Object.seal

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Object.seal prevents the addition or removal of properties from an object. Once an object has been sealed its property descriptors can’t be converted to another type. Unlike Object.freeze it does allow properties to be edited.

Attempts to do this operations on a sealed object will fail silently

var obj = { foo: 'foo', bar: function () { return 'bar'; } };

Object.seal(obj)

obj.newFoo = 'newFoo';
obj.bar = function () { return 'foo' };

obj.newFoo; // undefined
obj.bar(); // 'foo'

// Can't make foo an accessor property
Object.defineProperty(obj, 'foo', { 
    get: function () { return 'newFoo'; }
}); // TypeError

// But you can make it read only
Object.defineProperty(obj, 'foo', { 
    writable: false
}); // TypeError

obj.foo = 'newFoo';
obj.foo; // 'foo';

In strict mode these operations will throw a TypeError

(function () {
    'use strict';

    var obj = { foo: 'foo' };

    Object.seal(obj);

    obj.newFoo = 'newFoo'; // TypeError
}());

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