Mostly, they don’t

I absolutely agree. An even better structure wouldn’t have a raw password field on the user object at all.

In addition to the excellent points made by steventhedev and koper:

user.password = await hashPassword(user.password);

Just this one line of code alone is wrong.

1. It’s unclear, but quite likely that the type has changed here. Even in a duck typed language this is hard to manage and often leads to bugs.
2. Even without a type change, you shouldn’t reuse an object member like this. Dramatically better to have password and hashed_password so that they never get mixed up. If you don’t want the raw password available after this point, zero it out or delete it.
3. All of these style considerations apply 4x as strongly when it’s a piece of code that’s important to the security of your service, which obviously hashing passwords is.