validation.js

var peopleSchema = new mongoose.Schema({
	name: {
		type: String,
		required: true,
		minlength: 3,
		maxlength: 25
	},
	age: Number
});


peopleSchema.statics.testValidate = function(person) {
	return new Promise((res, rej) => {
		const pObj = new this(person);
		
		pObj.validate(err => {
			if (err) {
				return rej(err);
			} else
				return res('Success');
		});
	});
}

var People = mongoose.model('People', peopleSchema); 

function insertData() {
	var p = new People({
		name: '',
		age: 23
	});
	
	People.testValidate(p)
		.then(data => {
			console.log('OK', data);
		})
		.catch( err => {
			console.error('FAILED:',err)
		})		
		.finally(() => mongoose.connection.close())
}