Javascript Object.seal() does not throw an exception

Question

I wanted to emulate something like a fixed object, so that no new members can be added to an object. Object.seal(Obj) seemed the right way, but it does not throw an exception when I try to create the new member. The member is not created, but it happens in silence.

var O = { a: 111 }
Object.seal(O)
O.b = 222  <------ here the exception is expected (trying to add a member "b")
O.a = 333
console.log(O) // { a: 333 }

Why would somebody want this silent behavior, why an exception is not thrown?

Answer 1

The behaviour of an assignment to a sealed object changes with the browsers. The latest release of chrome, for example, behaves as you might expect. For practical purposes, it is safe to assume that adding a member to a sealed object only throws an exception when in strict mode.

;(function () {
    'use strict';
    var O = { a: 111 }
    Object.seal(O)
    O.b = 222
    O.a = 333
    console.log(O) // { a: 333 }
}());

This self-invoking anonymous function throws an error, as you expect. On old browser, unfortunately, you cannot rely on polyfills such as https://github.com/kriskowal/es5-shim

In fact, the seal method on the prototype of Object avoids a 'TypeError' exception, but fails silently when invoked.
From the documentation:

This should be fine unless you are depending on the safety and security provisions of this method, which you cannot possibly obtain in legacy engines.