Property decorators are prefixed to property declarations. The signature of PropertyDecorator in the TypeScript source code is this:

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

The following is a code snippet of a class with a property decorator applied to a property. In this code, the firstname property is decorated with the @hashify property decorator:

class Customer {  
  @hashify 
  public firstname: string; 
  public lastname: string; 
  constructor(firstname : string, lastname : string) {  
    this.firstname = firstname; 
    this.lastname = lastname; 
  } 
} 

Now, we will see the code snippet of the @hashify property decorator function:

function hashify(target: any, key: string)
 { 
  var _value = this[key];
  var getter = function ()
    { 
        return '#' + _value; 
    }; 
  var setter = function (newValue)
   { 
      _value = newValue; 
    }; 
  if (delete this[key])
 { 
    Object.defineProperty(target, key,
 { 
      get: getter, 
      set: setter, 
      enumerable: true, 
      configurable: true 
    }); 
  } 
} 

The _value variable holds the value of the property that is being decorated. Both the getter and setter functions will have access to the _value variable, and here, we can manipulate the _value variable by adding extra behaviors. I have concatenated # in getter to return hash tagged first name. Then, we delete the original property from the class prototype using the delete operator. A new property will be created with the original property name and the extra behavior.