Property Descriptors

    // Property Descriptors Methods and Usage
    Object.defineProperty(obj, propName, { })
    Object.defineProperties(obj, props)
    Object.getOwnPropertyNames(obj)
    Object.getOwnPropertyDescriptor(obj, prop)
    Object.getOwnPropertyDescriptors(obj)
    Object.keys(obj) - list of enumerable properties
    Object.values(obj) - list of enumerable properties
    
    obj.propertyIsEnumerable(prop)
    obj.hasOwnProperty(prop)
    
    Objects can be
    1. Extensible - new properties added
    2. Frozen - props cannot be changed in anyway
    3. Sealed - props cannot be deleted or configured but are still writable
    
    Object PROPERTIES can be
    1. Writable - change the value
    2. Enumerable - seen through a for…in loop
    3. Configurable - change the property descriptors
    
    Object.isExtensible(obj)
    Object.isFrozen(obj)
    Object.isSealed(obj)
    Object.preventExtensions(obj)
    Object.freeze(obj)
    Object.seal(obj)
    
    Descriptor Groups
    DATA		    ACCESSOR
    value		    get
    writable	    set
    configurable	configurable
    enumerable	    enumerable
    **************************************************/
    let log = console.log;
    let obj = {
    	name: 'Bob',
    	age: 45
    }; 

    for (let prop in obj){           // list all enumerable properties in the object
        log(prop);
    };

    Object.defineProperty(obj, 'test', {
    	value: 'Shagadelic',
    	writable: true,
    	configurable: true,
    	enumerable: false			// like a ‘behind the scenes’ property
     } );
    
    Object.defineProperty(obj, 'frank', {
    	get: ( ) => this.value,
    	set: (_val) => {
    	      this.value = _val + " baby!";
    	},
    	configurable: true,
    	enumerable: true					
     } );
    
    log(obj, obj.test, obj.frank);	// obj.frank returns undefined as no value has been set
    obj.frank = 'Shagadelic';
    log(obj.frank);					// returns Shagadelic baby!