using POST for every update.