Class decorators are declared before the class declaration. Class decorators can observe, modify, and replace the definition of a class by applying to the constructor of that class. The signature of ClassDecorator in TypeScript is as illustrated:

declare type ClassDecorator = <TFunction extends Function>(target: TFunction) => TFunction | void; 

Consider a Customer class, and we would like that class to be freezed. Its existing properties should not be removed and new properties should not be added.

We can create a separate class that can take any object and freeze it. We can then decorate the customer class with @freezed to prevent adding new properties or removing the existing properties from the class:

@freezed 
class Customer {  
  public firstName: string; 
  public lastName: string; 
  constructor(firstName : string, lastName : string) {  
    this.firstName = firstName; 
    this.lastName = lastName; 
  } 
} 

The preceding class takes four arguments in the firstname and lastname constructors. The following is the code snippet of the function written for the freezed decorator:

function freezed(target: any) { 
    Object.freeze(target); 
} 

Here, the freezed decorator takes the target, that is, the Customer class that is being decorated and freezes it when it is executed.