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Using Property Decorators in Typescript with a real example

Dany Paredes
ex .NET guy, my last years building solutions with Vue, Angular, Sass, and Typescript.
Updated on ・3 min read

I was talking about class decorators in typescript in my previous post, today is time for properties decorators, how to define and use it for writing clean and elegant code.

What is Property Decorator

The property decorator is a function, applied to the property declaration in our classes.

It gets the constructor function of the class and the name of the property as parameters and with this information, we can do funny and cool things, like change the default definition or modify our object instance like an add new properties or change data.

class User {
 @MyDecorator
 password: string;
}
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How to create my property decorator.

I build the Min property decorator, it checks if the property has a minimum length, if not the object instance will have a new the errors' property with a message.

I will explain step by step or you can scroll and read the full code, then lets go.

1- Declare the Min function as the decorator.

The decorator is a function, but because we use a factory the Min function needs the limit number and returns another function that expects the Object and the property key.

function Min(limit: number) {
  return function(target: Object, propertyKey: string) {
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2- Define functions for the getter and setter.

We need to define 2 functions for handling when the user needs to read or set the value of the property with the decorator.

The getter returns the value of the property himself.

let value : string;
    //the getter of the property to return the value.
    const getter = function() {
      return value;
    };
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The setter gets the value of the property when using it and handles the raise the validation.

 const setter = function(newVal: string) {

      if(newVal.length < limit) { 
//waiting for Object.define implementation.
      }
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3- Using the Object.defineProperty

The property error needs to be declared using Object.defineProperty a short story about Object.defineProperty is it helps to define properties.

The defineProperty methods take 3 parameters: the instance of the object, the property name, and an object with the configuration like the value or the getter and setter.

 Object.defineProperty(target, 'errors', {
          value: `Your password should be bigger than ${limit}`
        });
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4- Redefine the property using Object.defineProperty and our functions.

The next step is the key, redefine the property with the decorator and set the getter and setter hooks to work with our logic.

 Object.defineProperty(target, propertyKey, {
      get: getter,
      set: setter
    });
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Done!, you have a clear overview of each statement, then feel free to read the full code.

function Min(limit: number) {
  return function(target: Object, propertyKey: string) { 
    let value : string;
    const getter = function() {
      return value;
    };
    const setter = function(newVal: string) {
       if(newVal.length < limit) {
        Object.defineProperty(target, 'errors', {
          value: `Your password should be bigger than ${limit}`
        });
      }
      else {
        value = newVal;
      }      
    }; 
    Object.defineProperty(target, propertyKey, {
      get: getter,
      set: setter
    }); 
  }
}

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The decorator is ready, the class User will have a password property then using the Min decorator, I set the password minimum length is 8.

class User {
    username: string;
    @Min(8)
    password: string;
    constructor(username: string, password: string){
        this.username = username;
        this.password = password;
    }    
}
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When the property password is set in the constructor it calls our decorator and internal getter and setter hooks.

let danyUser = new User("dany", "pass");
console.log(danyUser);
console.log(danyUser.errors);
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Because the password doesn't fit with the requirements then the error property will be available and contains the value.

[nodemon] starting `node app.js`
User { username: 'dany' }
Your password should be bigger than 8
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That's it!

Hopefully, that will give you a bit of help with how and when using Property decorator in Typescript. If you enjoyed this post, share it!

Discussion (2)

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jwp profile image
John Peters

Very cool Dany! I recall many years ago when Java Spring came out and they talked about AOP at the time .NET didn't have anything close to it . Now we see this for .NET

All in all what you've shown here is really powerful. Thanks!

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bezael profile image
Bezael Pérez

Thank you so much for sharing!!