JavaScript

Question 1

object literal

Answer 1

var xxx = {}
I can literally see the object

Question 2

What is the purpose of variables?

Answer 2

to create a memory space for restore values

Question 3

How do you declare a variable?

Answer 3

var const let

Question 4

How do you initialize (assign a value to) a variable?

Answer 4

use = == ===

Question 5

What characters are allowed in variable names?

Answer 5

The period, the underscore, and the characters $, #, and @ can be used within variable names. For example, A. _$@#1 is a valid variable name.

Question 6

What does it mean to say that variable names are “case sensitive”?

Answer 6

JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

Question 7

What is the purpose of a string?

Answer 7

text information

Question 8

What is the purpose of a number?

Answer 8

numerical information

Question 9

What is the purpose of a boolean?

Answer 9

true or false

Question 10

What does the =, ==, === operator mean in JavaScript?

Answer 10

= assign variable

The equality operator (==) checks whether its two operands are equal, returning a Boolean result. Unlike the strict equality operator, it attempts to convert and compare operands that are of different types.

The strict equality operator (===) checks whether its two operands are equal, returning a Boolean result. Unlike the equality operator, the strict equality operator always considers operands of different types to be different.

Question 11

How do you update the value of a variable?

Answer 11

write the name, reassign value

Question 12

What is the difference between null and undefined?

Answer 12

null is created by human, means there’s empty for now but not long

Undefined is by machine. telling us item don’t have value

Question 13

Why is it a good habit to include “labels” when you log values to the browser console?

Answer 13

nice and clear on what are we log in

Question 14

Give five examples of JavaScript primitives.

Answer 14

undefined , null , boolean , string and number

bigint. for restore crazy big number (astro, bunisess data)
symbol.

Question 15

What data type is returned by an arithmetic operation?

Answer 15

number

Question 16

What is string concatenation?

Answer 16

var string += xxx

Question 17

What purpose(s) does the + plus operator serve in JavaScript?

Answer 17

plus things

Question 18

What data type is returned by comparing two values (<, >, ===, etc)?

Answer 18

boolean

Question 19

What does the += “plus-equals” operator do?
Exercise

Answer 19

ob += x
ob = ob + x

Question 20

What are objects used for?

Answer 20

{xx:pp, xx:pp} like dictionary

Question 21

What are object properties?

Answer 21

{xx:pp, xx:pp} xx is the property

Question 22

How do you remove a property from an object?

Answer 22

delete object.object property

Question 23

What are the two ways to get or update the value of a property?

Answer 23

object.property = value
object[‘property’] = value

Question 24

What are arrays used for?

Answer 24

restore a list can be count, loop

Question 25

How are arrays different from "plain" objects?

Answer 25

Objects represent a special data type that is mutable and can be used to store a collection of data (rather than just a single value). Arrays are a special type of variable that is also mutable and can also be used to store a list of values.

Question 26

What number represents the first index of an array?

Answer 26

array[0]

Question 27

What is the length property of an array?

Answer 27

how many number of item in array

Question 28

How do you calculate the last index of an array?

Answer 28

array.length-1

Question 29

What is a function in JavaScript?

Answer 29

a set of code we can reuse, and deal with information .

Question 30

Describe the parts of a function definition.

Answer 30

A function has three parts, a set of inputs, a set of outputs, and a rule that relates the elements of the set of inputs to the elements of the set of outputs in such a way that each input is assigned exactly one output.

Question 31

Describe the parts of a function call.

Answer 31

call with argument

Question 32

When comparing them side-by-side, what are the differences between a function call and a function definition?

Answer 32

call give argument, definition set pramater.

Question 33

What is the difference between a parameter and an argument?

Answer 33

parameter is the placeholder argument is the actually value

Question 34

Why are function parameters useful?

Answer 34

Parameters allow us to pass information or instructions into functions and procedures .

Question 35

What two effects does a return statement have on the behavior of a function?

Answer 35

return value close functiuon

Question 36

Why do we log things to the console?

Answer 36

debug

Question 37

What is a method?

Answer 37

JavaScript Methods: A JavaScript method is a property of an object that contains a function definition. Methods are functions stored as object properties.

Question 38

How is a method different from any other function?

Answer 38

A method, like a function, is a set of instructions that perform a task. The difference is that a method is associated with an object, while a function is not.

Question 39

How do you remove the last element from an array?

Answer 39

array.length-1

Question 40

How do you round a number down to the nearest integer?

Answer 40

Math.floor()

Question 41

How do you generate a random number?

Answer 41

var randomNumber = Math.random(); randomNumber = randomNumber * heroes.length;

Question 42

How do you delete an element from an array?

Answer 42

array.pop

Question 43

How do you append an element to an array?

Answer 43

array.push

Question 44

How do you break a string up into an array?

Answer 44

array.split(' ')

Question 45

Do string methods change the original string? How would you check if you weren't sure?

Answer 45

no, log

Question 46

Is the return value of a function or method useful in every situation?

Answer 46

no. sometimes just do some stuff, no return value

Question 47

Give 6 examples of comparison operators.

Answer 47

< > = && || >= <=

Question 48

What data type do comparison expressions evaluate to?

Answer 48

boolean

Question 49

What is the purpose of an if statement?

Answer 49

if something is true, do it

Question 50

Is else required in order to use an if statement?

Answer 50

no

Question 51

Describe the syntax (structure) of an if statement.

Answer 51

if (xxx) {do xxx}

Question 52

What are the three logical operators?

Answer 52

> = <

Question 53

How do you compare two different expressions in the same condition?

Answer 53

==

Question 54

What is the purpose of a loop?

Answer 54

loop through code, processing again and again until it meet expectations

Question 55

What is the purpose of a condition expression in a loop?

Answer 55

set loop times etc

Question 56

What does "iteration" mean in the context of loops?

Answer 56

loop through code, processing again and again until it meet expectations

Question 57

When does the condition expression of a while loop get evaluated?

Answer 57

until give situations is false

Question 58

When does the initialization expression of a for loop get evaluated?

Answer 58

until give situations is break

Question 59

When does the final expression of a for loop get evaluated?

Question 60

Besides a return statement, which exits its entire function block, which keyword exits a loop before its condition expression evaluates to false?

Answer 60

break

Question 61

How do you iterate through the keys of an object?

Answer 61

use for..in.. loop

Question 62

How do you deserialize a JSON string into a data structure using JavaScript?

Answer 62

JSON.parse

Question 63

serialization and deserialization?

Answer 63

Converting a string to a native object is called deserialization, while converting a native object to a string so it can be transmitted across the network is called serialization. serialization : make array to JSON string deserialization: make JSON string to array

Question 64

What is JSON?

Answer 64

The JSON object contains methods for parsing JavaScript Object Notation (JSON) and converting values to JSON. It can't be called or constructed.

Question 65

Why are serialization and deserialization useful?

Answer 65

serialization is easy to sent, turn data to easy to transfer deserialization is easy to work with

Question 66

How do you serialize a data structure into a JSON string using JavaScript?

Answer 66

JSON.stringify()

Question 67

How do you store data in localStorage?

Answer 67

keyname

Question 68

How do you retrieve data from localStorage?

Answer 68

getItem('key', value)

Question 69

What data type can localStorage save in the browser?

Answer 69

string

Question 70

When does the 'beforeunload' event fire on the window object?

Answer 70

refresh page, close tab, etc (anything make the page closed)

Question 71

What is a method?

Answer 71

A method is a function which is a property of an object.

Question 72

How can you tell the difference between a method definition and a method call?

Answer 72

method definition: function assign to proprety in the object method call: call the function assign to the object property with parameter

Question 73

Describe method definition syntax (structure).

Question 74

Describe method call syntax (structure).

Answer 74

object dot

Question 75

How is a method different from any other function?

Answer 75

dot notation

Question 76

What is the defining characteristic of Object-Oriented Programming?

Answer 76

objects can contain both data (as properties) and behavior (as methods).

Question 77

What are the four "principles" of Object-Oriented Programming?

Answer 77

Abstraction Encapsulation Inheritance Polymorphism

Question 78

What is "abstraction"?

Answer 78

being able to work with (possibly) complex things in simple ways.

Question 79

What does API stand for?

Question 80

What is the purpose of an API?

Answer 80

tools allow user interface programs with data

Question 81

What is this in JavaScript?

Answer 81

the object you are currently working with

Question 82

What does it mean to say that this is an "implicit parameter"?

Answer 82

implicit: presented but not stated

Question 83

When is the value of this determined in a function; call time or definition time?

Answer 83

call time. cause when definition time the parameter dose not have value

Question 84

How can you tell what the value of this will be for a particular function or method definition?

Answer 84

you can't

Question 85

How can you tell what the value of this is for a particular function or method call?

Answer 85

if is xxx.jj() this is xxx if is xxx() this is window

Question 86

window

Answer 86

window is a object, everything is in window object