In JavaScript, there are numerous ways to protect an Object from being changed.

With const you can not re-assign the object, but you can still change the value of a property, delete a property or create a new property.

// create object
const person = {
    firstName: "Johnny",
    lastName: "Shay"
};

The Object.preventExtensions() method prevents adding properties to an object.

// create object
const person = {
    firstName: "Johnny",
    lastName: "Shay"
};

// prevent extensions
Object.preventExtensions(person);

// test error
let text = "";
try {
    person.age = 12;
}
catch (err) {
    text = err;
}

document.getElementById("demo").innerHTML = text;

Error:

TypeError: Cannot add property age, object is not extensible

The Object.preventExtensions() method will also prevent items from being added to an array within an object.

// create object
const cars = ["Lamborghini", "Ferrari", "Maserati"];

// prevent extensions
Object.preventExtensions(cars);

// test error
let text = "";
try {
    cars.push("Alfa Romeo");
}
catch (err) {
    text = err;
}

document.getElementById("demo").innerHTML = text;

Error:

TypeError: Cannot add property 3, object is not extensible

The Object.isExtensible() method is used to check if an object is extensible. It will return “true” if an object is extensible, and “false” if it is not.

// create object
const person = {
    firstName: "Johnny",
    lastName: "Shay"
};

// prevent extensions
Object.preventExtensions(person);

// test
let answer =  Object.isExtensible(person)
document.getElementById("demo").innerHTML = answer; // will return false

• Prevents additions or deletions of new properties
• Makes existing properties non-configurable
• Can be used to check if an object is sealed

"use strict"

// create object
const person = {
    firstName: "Johnny",
    lastName: "Shay"
};

// seal object
Object.seal(person)

// test
let text = "";
try {
    delete person.lastName;
    text = Object.values(person);
}
catch (err) {
    text = err;
}

document.getElementById("demo").innerHTML = text;

Error:

TypeError: Cannot delete property 'lastName' of #

The Object.isSealed() method is used to check if an object is sealed. It will return “true” if an object is sealed, and “false” if it is not.

// create object
const person = {
    firstName: "Johnny",
    lastName: "Shay"
};

// seal object
Object.seal(person);

// test
let answer =  Object.isSealed(person)
document.getElementById("demo").innerHTML = answer; // will return true

The Object.freeze() method prevents any changes to an object, making the object read only. No modification, addition or deletion of properties are allowed.

"use strict"

// create object
const person = {
    firstName: "Johnny",
    lastName: "Shay"
};

// freeze object
Object.freeze(person)

// test
let text = "";
try {
    person.lastName = "Chicken";
    text = Object.values(person);
}
catch (err) {
    text = err;
}

document.getElementById("demo").innerHTML = text;

Error:

TypeError: Cannot assign to read only property 'lastName' of object '#'

The Object.isFrozen() method is used to check if an object is frozen. It will return “true” if an object is frozen, and “false” if it is not.

// create object
const person = {
    firstName: "Johnny",
    lastName: "Shay"
};

// freeze object
Object.freeze(person);

// test
let answer = Object.isFrozen(person);
document.getElementById("demo").innerHTML = answer; // will return true

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