Object.create()

Object.create(proto)
Object.create(proto, propertiesObject)

oco = Object.create( {} );   // create a normal object
ocn = Object.create( null ); // create a "null" object

> console.log(oco) // {} -- Seems normal
> console.log(ocn) // {} -- Seems normal here too, so far

oco.p = 1; // create a simple property on normal obj
ocn.p = 0; // create a simple property on "null" obj

> console.log(oco) // {p: 1} -- Still seems normal
> console.log(ocn) // {p: 0} -- Still seems normal here too. BUT WAIT...

> "oco is: " + oco // shows "oco is: [object Object]"

> "ocn is: " + ocn // throws error: Cannot convert object to primitive value

> alert(oco) // shows [object Object]
> alert(ocn) // throws error: Cannot convert object to primitive value

> oco.toString() // shows [object Object]
> ocn.toString() // throws error: ocn.toString is not a function

> oco.valueOf() // shows {}
> ocn.valueOf() // throws error: ocn.valueOf is not a function

> oco.hasOwnProperty("p") // shows "true"
> ocn.hasOwnProperty("p") // throws error: ocn.hasOwnProperty is not a function

> oco.constructor // shows "Object() { [native code] }"
> ocn.constructor // shows "undefined"

// display top-level property name:value pairs of given object
function ShowProperties(obj){
  for(var prop in obj){
    console.log(prop + ": " + obj[prop] + "\n" );
  }
}

ob={}; ob.po=oco; ob.pn=ocn; // create a compound object using the test objects from above as property values

> ShowProperties( ob ) // display top-level properties
- po: [object Object]
- Error: Cannot convert object to primitive value

Note that only first property gets shown.

ob={}; ob.pn=ocn; ob.po=oco; // create same compound object again, but create same properties in different order

> ShowProperties( ob ) // display top-level properties
- Error: Cannot convert object to primitive value

Note that neither property gets shown.

var obj = Object.create({ a: 1, b: 2 });

> console.log(Object.entries(obj)); // shows "[]"

ocn = Object.create( null ); // create "null" object (same as before)

ocn.toString = Object.toString; // since new object lacks method then try assigning it directly from standard-object

> ocn.toString // shows "toString() { [native code] }" -- missing method seems to be there now
> ocn.toString == Object.toString // shows "true" -- method seems to be same as the standard object-method

> ocn.toString() // error: Function.prototype.toString requires that 'this' be a Function

ocn = Object.create( null ); // create "null" object (same as before)

ocn.prototype.toString = Object.toString; // Error: Cannot set property 'toString' of undefined

ocn.prototype = {};                       // try to create a prototype
ocn.prototype.toString = Object.toString; // since new object lacks method then try assigning it from standard-object

> ocn.toString() // error: ocn.toString is not a function

ocn = Object.create( null );        // create "null" object (same as before)
Object.setPrototypeOf(ocn, Object); // wrong; sets new object's prototype to the Object() function

> ocn.toString() // error: Function.prototype.toString requires that 'this' be a Function

ocn = Object.create( null ); // create "null" object (same as before)

ocn.toString = toString; // since new object lacks method then assign it directly from generic version

> ocn.toString() // shows "[object Object]"
> "ocn is: " + ocn // shows "ocn is: [object Object]"

ob={}; ob.pn=ocn; ob.po=oco; // create a compound object (same as before)

> ShowProperties(ob) // display top-level properties
- po: [object Object]
- pn: [object Object]

ocn = Object.create( null );                  // create "null" object (same as before)
Object.setPrototypeOf(ocn, Object.prototype); // set new object's prototype to the "generic" object (NOT standard-object)

> ocn.valueOf() // shows {}
> ocn.hasOwnProperty("x") // shows "false"
> ocn.constructor // shows "Object() { [native code] }"

// ...and all the rest of the properties and methods of Object.prototype.

// Shape - superclass
function Shape() {
  this.x = 0;
  this.y = 0;
}

// superclass method
Shape.prototype.move = function(x, y) {
  this.x += x;
  this.y += y;
  console.info('Shape moved.');
};

// Rectangle - subclass
function Rectangle() {
  Shape.call(this); // call super constructor.
}

// subclass extends superclass
Rectangle.prototype = Object.create(Shape.prototype);

//If you don't set Rectangle.prototype.constructor to Rectangle,
//it will take the prototype.constructor of Shape (parent).
//To avoid that, we set the prototype.constructor to Rectangle (child).
Rectangle.prototype.constructor = Rectangle;

var rect = new Rectangle();

console.log('Is rect an instance of Rectangle?', rect instanceof Rectangle); // true
console.log('Is rect an instance of Shape?', rect instanceof Shape); // true
rect.move(1, 1); // Outputs, 'Shape moved.'

var o;

// create an object with null as prototype
o = Object.create(null);

o = {};
// is equivalent to:
o = Object.create(Object.prototype);

// Example where we create an object with a couple of
// sample properties. (Note that the second parameter
// maps keys to *property descriptors*.)
o = Object.create(Object.prototype, {
  // foo is a regular 'value property'
  foo: {
    writable: true,
    configurable: true,
    value: 'hello'
  },
  // bar is a getter-and-setter (accessor) property
  bar: {
    configurable: false,
    get: function() { return 10; },
    set: function(value) {
      console.log('Setting `o.bar` to', value);
    }
/* with ES2015 Accessors our code can look like this
    get() { return 10; },
    set(value) {
      console.log('Setting `o.bar` to', value);
    } */
  }
});

function Constructor() {}
o = new Constructor();
// is equivalent to:
o = Object.create(Constructor.prototype);
// Of course, if there is actual initialization code
// in the Constructor function,
// the Object.create() cannot reflect it

// Create a new object whose prototype is a new, empty
// object and add a single property 'p', with value 42.
o = Object.create({}, { p: { value: 42 } });

// by default properties ARE NOT writable,
// enumerable or configurable:
o.p = 24;
o.p;
// 42

o.q = 12;
for (var prop in o) {
  console.log(prop);
}
// 'q'

delete o.p;
// false

// to specify an ES3 property
o2 = Object.create({}, {
  p: {
    value: 42,
    writable: true,
    enumerable: true,
    configurable: true
  }
});
/* is not equivalent to:
This will create an object with prototype : {p: 42 }
o2 = Object.create({p: 42}) */