Objects

Question 1

JS objects are simple collections of named … with ….

Answer 1

JavaScript objects are simple collections of named properties with values

Question 2

JavaScript inheritance uses ….

Answer 2

Prototypes and a prototype chain, known as prototypal inheritance

Question 3

Every object can have a reference to a … but it differs from the … on a …

Answer 3

Prototype, prototype property, function

Question 4

We can define the prototype of an object by using the … method

Answer 4

Object.setPrototypeOf()

Question 5

Every function has a …. property

Answer 5

Prototype property, not to be confused with the __proto__ / [[Prototype]] link on an object.

Question 6

A function’s prototype property has a … property pointing back to itself

Answer 6

constructor

Question 7

What is returned from the Person function?

function Person() {}

const x = new Person();

Answer 7

An object - an instance of the Person ‘class’.

Question 8

The ES6 class syntax is really what under the hood?

Answer 8

Syntactic sugar for the constructor function / new keyword technique for prototypal inheritance

Question 9

What does the super() call do?

Answer 9

It calls the parent class’s constructor function

Question 10

Which of the following properties points to an object that will be searched if the target object doesn’t have the searched-for property?

• a class
• b instance
• c prototype
• d pointTo

Answer 10

c) prototype

Question 11

After running the following code, what will ninja.constructor point to?

function Person() {}

function Ninja() {}

Ninja.prototype = new Person();

const ninja = new Ninja();

Answer 11

ninja.constructor will point to Person()

Question 12

The Object.setPrototypeOf() method sets the … of a specified object to … or …

Answer 12

The Object.setPrototypeOf() method sets the prototype (i.e., the internal [[Prototype]] property) of a specified object to another object or null.

Question 13

What are the parameters a & b in the following example?

Object.setPrototypeOf(a, b)

Answer 13

a: target object - the object which is to have its prototype set
b: prototype object - the object’s new prototype (an object or null)

Question 14

What is the return value of Object.setPrototypeOf()?

Answer 14

The specified object that has its prototype changed

Question 15

What is printed to the console?

const a = { speed: 100 };

const x = Object.setPrototypeOf({}, a);

console.log(x.speed);

Answer 15

100

Question 16

How many parent object’s can the [[Prototype]] property hold?

Answer 16

1

Question 17

The Object.create method creates a new …

Answer 17

Object

Question 18

How many parameters does the Object.create() method take?

Answer 18

At least 1, the prototype object. There is also an optional properties object:

Object.create(proto[, propertiesObject])

Question 19

What are the paramters a & b?

Object.create(a,b);

Answer 19

a: prototype object - the object that will be used as the prototype
b: properties object - an optional object

Question 20

What is the return value of Object.create?

Answer 20

A new object with the specifed prototype object and properties

Question 21

Can inheritance using Object.create be detected by instanceof? Why?

Answer 21

No. instanceof works by checking prototype constructors, and Object.create does not use constructors.

Question 22

What can we use instead of instanceof to check prototypal inheritance using Object.create?

Answer 22

parentObject.isPrototypeOf(child) // boolean

Question 23

Do objects have prototypes?

Answer 23

Yes

Question 24

What is the difference between a function’s prototype and an object’s prototype?

Answer 24

All objects have a prototype, refered to by the __proto__ / chain property *. Functions are objects. Functions have prototypes.

However - this is confusing because there is a __proto__ (dunder proto) and an explicit .prototype property. Functions have both of these.

Foo.prototype is an explicit property of Foo:

Foo.prototype === Foo.__proto__ // false

[[Prototype]] is the actual object that is used in the prototypal lookup chain. Try to think of the underscores as being a chain __proto__ (the __proto__ is not supported by all engines).

* Objects can be created with their prototypes deliberately set to null, in which case searching their prototype returns undefined.

Question 25

Does JavaScript have private object properties?

Answer 25

No.

Question 26

Name 3 ways to mimic privacy within JavaScript

Answer 26

1. ) Via closure
2. ) Via Object.defineProperty creating setters & getters
3. ) Via Symbols