Javascript Object.seal()

The JavaScript Object.seal() method seals the given object.

The seal() method prevents new properties from being added to the object and marks all the existing properties as non-configurable.

The syntax of the seal() method is:


The seal() method, being a static method, is called using the Object class name.

seal() Parameters

The seal() method takes in:

  • obj - The object that is to be sealed.

Return value from seal()

  • Returns the object being sealed.

Example: Using seal()

let obj = {
  foo: "bar",
  func: function () {},

// before sealing, properties can be added, modified, or removed = "JavaScript";
obj.value = 5;
delete obj.func;

// sealing the object
o = Object.seal(obj);

// can still change property values = "bar1";

// no other change
// fails silently
obj.foo1 = "bar";

console.log(obj); // { foo: 'bar1', value: 5 }

// cannot convert data property to accessors or vice versa
Object.defineProperty(obj, "foo", {
  get: function () {
    return "g";
// TypeError Cannot redefine property: foo


{ foo: 'bar1', value: 5 }
TypeError Cannot redefine property: foo


  • By default, objects are extensible (new properties can be added to them). The sealing of objects makes properties on objects fixed and immutable. The values of present properties can still be changed as long as they are writable.
  • Object.isSealed() can be used to check if an object is sealed or not.
  • Attempt to convert data property to accessor or vice versa will fail silently or throw TypeError.

Recommended Reading: JavaScript Object isSealed()

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