JavaScript Basics: Object.freeze and Object.isFrozen

In the previous posts, I discussed about two levels of object protection: Object.preventExtensions and Object.seal. The last level of protection is Object.freeze, which also makes all the properties unchangeable. You cannot add a new property, you cannot delete an existing property, you cannot reconfigure it, and you cannot change its value.

The following code snippet demonstrates the effects of Object.freeze:

    let myObj = {
        name: "Ali",
    };
    Object.freeze(myObj);
    
    console.log(Object.isFrozen(myObj)); // true

    // You cannot add new properties. This fails silently.
    myObj.age = 19;
    console.log(myObj.age); // undefined

    // You cannot reconfigure existing properties.
    try {
        Object.defineProperty(myObj, "name", {
            get() {
                return "Susan";
            },
        });
        console.log(myObj.name);
    } catch (e) {
        console.log(e); // TypeError: Cannot redefine property: name
    }

    // You cannot change property values.
    myObj.name = "Susan";
    console.log(myObj.name); // Ali
    try {
        Object.defineProperty(myObj, "name", {
            value: "Aria",
        });
        console.log(myObj.name); // Ali
    } catch(e) {
        console.log(e); // TypeError: Cannot redefine property: name
    }

    // You can add new properties to the prototype.
    myObj.__proto__.age = 14;
    console.log(myObj.age); // 14

    // You cannot change the prototype.
    try {
        Object.setPrototypeOf(myObj, {
            grade: 9,
        });
        console.log(myObj.grade);
    } catch (e) {
        console.log(e); // TypeError: # is not extensible
    }
    try {
        myObj.__proto__ = {
            grade: 9,
        };
        console.log(myObj.grade);
    } catch (e) {
        console.log(e); // TypeError: # is not extensible
    }

In short, Object.freeze prevents extensions to the object and makes all the existing properties non-configurable, and also prevents the values of the existing properties from being changed. In this way, the object becomes immutable.

JavaScript Basics: Object.freeze and Object.isFrozen

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