Object.defineProperty()

gistfile1.js

var obj4 = {}; // create a new empty object

Object.defineProperty(obj4, 'x', {
    value : 'Some generic string',
    writable : true,
    enumerable : true,
    configurable : true
});

obj4.x; // returns "Some generic string"
Object.keys( obj4 ); // returns ["x"]

obj4.x = 'Another generic test string';
obj4.x; // returns "Another generic test string"

// since we have set configurable to true we can alter the property and its
// attributes can be changed. One thing to note is that if this property was set to false then trying to 
// make updates to it other than changing the writable property will fail and attempting to delete a property will also fail
Object.defineProperty(obj4, 'x', {
    value : 'And again some generic string',
    writable : true,
    enumerable : false,
    configurable : false
});

obj4.x; // returns "And again some generic string"
Object.keys( obj4 ); // returns []

// this will result in a TypeError because we cannot redefine the property: x once configurable has been set to false
Object.defineProperty(obj4, 'x', {
    value : 'And again some generic string',
    configurable : true
});

// we can however set the writable property once the configurable property has been set to false
Object.defineProperty(obj4, 'x', {
    writable : false
});

obj4.x = 'A string that wont be set';
obj4.x; // returns "And again some generic string" since we can no longer write to it

// adding properties and default values
// its important to note the following:

var obj5 = {
    x : true
};

// is equivalent to:

Object.defineProperty(obj5, 'x', {
    value : true,
    writable : true,
    configurable : true,
    enumerable : true
});

// but...

Object.defineProperty(obj6, 'x', { value : true });

// is equivalent to:

Object.defineProperty(obj6, 'x', { 
    value : true,
    writable : false, 
    configurable : false,
    enumerable : false
});


// Example of using defineProperty with an accessor property descriptor which can be useful when you
// want to execute some code each time a property is set or retrieved

var obj7 = {}
    , y = 100;

Object.defineProperty(obj7, 'y', {
    get : function() {
        console.log('getting property');
        return y;
    },
    set : function( value ) {
        console.log('setting property');
        y = value;
    },
    enumerable : true,
    configurable: true
});

obj7.y; // returns