TypeScript - Parameter Decorator

[Last Updated: Aug 11, 2020]

A parameter decorator is applied on a method/constructor parameter declaration

The parameter decorator function is called with the following two arguments:

  • Either the constructor function of the class for a static member, or the prototype of the class for an instance member.
  • The name of the member (method).
  • The ordinal index of the parameter in the function's parameter list.

Just like Property Decorators, a Property Descriptor is not provided as an argument to a parameter decorator function.

The return value from the decorator function is ignored.

A parameter decorator can only be used to observe that a parameter has been declared on a method.


In following example we will use parameter decorator '@notNull' to register the target parameter for not-null validation, but as this decorator is only called during loading time (not during when method is called) we will also need method decorator '@validate' which will intercept method call and will perform required validation.


function notNull(target: any, propertyKey: string, parameterIndex: number) {
    console.log("param decorator notNull function invoked ");
    Validator.registerNotNull(target, propertyKey, parameterIndex);

function validate(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    console.log("method decorator validate function invoked ");
    let originalMethod = descriptor.value;
    //wrapping the original method
    descriptor.value = function (...args: any[]) {//wrapper function
        if (!Validator.performValidation(target, propertyKey, args)) {
            console.log("validation failed, method call aborted: " + propertyKey);
        let result = originalMethod.apply(this, args);
        return result;

class Validator {
    private static notNullValidatorMap: Map<any, Map<string, number[]>> = new Map();

    //todo add more validator maps
    static registerNotNull(target: any, methodName: string, paramIndex: number): void {
        let paramMap: Map<string, number[]> = this.notNullValidatorMap.get(target);
        if (!paramMap) {
            paramMap = new Map();
            this.notNullValidatorMap.set(target, paramMap);
        let paramIndexes: number[] = paramMap.get(methodName);
        if (!paramIndexes) {
            paramIndexes = [];
            paramMap.set(methodName, paramIndexes);

    static performValidation(target: any, methodName: string, paramValues: any[]): boolean {
        let notNullMethodMap: Map<string, number[]> = this.notNullValidatorMap.get(target);
        if (!notNullMethodMap) {
            return true;
        let paramIndexes: number[] = notNullMethodMap.get(methodName);
        if (!paramIndexes) {
            return true;
        let hasErrors: boolean = false;
        for (const [index, paramValue] of paramValues.entries()) {
            if (paramIndexes.indexOf(index) != -1) {
                if (!paramValue) {
                    console.error("method param at index " + index + " cannot be null");
                    hasErrors = true;
        return !hasErrors;

class Task {
    run(@notNull name: string): void {
        console.log("running task, name: " + name);

console.log("-- creating instance --");
let task: Task = new Task();
console.log("-- calling Task#run(null) --");;
console.log("-- calling Task#run('test') --");"test");
param decorator notNull function invoked
method decorator validate function invoked
-- creating instance --
-- calling Task#run(null) --
method param at index 0 cannot be null
validation failed, method call aborted: run
-- calling Task#run('test') --
running task, name: test

Example Project

Dependencies and Technologies Used:

  • Typescript 3.3.3333
Parameter decorators examples Select All Download
  • typescript-parameter-decorator-examples
    • Ex1ParameterDecorator.ts

    See Also