Using Property Decorators in Typescript with a real example

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;
}

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 let's 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) {

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;
    };

The setter gets the value of the property when using it and handles the validation.

 const setter = function(newVal: string) {

      if(newVal.length < limit) { 
//waiting for Object.define implementation.
      }

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}`
        });

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
    });

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
    }); 
  }
}

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;
    }    
}

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);

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

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!

Photo by Ferenc Almasi on Unsplash

Comments

[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!

[Marcel A]

This does not work. All instances of the User class do have the same value for Password! Here is an example: TypeScript Playground

[Navjot]

There is a reason where every instance of User class is having the same value for the password. The article author is binding the properties to the target argument in the decorator function. According to the docs the target parameter in the decorator function is the prototype of the class (in this case User.prototype). So when the author did Object.defineProperty(target, propertyKey, { get: getter, set: setter }), then the property is being bound to the User.prototype object and not the current object. Since, the property is defined on the prototype it is available to all the instances of that class.

[ahuigo]

For es2022 or above, useDefineForClassFields is default to be true.
This causes that the property of instance(new User()) is independent from its User.prototype.

To make this example work only, we should set "useDefineForClassFields":false.

My example playground

[Jamie Miller]

Hi Marcel,

I came up for a solution with this problem. One should set a value on the object themselves using a unique symbol rather than a value that is accessed on the prototype.

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

}
});
}
}

[Dany Paredes]

Thanks marcel for you feedback, the article was wrote one year and half, so I need take time for remmember, nowdays most of people use npmjs.com/package/reflect-metadata. If you found why is not working or someidea leave a message or link to share, When return from my holidays I promise update the article.

[Jamie Miller]

Hi Danny,

I came up for a solution with this problem. One should set a value on the object themselves using a unique symbol rather than a value that is accessed on the prototype.

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

}
});
}
}

[José Thomaz]

any solution for this issue?

[Jamie Miller]

Hi Jose,

I came up for a solution with this problem. One should set a value on the object themselves using a unique symbol rather than a value that is accessed on the prototype.

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

}
});
}
}

[Siddharth Kumar]

with TS5.0, I'm getting below error:

Unable to resolve signature of property decorator when called as an expression. Argument of type 'ClassFieldDecoratorContext<User, string> & { name: "password"; private: false; static: false; }' is not assignable to parameter of type 'string'.ts(1240)

Anyone has been able to resolve it?

[Philippe Vaillancourt]

Could other decorators with a similar implementation be used along with this one? Something like this:

class User {
    username: string;
    @Min(8)
    @Max(12)
    @SpecialChar(2)
    password: string;
    constructor(username: string, password: string){
        this.username = username;
        this.password = password;
    }    
}

[Dany Paredes]

Yes, you can use nested decorators :D

[Bezael Pérez]

Thank you so much for sharing!!