Method Decorators

Method Decorators

A Method Decorator is declared just before a method declaration. The decorator is applied to the Property Descriptor for the method, and can be used to observe, modify, or replace a method definition. A method decorator cannot be used in a declaration file, on an overload, or in any other ambient context (such as in a declare class).

The expression for the method decorator will be called as a function at runtime, with the following three arguments:

1. Either the constructor function of the class for a static member, or the prototype of the class for an instance member.
2. The name of the member.
3. The Property Descriptor for the member.

NOTE The Property Descriptor will be undefined if your script target is less than ES5.

If the method decorator returns a value, it will be used as the Property Descriptor for the method.

NOTE The return value is ignored if your script target is less than ES5.

The following is an example of a method decorator (@enumerable) applied to a method on the Greeter class:

class Greeter {
  greeting: string;
  constructor(message: string) {
    this.greeting = message;
  }

  @enumerable(false)
  greet() {
    return "Hello, " + this.greeting;
  }
}

We can define the @enumerable decorator using the following function declaration:

function enumerable(value: boolean) {
  return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
    descriptor.enumerable = value;
  };
}

The @enumerable(false) decorator here is a decorator factory. When the @enumerable(false) decorator is called, it modifies the enumerable property of the property descriptor.