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"])