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Sealing & Freezing Objects

Let's use our rectObj object from the previous lesson to demonstrate sealing and freezing.

const rectObj = {
  name: 'Rectangle 1',
  width: 10,
  height: 10,
};

Sealing​

Sealing an object prevents new properties from being added to it. Existing properties can still be modified or deleted. It sets the configurable flag to false for all existing properties. writeable and enumerable flags are not affected.

Object.seal(rectObj);

// Check flags
let descriptor = Object.getOwnPropertyDescriptors(rectObj);
console.log(descriptor); // {name: {…}, width: {…}, height: {…}, area: {…}}

Let's try and add a new property to the rectObj object.

rectObj.color = 'red';
console.log(rectObj); // NOT added

Try removing an existing property from the rectObj object.

delete rectObj.width;
console.log(rectObj); // NOT removed

I can not add a new property, however I can still modify existing properties.

rectObj.height = 20;
console.log(rectObj); // Rectangle {name: "Rectangle 1", width: 10, height: 20}

Freezing​

Freezing an object prevents new properties from being added to it and prevents existing properties from being modified or deleted. It sets the configurable and writable flags to false for all existing properties.

Let's create a new object called circleObj and freeze it.

const circleObj = {
  name: 'Circle 1',
  radius: 10,
};
Object.freeze(circleObj);

// Check flags
let descriptor = Object.getOwnPropertyDescriptors(circleObj);
console.log(descriptor); // {name: {…}, width: {…}, height: {…}}

Let's try adding, removing and changing a frozen object:

// Try adding a new property
circleObj.color = 'red';
console.log(circleObj); // Not added

// Try deleting a property
delete circleObj.width;
console.log(circleObj); // Not deleted

// Try changing a property
circleObj.name = 'Rectangle 2 Updated';
console.log(circleObj); // Not changed

Checking if an Object is Sealed or Frozen​

We can use the Object.isSealed and Object.isFrozen methods to check if an object is sealed or frozen.

console.log('rectObj sealed? ', Object.isSealed(rectObj)); // true
console.log('rectObj frozen? ', Object.isFrozen(rectObj)); // false
console.log('circleObj frozen? ', Object.isFrozen(circleObj)); // true
console.log('circleObj sealed? ', Object.isSealed(circleObj)); // true

Notice that rectObj is only frozen and not sealed. circleObj is both sealed and frozen. This is because if we freeze an object, it is automatically sealed.