Preventing Addition of New Properties to a Javascript Object with Object.seal()

The Object.seal() method prevents addition of new properties to a Javascript object. However values of existing properties can be changed if required.

Object.seal() is meant for cases where several developers are working on a single Javascript application, and data needs to be passed around. This can be used by a developer to "seal" his data before passing it to the other developer. This prevents the data from being polluted by other developers.

var test = {
    a: 34,
    b: 67
};

// new property is added
test.c = 45;

// object is now sealed
Object.seal(test);

// addition of new properties cannot be done now
test.d = 57;

// existing properties can still be changed
test.b = 88;

// Object {a: 34, b: 88, c: 45}
console.log(test);

Checking whether a Given Object is Sealed or Not

The Object.isSealed() method can check whether a given object is sealed or not. If sealed, it returns a true, and false otherwise.

var test = {
    a: 34,
    b: 67
};

// false
console.log(Object.isSealed(test));

// object is now sealed
Object.seal(test);

// true
console.log(Object.isSealed(test));

Catching Error if Attempt is Made to Add New Property to a Sealed Object

If new properties are added to a sealed object, they will fail silently.

However in strict mode, if new properties are added, they will throw a TypeError Exception. This can be caught within a try .. catch block.

"use strict";

var test = {
    a: 34,
    b: 67
};

Object.seal(test);

// exception will be thrown in strict mode
try {
    test.c = 50;   
}
catch(error) {
    // can't define property "c": Object is not extensible
    console.log(error.message);
}