Key feature that make Plumier different than other TypeScript backend framework is its ability to perform metaprogramming. Plumier has its own reflection (introspection) library named @plumier/reflect, that makes metaprogramming possible.

Metaprogramming in Plumier increases reusability of custom extension, because it can access information about current controller and action handles the request and the location of custom extension applied.


Metadata is specialized class contains information about current request metadata, such as controller object graph, action object graph, action parameters etc. It has properties below

  1. actionParams current action parameters, contains information about action parameters values used to execute the action.
  2. controller current controller object graph, contains information about controller name, decorators, methods, constructor etc.
  3. action current action object graph, contains information about action name, parameters, decorators etc.
  4. current metadata information where the appropriate decorator applied, can be Class metadata, Method metadata, Property metadata or Parameter metadata. For global middleware the current property will be undefined.

Access The Metadata#

Metadata object accessible through all custom extension, it accessible by metadata property.


Invocation object has metadata property, you can access it like below

const myCustomMiddleware:CustomMiddlewareFunction = ({ metadata, proceed }) => {
// process metadata
return proceed()

Or accessible from class style middleware

class MyCustomMiddleware implements CustomMiddleware {
async execute({ metadata, proceed }: Invocation) {
// process metadata
return proceed()

Custom Authorizer#

Metadata can be accessed from AuthorizationContext class like below

const myCustomAuthorizer: CustomAuthorizerFunction = ({ metadata }) => {
// process metadata
return true

Or accessible from class style authorizer

class MyCustomAuthorizer implements CustomAuthorizer {
authorize({ metadata }: AuthorizationContext) {
// process metadata
return true

Custom Validator#

Metadata can be accessed from ValidatorContext class like below

const myCustomValidator:CustomValidatorFunction = (val, { metadata }) => {
// process metadata

Or accessible from class style validator

class MyCustomValidator implements CustomValidator {
validate(value: any, { metadata } : ValidatorContext) {
// process metadata

Custom Binder#

Metadata accessible from the second parameter of custom binder

const myCustomBinder:CustomBinderFunction = (ctx, metadata) => {
// process metadata

Controller and Action Object Graph#

Metadata object contains information of current controller and action handle the request, Access them like below

const controllerName =
const actionName =

Access the parameter names of the action

const actionParameterNames = =>

Access decorator applied to the Controller or Action

const controllerDecorators = metadata.controller.decorators
const actionDecorators = metadata.action.decorators

Action Params#

Action parameter useful to get information about current action parameters and their values. For example with controller below

class AnimalController {
get(id:string, breed:string){

With the request GET /animal/12345?breed=canine you can access the values of the parameter from metadata object like below

const id = metadata.actionParams.get("id") //result id = 12345
const breed = metadata.actionParams.get("breed") //result breed = canine

You can access the parameter by its index like below

const id = metadata.actionParams.get(0) //result id = 12345
const breed = metadata.actionParams.get(1) //result breed = canine

To increase your app robustness it is necessary to check if current action handles the request has specific parameter. You can do that like below

throw new Error("Applied method doesn't have 'id' parameter")