section8.1.md

Section 8.1: Generating metadata using a class decorator

This time we are going to declare a class decorator that will add some metadata to a class when we applied to it:

function addMetadata(target: any) {
  // Add some metadata
  target.__customMetadata = {
    someKey: "someValue"
  };
  // Return target
  return target;
}

// We can then apply the class decorator:
@addMetadata
class Person {
  private _name: string;
  public constructor(name: string) {
    this._name = name;
  }
  public greet() {
    return this._name;
  }
}

function getMetadataFromClass(target: any) {
  return target.__customMetadata;
}

console.log(getMetadataFromClass(Person));

The decorator is applied when the class is declared not when we create instances of the class. This means that the metadata is shared across all the instances of a class:

function getMetadataFromInstance(target: any) {
  return target.constructor.__customMetadata;
}

let person1 = new Person("John");
let person2 = new Person("Lisa");

console.log(getMetadataFromInstance(person1));
console.log(getMetadataFromInstance(person2));