Check if property is decorated with specific annotation - Typescript

Question

How can I determine if a specific property is decorated with a specific annotation?

For example this class:

class A {
    @DecoratedWithThis
    thisProp: number;
}

How can I know that thisProp is decorated with DecoratedWithThis?

My use case: I use the class from another file to generate code and HTML for the properties.

Could you imagine of another solution?

Answer 1

There is no native way that I know of in typescript, but there is a way to do it using 'reflect-metadata' library https://github.com/rbuckton/reflect-metadata which is a proposed extension to native reflection api.

Decorators are just functions that get executed when your class is defined. in your case you want to create a property decorator

function (target, propertyKey) {}

Where target is your Object.prototype and name is the property name

Per Reflect Metadata Documentation

// define metadata on an object or property
Reflect.defineMetadata(metadataKey, metadataValue, target, propertyKey);

// get metadata value of a metadata key on the prototype chain of an object or property
let result = Reflect.getMetadata(metadataKey, target, propertyKey);

You can solve your problem by adding metadata to your property that you can read later on to check if that property has been decorated.

Example Implementation

import 'reflect-metadata';

const metadataKey = 'MyDecorator';

function MyDecorator(target, propertyKey) {
    Reflect.defineMetadata(metadataKey, true, target, propertyKey);
}

function myDecoratorUsingClass<T>(type:  Type<T>, propertyKey: string) {
    return myDecoratorUsingInstance(new type(), propertyKey);
}

function myDecoratorUsingInstance<T>(instance: T, propertyKey: string) {
  return !!Reflect.getMetadata(metadataKey, instance, propertyKey);
}

class MyClass {
   @MyDecorator
   property1;
   property2;
}

console.log(myDecoratorUsingClass(MyClass, 'property1')); // true
console.log(myDecoratorUsingClass(MyClass, 'property2')); // false

const instance = new MyClass();
console.log(myDecoratorUsingInstance(instance, 'property1')); // true
console.log(myDecoratorUsingInstance(instance, 'property2')); // false

NOTE To use myDecoratorUsingClass your class should have a constructor where all the parameters are optional (or no parameters)