Chapter 6: Objects

Question 1

Any value in JavaScript that is not a string, a number, a Symbol, or
true, false, null, or undefined is an _____

Answer 1

object

Question 2

How to create an object

Answer 2

Object.create()

Question 3

In its simplest form, an ____ ____ is a commaseparated list of colon-separated name:value pairs, enclosed within
curly braces

Answer 3

object literal
let empty = {}; // An object with no
properties
let point = { x: 0, y: 0 }; // Two numeric
properties
let p2 = { x: point.x, y: point.y+1 }; // More complex

Question 4

let o = new Object();

Answer 4

// Create an empty object: same as {}.

Question 5

let a = new Array();

Answer 5

// Create an empty array: same as [].

Question 6

let r = new Map();

Answer 6

// Create a Map object for key/value

Question 7

Example of object creation

Answer 7

let o1 = Object.create({x: 1, y: 2});

Question 8

You can pass null to create a new object that does not have a
prototype, but if you do this, the newly created object will not _____
anything, not even basic methods like toString()

Answer 8

inherit

Question 9

Creating any empty object

Answer 9

let o3 = Object.create(Object.prototype); // o3 is like {} or
new Object().

Question 10

A collection of related properties and/or methods

Answer 10

objects

object = {key:value, function()}

Question 11

To obtain the value of a property, use the _____ or ______ ______

Answer 11

dot (.), square bracket ([])

Question 12

If
using square brackets, the value within the brackets must be an
expression that evaluates to a ______ that contains the desired property

Answer 12

string

Question 13

Are these two expressions the same?
object.property
object[“property”]

Answer 13

yes

Question 14

Example

Answer 14

let unitcircle = { r: 1 }; // An object to inherit
from
let c = Object.create(unitcircle); // c inherits the property r
c.x = 1; c.y = 1; // c defines two properties of its own
c.r = 2; // c overrides its
inherited property
unitcircle.r // => 1: the prototype is
not affected

Question 15

Property access expressions will fail if the lefthand side of the. is
____ or _______.

Answer 15

null, undefined

Question 16

Attempting to set a property on ____ or ______ also causes a
TypeError

Answer 16

null, undefined

Question 17

The delete operator (§4.13.4) removes a ______ from an object

Answer 17

property

Question 18

delete does not operate on the value of the property but on the
property itself

Answer 18

True
delete book.author; // The book object now has no
author property.
delete book[“main title”]; // Now it doesn’t have “main
title”, either.

Question 19

The ___________ method of an object tests whether that
object has an own property with the given name. It returns false for
inherited properties:

Answer 19

hasOwnProperty()
let o = { x: 1 };
o.hasOwnProperty(“x”) // => true: o has an own
property x
o.hasOwnProperty(“y”) // => false: o doesn’t have a
property y
o.hasOwnProperty(“toString”) // => false: toString is an
inherited property

Question 20

____ can distinguish between
properties that do not exist and properties that exist but have been set to
undefined

Answer 20

in

Ex:
let o = { x: undefined }; // Property is explicitly set to
undefined
o.x !== undefined // => false: property exists but
is undefined
o.y !== undefined // => false: property doesn’t even
exist
“x” in o // => true: the property exists
“y” in o // => false: the property doesn’t
exist
delete o.x; // Delete the property x
“x” in o // => false: it doesn’t exist
anymore

Question 21

There are four functions you can use to get an
array of property names

Answer 21

Object.keys()
Object.getOwnPropertyNames()
Object.getOwnPropertySymbols()
Reflect.ownKeys() –> returns all own property names both enumerable , and obth string and Symbol

Question 22

________ copies properties with ordinary property get and
set operations, so if a source object has a getter method or the target
object has a setter method, they will be invoked during the copy, but
they will not themselves be copied.

Answer 22

Object.assign

Question 23

Example of Object.assign()

Answer 23

Object.assign(o, defaults); // overwrites everything in o
with defaults

Question 24

As discussed earlier, all JavaScript objects (except those explicitly
created without a prototype) inherit properties from
__________

Answer 24

Object.prototype()

Question 25

Every object created using {} or new Object() automatically links to ________

Answer 25

Object.prtotype

Question 26

The ______ method takes no arguments; it returns a string that somehow represents the value of the object on which it is invoked

Answer 26

toString()

Question 27

. The purpose of this method is to return a localized string representation of the object

Answer 27

toLocaleString()

Question 28

The _______ method is much like the toString() method, but it is called when JavaScript needs to convert an object to some primitive type other than a string—typically, a number

Answer 28

valueOf()

Question 29

Another example of object

Answer 29

let square = { area: function() { return this.side * this.side; }, side: 10 }; square.area() // => 100

Question 30

Ways to access object properties

Answer 30

1. object.property .2. Bracket Notation (object["property"])

Question 31

Accessor properties are represented by “getter” and “setter” methods. In an object literal they are denoted by ____ and ____

Answer 31

get, set let user = { name: "John", surname: "Smith", get fullName() { return `${this.name} ${this.surname}`; } }; alert(user.fullName); // John Smith

Question 32

let a = {}; let b = {}; // two independent objects alert( a == b );

Answer 32

// false

Question 33

Object.assign(dest, ...sources)

Answer 33

The first argument dest is a target object. Further arguments is a list of source objects. It copies the properties of all source objects into the target dest, and then returns it as the result.

Question 34

The call structuredClone(object) clones the object with all nested properties.

Answer 34

let user = { name: "John", sizes: { height: 182, width: 50 } }; let clone = structuredClone(user); alert( user.sizes === clone.sizes ); // false, different objects // user and clone are totally unrelated now user.sizes.width = 60; // change a property from one place alert(clone.sizes.width); // 50, not related

Question 35

To access the object, a method can use the ___ keyword

Answer 35

this

Question 36

In JavaScript, keyword this behaves unlike most other programming languages. It can be used in any function, even if it’s not a ______ of an _____

Answer 36

method, object

Question 37

Arrow functions have no this

Answer 37

true

Question 38

When a function is executed with new, it does the following steps:

Answer 38

A new empty object is created and assigned to this. The function body executes. Usually it modifies this, adds new properties to it. The value of this is returned.