Develop Reference

Correct way to pass prototype to an object

My understanding of prototypes is as follows:
let Animal = function() {
this.bark = "woof";
}
Animal.prototype.barkLoud = function() {
return this.bark.toUpperCase();
}
let x = new Animal();
x.barkLoud() = "WOOF";
Everything above makes sense to me but then I saw a tutorial what seemingly appears to be 2 different ways to pass prototypes to an object. Are these the same thing? If so, which approach is better:
let obj = {
age: 45;
__proto__: Animal
}
vs
let obj = {
age: 45;
}
obj.prototype = Object.create(Animal.protoype);

As a rule of thumb, the more underscore (_) characters you see around a property name in JS, the more of implementation details it is - and the more discouraged you are from even querying it, let alone attempting to modify.
While __proto__ is indeed supported by all the existing browsers, its usage to set up a prototype is not recommended. Use Object.create() instead.
BTW, two ways you've showed are not even equivalent. See, __proto__ refers to the prototype object, but Animal is not the one - it's a function. Animal.prototype is an object that will be used as a prototype (__proto__ value) for all the objects created by this function with new` operator.
So the first object won't be able to resolve barkLoud name from the prototype chain:
> obj.barkLoud // undefined

__proto__ way is deprecated by the way and I've never seen it implemented anywhere.
Go with Object.create as the official and recommended way to create/assign prototype of existing object to the newly created object.

How can I prevent javascript assignment from creating ownProperty

Here is an example:
var cat1 = Object.create({
name: "mia"
});
cat1.hasOwnProperty('name') // false
cat1.name = "haha";
cat1.hasOwnProperty('name') // true
This is rather surprising to me.
1) What is the design intention here?
2) How can I use = without create new properties?

What is the design intention here?
I didn't design JavaScript, but my guess would be to restrict the "scope" of mutations. Imagine you had two objects:
var proto = {name: "mia"};
var cat1 = Object.create(proto);
var cat2 = Object.create(proto);
If assignment to cat1.name would not create a new property, but update the prototype property instead, then cat2.name would suddenly be updated as well.
In other words, if assignment was updating prototype properties instead of the object's own properties, other objects could be affected by the change, without you even knowing it.
How can I use = without create new properties?
You cannot. You could assign to the prototype explicitly, but that requires you to know that the property is defined on the prototype:
Object.getPrototypeOf(cat1).name = 'haha';

hasOwnProperty shows properties defined on the object. At the same time:
Object.create({
name: "mia"
});
Creates new object with prototype {name: "mia"}. This means that property name will be defined for prototype not for object. Using prototypes makes some sort of optimization. Different object share the same logic from the prototype. Also the methods are not duplicated, so less memory is used.
When you defines own property by = you are showdowing the property with the same name from the prototype, so as #Felix Kling indicated you are not changing property in the prototype and other objects that use the same prototype will not be affected. You also can use Object.defineProperty(obj, prop, descriptor) instead of = in order to define own property of the object, but you can not use = on object without defining new properties you can do that on the prototype directly like this:
cat1.__proto__.name="other name" //avoid, this is not the best practive at all

By design, When you create object from a prototype, the new objects will share the properties with prototype object until you assign value explicitly to that property on that object, once its assigned since then the object will maintain its own state. below is sample code for the same.
var x = {"name":"mia"};
var o1= Object.create(x);
var o2= Object.create(x);
console.log(x.name,o1.name,o2.name); //output: mia,mia,mia
x.name="xxx";
console.log(x.name,o1.name,o2.name); //output: xxx,xxx,xxx
o1.name="yyy";
console.log(x.name,o1.name,o2.name); //output: xxx,yyy,xxx

hasOwnProperty when to use, and when not needed?

It is recommended to always use hasOwnProperty, but in many cases this is not needed.
For example consider the following code:
var object = JSON.parse(somejsontext);
for(var prop in object) {
console.log(object[prop]);
}
I know in this case that prop is part of the object, it is explict define by the for..in.
But according to MOZ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty we should use it to avoid iterating over non-enumarable props, with this example:
var buz = {
fog: 'stack'
};
for (var name in buz) {
if (buz.hasOwnProperty(name)) {
console.log('this is fog (' + name + ') for sure. Value: ' + buz[name]);
}
else {
console.log(name); // toString or something else
}
}
But testing this code actually, never goes to the else.
So when does make sense the use of hasOwnProperty ?
UPDATE: Considering the choosen answer, we can safetly avoid the use of hasOwnProperty in this cases:
- Object js has not been extended by any javascript library or by our code
- Object is a simple code that we have control on

The problem arises when a prototype includes enumerable properties that your code did not anticipate. For example, suppose this ran just before your example:
Object.prototype.foobar = "hello";
In this case, iterating over buz would include the enumerable foobar prototype property. This hasOwnProperty pattern allows your code to differentiate between properties that are directly on the object, versus properties that are inherited from a prototypal ancestor.
The problem is not "enumerating over non-enumerable properties" (that's by definition not possible, unless you explicitly get them via getOwnPropertyNames for each level of the prototype hierarchy), but rather enumerating over inherited properties. This is a concern when using libraries that may add enumerable properties to high-level prototypes, exactly as I have illustrated above.
If you want to add a property to a prototype without causing that property to be enumerated, you can make a non-enumerable property with Object.defineProperty:
Object.defineProperty(Object.prototype, "foobar", {
value: "hello",
enumerable: false,
writeable: true,
configurable: true
});
Such a property would not appear in a for..in loop on buz (or in a for..in loop directly on Object.prototype, either).

Object.defineProperties/create really needed?

Some projects use Object.create() or Object.defineProperties() function. I wonder is is recommended? Whats the difference between
x = Object.create(null);
vs
x = {}
And
x = {}
x.__proto__.hello = function() {
console.log("hello");
}
vs
x = Object.create(null);
Object.defineProperty(x, "hello", {
value: function() {
console.log("hello");
}
});
defineProperty/create seems very verbose and long to me. When/Why do I use them? Perhaps the good might be to enforce getters/setters/overriding properties?

There is a huge difference. Have a look at the docs!
Object.create does create an Object that inherits from the first argument, null in your case. In contrast, {} - or new Object() - creates a new object that inherits from Object.prototype.
__proto__ is non-standard and should not be used. However, in your case you just do Object.prototype.hello = function() {…};. Never extend that object with enumerable properties, never ever!
Object.defineProperty does define a property on an object with a special descriptor object. The enumerable, configurable and writable attributes default to false, which means that you wont be able to delete x.hello for example, or assign any other value.
Your first snippet creates a plain object, which inherits a hello method from Object.prototype, while your second snippet creates an object inheriting from nothing and having a non-editable hello property. I don't see much relatedness.

Does a new object in Javascript have a prototype property?

This is a purely trivial question for academic value:
If I create a new object, either by doing:
var o = { x:5, y:6 };
or
var o = Object.create({ x:5, y:6 });
when I query the o.prototype property, I get undefined. I thought that any newly created object automatically inherits the Object.prototype prototype.
Furthermore, invoking toString(), (a method of Object.prototype) on this object works just fine, implying that o does inherit from Object.prototype. So why do I get undefined?

There is a difference between instances and their constructors.
When creating an object like {a: 1}, you're creating an instance of the Object constructor. Object.prototype is indeed available, and all functions inside that prototype are available:
var o = {a: 1};
o.hasOwnProperty === Object.prototype.hasOwnProperty; // true
But Object.create does something different. It creates an instance (an object), but inserts an additional prototype chain:
var o = {a: 1};
var p = Object.create(o);
The chain will be:
Object.prototype - o - p
This means that:
p.hasOwnProperty === Object.prototype.hasOwnProperty; // true
p.a === o.a; // true
To get the prototype "under" an instance, you can use Object.getPrototypeOf:
var o = {a: 1};
var p = Object.create(o);
Object.getPrototypeOf(p) === o; // true
Object.getPrototypeOf(o) === Object.prototype; // true
(Previously, you could access an instance's prototype with o.__proto__, but this has been deprecated.)
Note that you could also access the prototype as follows:
o.constructor === Object; // true
So:
o.constructor.prototype === Object.prototype // true
o.constructor.prototype === Object.getPrototypeOf(o); // true
This fails for Object.create-created objects because they do not have a constructor (or rather, their constructor is Object and not what you passed to Object.create because the constructor function is absent).

Not a direct answer, but knowledge that everybody, who deal with inheritance in Javascript, should have.
Prototype inheritance in Javascript is a tricky concept. Up until now, it has been impossible to create an empty object (by empty I mean lacking even properties form Object via prototype). So this means that creating a new object always had a link to the original Object prototype. However, according to the specification, the prototype chain of an object instance isn't visible, but some vendors have decided to implement their own proprietary object properties so that you could follow it, but it's highly recommended not to use it in production code.
The following sample code demonstrates just two ways of creating an object instance.
var someObject = {};
var otherObject = new Object();
var thirdObject = Object.create({});
Even though you don't manually add object properties to empty curly braces, you still get automatically added prototype chain. The same goes for the second example. To visualize it better, you can type those lines into Chrome console and then enter either someObject, otherObject or thirdObject to see details. Chrome shows the prototype chain by adding a proprietary property __proto__ which you can expand to see what is inherited and where it's from. If you executed something like
Object.prototype.sayHello = function() {
alert('hello');
};
you would be able to call it on all instances, by executing otherObject.sayHello().
However, using something that was implemented quite recently (therefore not supported by all browsers), you can actually create a truly empty object instance (doesn't inherit even from Object itself).
var emptyObject = Object.create(null);
When you enter it into Chrome console and then expand the emptyObject to see it's prototype chain, you can see that it doesn't exist. So even if you implemented the sayHello function to Object prototype, it would be impossible to call emptyObject.sayHello() since emptyObject does not inherit from Object prototype.
Hope it helps a bit with the general idea.

JavaScript has two types of objects: function object and non-function object. Conceptually, all objects have a prototype (NOT A PROTOTYPE PROPERTY). Internally, JavaScript names an object's prototype as [[Prototype]].
There are two approaches to get any object (including non-function object)'s [[prototype]]: the Object.getPrototypeOf() method and the __proto__ property. The __proto__ property is supported by many browsers and Node.js. It is to be standardized in ECMAScript 6.
Only a function (a callable) object has the prototype property. This prototype property is a regular property that has no direct relationship with the function's own [[prototype]]. When used as a constructor ( after the new operator), the function's prototype property will be assigned to the [[Prototype]] of a newly created object. In a non-function object, the prototype property is undefined . For example,
var objectOne = {x: 5}, objectTwo = Object.create({y: 6});
Both objectOne and objectTwo are non-function objects therefore they don't have a prototype property.