decorators-basics-in-typescript.md

Decorators # A Decorator is a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter. Decorators use the form @expression, where expression must evaluate to a function that will be called at runtime with information about the decorated declaration.

For example, given the decorator @sealed we might write the sealed function as follows:

function sealed(target) {
  // do something with 'target' ...
}

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

@sealed
class Greeter {
  greeting: string
  constructor(message: string) {
    this.greeting = message
  }
  greet() {
    return "Hello, " + this.greeting
  }
}

// We can define the @sealed decorator using the following function declaration:

function sealed(constructor: Function) {
  Object.seal(constructor)
  Object.seal(constructor.prototype)
}

When @sealed is executed, it will seal both the constructor and its prototype.

Further Reading

https://www.typescriptlang.org/docs/handbook/decorators.html