This works fine for the registration process...

// Hash/Salt Password
AccountSchema.pre('save', function(next) {
  const account = this;
  bcrypt.hash(account.password, 10, function(err, hash) {
    if (err) return next(err);
    account.password = hash;
    next();
  });
});

...however, if a registered user wants to change their password, I can't seem to bcrypt when updating the document.

I'm looking for a direct solution, using a similar approach to the example above.

UPDATE:

Oddly enough, I found a solution. This works...

AccountSchema.pre('findOneAndUpdate', function(next) {
    const account = this.getUpdate();
    bcrypt.hash(account.password, 10, (err, hash) => {
        this.getUpdate().password = hash;
        next();
    })
});

...while this doesn't...

AccountSchema.pre('findOneAndUpdate', function(next) {
    const account = this.getUpdate();
    bcrypt.hash(account.password, 10, function(err, hash) {
        if (err) return next(err);
        this.getUpdate().password = hash;
        next();
    })
});

...but I have no idea why. The difference is basically the fat arrow plus error handling, and I'm very confused about how it would make such a big difference - since pre('save', ... ) had zero issues with function(err, hash) or if (err) return next(err);.

If anyone can explain why, it would be greatly appreciated.

() => 'foo';

function () { return 'foo'; }.bind(this)