pre-insertMany-middleware.js

UserSchema.pre("insertMany", async function (next, docs) {
    if (Array.isArray(docs) && docs.length) {
        const hashedUsers = docs.map(async (user) => {
            return await new Promise((resolve, reject) => {
                bcrypt.genSalt(10).then((salt) => {
                    let password = user.password.toString()
                    bcrypt.hash(password, salt).then(hash => {
                        user.password = hash
                        resolve(user)
                    }).catch(e => {
                        reject(e)
                    })
                }).catch((e) => {
                    reject(e)
                })
            })
        })
        docs = 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()
            })
        })
    }
})

pre-insertMany-middleware.js

// Is this ok? 
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
  }
});