Custom Validator

Custom validator can be created using @val.custom decorator, you can wrap the @val.custom inside a function and make a new validator decorator, and provide logic on the Validator function callback. Validator function signature is like below:

(value: string, info: ValidatorContext) => string | AsyncValidatorResult[] | undefined | Promise<AsyncValidatorResult[] | string | undefined>
  • value is the current value that will be validated. value will always of type string
  • info is the context information required for validation see below
  • return value: return error message if not valid, or return undefined for valid value.

Signature of the ValidatorContext is like below

interface ValidatorContext {
name: string,
ctx: Context,
parent?: { type: Class, decorators: any[] }
  • name name of the current validating property or parameter
  • ctx Koa context of current request
  • parent parent class of current validation property, can be undefined if the current validating is a method parameter


For example we will create an age restriction validator which restrict only 18+ age allowed.

import { val } from "@plumier/validator";
export async function is18plus(){
return val.custom(val => parseInt(val) < 18 : "Should greater than 18 years old" : undefined)

Then you can use our new validator like below:

class User {
public age:number

The info parameter of the validator function useful when you need to validate value that require request context parameter such as body, params etc.

Class Validation#

Sometime its not possible to validate value only on single property, but require multiple properties. Real world example is the confirm password.

function checkConfirmPassword() {
return val.custom((x, info) => {
if(x.password !== x.confirmPassword)
return val.result("confirmPassword", "Password is not the same")
class User {
public password: string,
public confirmPassword: string
) { }
class UsersController {
get(@checkConfirmPassword() model: User) { }

Separate Decorator and Its Implementation#

Putting validator implementation inside decorator is simple and easy to read, but in some case it might cause circular dependency issue. You can use dependency resolver to solve this issue, by register the validator classes by ID.

The first step, create a class implements CustomValidator interface like below.

import { CustomValidator, ValidatorContext, DefaultDependencyResolver } from "plumier"
//create instance of DefaultDependencyResolver globally
const resolver = new DefaultDependencyResolver()
//register the custom authorizer with the ID
export class Is18PlusValidator implements CustomValidator {
validate(value: any, info: ValidatorContext)
if(parseInt(val) < 18)
return "Should greater than 18 years old"

Register the created resolver into the Plumier application

import { Plumier, WebApiFacility } from "plumier"
const app = new Plumier()
.set(new WebApiFacility({ dependencyResolver: resolver }))
//other facilities or middlewares

Then use the ID on each authorization applied.

class User {
//use the ID here, Plumier will use resolver
//to create instance of the custom authorizer
//then execute it
public age:number

This functionality work well with dependency injection, register the custom validator by name/id and plumier will automatically pass the ID into the custom dependency resolver.