Mongoose中isModified何时返回false？为何我的代码始终返回true？

Let's break down what's happening here and show you when it will return false.

First: Why your current code always returns true #

Your code is creating a new document from scratch:

let user = new User; 
user.email = req.body.email; 
user.displayName = req.body.displayName; 
user.avatar = req.body.avatar; 
user.password = req.body.password; 
user.save(/* ... */);

For brand-new Mongoose documents, isModified() will always return true because you're initializing fields that were previously unset (the empty document has no values for these fields). Setting any field on a new document counts as a "modification" relative to its initial state.

When isModified() returns false #

This method returns false when you're working with an existing document fetched from the database and you don't change any of its fields before calling save(). Here's a concrete example:

// Fetch an existing user from the database
User.findById(someExistingUserId, (err, user) => {
  if (err) {
    return res.status(500).send({ Message: `Error fetching user: ${err}` });
  }

  // Don't modify any fields here
  user.save((err, userStored) => {
    // In your pre('save') hook, this.isModified() will return false now!
    if (err) return res.status(500).send({ Message: `Error saving: ${err}` });
    res.status(200).send({ user: userStored });
  });
});

Another scenario: if you modify a field but then revert it back to its original value before saving:

User.findById(someExistingUserId, (err, user) => {
  const originalName = user.displayName;
  
  // Modify the field (isModified() becomes true)
  user.displayName = "Temporary Name";
  console.log(user.isModified()); // Logs true

  // Revert to original value
  user.displayName = originalName;
  console.log(user.isModified()); // Logs false

  // Save now - pre('save') hook will see isModified() = false
  user.save();
});

Quick recap of how isModified() works #

Mongoose tracks changes to a document from two states:

1. For new documents: the initial empty state (all fields unset)
2. For fetched documents: the state as it was when loaded from the database

Any change from that baseline triggers isModified() to return true; no changes (or reverting changes) make it return false.

内容的提问来源于stack exchange，提问作者Jamo