Parameter decorators

Parameter decorators are prefixed to parameter declarations, and they are applied to a function for a class constructor or a method declaration. The signature of ParameterDecorator is as follows:

declare type ParameterDecorator = (target: Object, propertyKey:  
   string | symbol, parameterIndex: number) => void; 

Now, let's define the Customer class and use a parameter decorator to decorate a parameter in order to make it required and validate whether the value has been served:

class Customer { 
    constructor() {  } 
  
    getName(@logging name: string) { 
        return name; 
   } 
} 

Here, the name parameter has been decorated with @logging. The parameter decorator implicitly takes three inputs, namely prototype of the class that has this decorator, the name of the method that has this decorator, and the index of the parameter that is being decorated. The logging function implementation of the parameter decorator is as follows:

function logging(target: any, key : string, index : number) { 
  
  console.log(target); 
  console.log(key); 
  console.log(index); 
} 

Here, target is the class that has the decorator, key is the function name, and index contains the parameter index. This code just logs target, key, and index to the console.