使用mongoose向现有模式添加验证器

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mydatabase', { useNewUrlParser: true });

const userSchema = new mongoose.Schema({
  name: String,
  age: Number
});

userSchema.pre('save', function(next) {
  if (this.name.length < 2 || this.name.length > 20) {
    throw new Error('Name must be between 2 and 20 characters');
  }
  next();
});

const User = mongoose.model('User', userSchema);

const user = new User({
  name: 'John',
  age: 25
});

user.save()
  .then(() => console.log('User saved successfully'))
  .catch(err => console.error(err));