Object.seal()

const object = {
  foo: 42,
};

Object.seal(object);
object.foo = 33;
console.log(object.foo);
// Expected output: 33

delete object.foo; // Cannot delete when sealed
console.log(object.foo);
// Expected output: 33

Object.seal(obj)

const obj = {
  prop() {},
  foo: "bar",
};

// New properties may be added, existing properties
// may be changed or removed.
obj.foo = "baz";
obj.lumpy = "woof";
delete obj.prop;

const o = Object.seal(obj);

o === obj; // true
Object.isSealed(obj); // true

// Changing property values on a sealed object
// still works.
obj.foo = "quux";

// But you can't convert data properties to accessors,
// or vice versa.
Object.defineProperty(obj, "foo", {
  get() {
    return "g";
  },
}); // throws a TypeError

// Now any changes, other than to property values,
// will fail.
obj.quaxxor = "the friendly duck";
// silently doesn't add the property
delete obj.foo;
// silently doesn't delete the property

//  and in strict mode such attempts
// will throw TypeErrors.
function fail() {
  "use strict";
  delete obj.foo; // throws a TypeError
  obj.sparky = "arf"; // throws a TypeError
}
fail();

// Attempted additions through
// Object.defineProperty will also throw.
Object.defineProperty(obj, "ohai", {
  value: 17,
}); // throws a TypeError
Object.defineProperty(obj, "foo", {
  value: "eit",
}); // changes existing property value

Object.seal(1);
// TypeError: 1 is not an object (ES5 code)

Object.seal(1);
// 1                             (ES2015 code)