pre-insertMany-middleware.js

const SALT_WORK_FACTOR = 10;
UserSchema.pre('insertMany', async (next, users) => {
  if (Array.isArray(users) && users.length > 0) {
    const hashedUsers = users.map(async (user) => {
      return await new Promise((resolve, reject) => {
        bcrypt
          .genSalt(SALT_WORK_FACTOR)
          .then((salt) => {
            bcrypt
              .hash(user.password.toString(), salt)
              .then((hash) => {
                user.password = hash;
                resolve(user);
              })
              .catch((hashError) => {
                reject(hashError);
              });
          })
          .catch((saltError) => {
            reject(saltError);
          });
      });
    });
    users = await Promise.all(hashedUsers);
    next();
  } else {
    return next(new Error('User list should not be empty')); // lookup early return pattern
  }
});

pre-save-middleware.js

UserSchema.pre('save', function (next) {
    let user = this
    if (!user.isModified("password")) {
        next()
    } else {
        bcrypt.genSalt(10, function (err, salt) {
            if (err) return next(err)

            bcrypt.hash(user.password, salt, function (err, hash) {
                if (err) return next(err)

                user.password = hash
                next()
            })
        })
    }
})