Decorators in TypeScript

06 Aug 2018

Intermediate

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Decorators

Decorators are simply functions that are prefixed @ symbol, and can be attach to a class declaration, method, accessor, property, or parameter. The decorators supply information about the class, method, property or parameters.

Decorators are proposed for future version of JavaScript and they are available as an experimental feature of TypeScript.

There are following types of decorators, you can use with TypeScript:

Class Decorators
Method Decorators
Accessor Decorators
Property Decorators
Parameter Decorators

Class Decorator

A Class Decorator is attached to a class declaration and tell about the class behaviors. The class decorator is applied to the constructor of the class and can be used to observe, modify, or replace a class definition.

classdecorator.ts

@sealed
class Employee {
 constructor(private firstName: string, private lastName: string) { }
 showDetails() {
 return this.firstName + ", " + this.lastName;
 }
}

In above example, @sealed decorator will seal both the constructor and its prototype so that you cannot inherit the class.

Method Decorator

A method Decorator is attached to the methods of a class.

methoddecorator.ts

class ItemList {
 itemArray: Array;
 constructor() {
 this.itemArray = [];
 }
 @log
 Add(item: string): void {
 this.itemArray.push(item);
 }
 GetAll(): Array {
 return this.itemArray;
 }
}

In above example, @log decorator will log the new item entry.

Property Decorator

A property Decorator is attached to the properties of a class.

propertydecorator.ts

class Company {
 @ReadOnly 
 name: string = "Dot Net Tricks";
}

let company = new Company();
t.name = 'TCS'; // you can't change it's name
console.log(t.name); // 'Dot Net Tricks'

In above example, @ReadOnly decorator will make the name property as readonly, so you can’t change its value.

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