JavaScript – Object defineProperty

In JavaScript, there are different ways to define properties to objects.

For example, if we have a Customer class, we can define properties using the this keyword, and also Object.defineproperty:

function Customer (firstname, lastname, email, phonenumber) {
    this.firstname = firstname,
    this.lastname = lastname,
    this.email = email,
    Object.defineProperty(this, "phonenumber", {
        writable: true,
        enumerable: true,
        configurable: false,
        value: phonenumber
    });
}

//Print out Customer
var Customer00001 = new Customer("Bob", "Smith", "bob@test.com", "111-222-7777")
console.log(Customer00001);

Running this produces:

With Object.defineProperty, we can be more granular with the property, setting:

• writable
• emumerable
• configurable
• value

With writable, consider the code where we update the phone number:

function Customer (firstname, lastname, email, phonenumber) {
    this.firstname = firstname,
    this.lastname = lastname,
    this.email = email,
    Object.defineProperty(this, "phonenumber", {
        writable: true,
        enumerable: true,
        configurable: false,
        value: phonenumber
    });
}

//Print out Customer
var Customer00001 = new Customer("Bob", "Smith", "bob@test.com", "111-222-7777")
Customer00001.phonenumber = "Updated: 222-111-4455";
console.log(Customer00001);

This produces:

However, with writable: false, the updated phone number would not be written to the object. If we have strict mode turned on, we would see “Uncaught TypeError: Cannot assign to read only property”:

If we set enumerable: false,

function Customer (firstname, lastname, email, phonenumber) {
    this.firstname = firstname,
    this.lastname = lastname,
    this.email = email,
    Object.defineProperty(this, "phonenumber", {
        writable: true,
        enumerable: false,
        configurable: false,
        value: phonenumber
    });
}

//Print out Customer
var Customer00001 = new Customer("Bob", "Smith", "bob@test.com", "111-222-7777")
for (var i in Customer00001)
{
    console.log(Customer00001[i]);
}

This produces below, with the phone number not displayed:

Now with enumerable: false. Let’s say we set this to false, if we try to change another property, we will receive an error “Cannot redefine property”:

function Customer (firstname, lastname, email, phonenumber) {
    this.firstname = firstname,
    this.lastname = lastname,
    this.email = email,
    Object.defineProperty(this, "phonenumber", {
        writable: true,
        enumerable: true,
        configurable: false,
        value: phonenumber
    });
}

//Print out Customer
var Customer00001 = new Customer("Bob", "Smith", "bob@test.com", "111-222-7777")
Object.defineProperty(Customer00001, "phonenumber", {enumerable: false});

You can also use Object.defineProperties to define multiple properties at once:

function Customer (firstname, lastname, email, phonenumber, website) {
    this.firstname = firstname,
    this.lastname = lastname,
    this.email = email,
    Object.defineProperties(this, {
        "phonenumber": {
        writable: true,
        enumerable: true,
        configurable: true,
        value: phonenumber
    },
    "website": {
        writable: true,
        enumerable: true,
        configurable: true,
        value: website
    }});
}

//Print out Customer
var Customer00001 = new Customer("Bob", "Smith", "bob@test.com", "111-222-7777", "www.test.com");
console.log(Customer00001);

This displays:

ABOUT CARL DE SOUZA

Carl de Souza is a developer and architect focusing on Microsoft Dynamics 365, Power BI, Azure, and AI.

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