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Learning Fuze > Javascript > Flashcards

Javascript Flashcards

1
Q

What is the purpose of variables?

A

Used to store data

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2
Q

How do you declare a variable?

A

var keyword + variable name

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3
Q

How do you initialize a variable?

A

By using the assignment operator ( = ) and giving it value

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4
Q

What characters are allowed in variable names?

A

Letters, numbers, dollar signs, and underscores

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5
Q

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

A

Uppercase and lowercase variables with the same name will have different values

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6
Q

What is the purpose of a string?

A

Used to store data of letters and words.
Used to add new content into a page
Can contain HTML markup

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7
Q

What is the purpose of a number?

A

Used for calculations or tasks such as determining size of screen, moving the position of an element on a page, or setting the amount of time an element should take to fade it

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8
Q

What is the purpose of a boolean?

A

Helpful in determining which part of a script should run

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9
Q

What does the = operator mean in javascript?

A

the assignment operator.

states that you are going to assign value to a variable

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10
Q

How do you update the value of a variable?

A

Reassign it.

Variable name + equal sign +new value

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11
Q

What is the difference between null and undefined?

A

Null is an assigned value (means nothing)
Undefined means a variable has been declared but not yet defined
Null is an object, undefined is undefined

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12
Q

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

A

For clarity - to keep your code clean and help you fix issues

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13
Q

Give 5 examples of javascript primitives:

A
  1. String
  2. Number
  3. Boolean
  4. Undefined
  5. Null
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14
Q

What data type is returned by an arithmetic operation?

A

Numbers

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15
Q

What is string concatenation?

A

Adding two strings together with the + symbol

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16
Q

What purpose does the + operator serve in javascript

A

Addition with numbers, concatenation with strings or strings & numbers

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17
Q

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

A

Boolean

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18
Q

What does the += “plus equal” operator do

A

assigns the value of the right operand to a variable and assigns the result to the variable
x+=5 is the same as x= x+5

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19
Q

What are objects for?

A

Grouping together a set of variables and functions to create a model of something
Objects store related data

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20
Q

What are object properties?

A

The variables of the object

collection of key: value pairs

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21
Q

Describe object literal notation

A

Key value pairs that are separated from each other with a colon and separated from other properties and methods with a comma and then stored in a variable inside curly braces

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22
Q

How do you remove a property from an object?

A

delete keyword followed by object name period and property name

delete object.property

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23
Q

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

A 
dot notation: var name = object.newproperty
bracket: var name = object['property']
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24
Q

What are arrays used for?

A

storing lists or sets of values

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25
Q

Describe the array literal notation

A

var keyword followed by the name of the array, equal sign, then the lists of values within square brackets.
each value is separated by a comma

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26
Q

How are arrays different from ‘plain’ objects?

A

arrays can store a series of objects and remember their order

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27
Q

What number represents the first index of an array?

A

0

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28
Q

What is the length property of an array?

A

holds the number of items in the array

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29
Q

How do you calculate the last index of an array?

A

array.length -1

the length of the array - 1

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30
Q

What is a function in Javascript?

A

collection of code that is reusable and helps make the code easier to read
allow you to package up code for use later in your program

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31
Q

Describe the parts of a function definition

A
  1. function keyword
  2. an optional name
  3. zero or more parameters
  4. a code block
  5. an optional return statement
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32
Q

Describe the parts of a function call

A

function named followed by ( ) parentheses with zero or more arguments

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33
Q

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

A
  • calling causes the functions code block to be executed
  • definition has parameters, calling has arguments with ( )
  • definition includes the function keyword, code block & return statement
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34
Q

What is the difference between a parameter and an argument?

A

parameters occur in the definition, while arguments occur when called
parameters are placeholders, arguments are values

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35
Q

Why are function parameters useful?

A

when a function is called, the parameters in its definition take on the values of the arguments that were passed

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36
Q

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

A

causes the function to produce a value

exits the function

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37
Q

Why do we log things to the console?

A

as a debugging tool - easy way to inspect your variables in the browser

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38
Q

What is a method?

A

a function which is a property of an object

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39
Q

How is a method different from any other function?

A

methods are associated with an object

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40
Q

How do you remove the last element from an array?

A

pop ( ) method

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41
Q

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

A

Math.floor ( )

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42
Q

How do you generate a random number?

A

Math.random ( )

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43
Q

How do you delete an element from an array?

A

splice( )

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44
Q

How do you append an element to an array?

A

push( )

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45
Q

How do you break a string up into an array?

A

split ( )

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46
Q

Do string methods change the original string?

How do you check if you werent sure?

A

no

MDN or the console

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47
Q

Roughly how many string methods are there according to the MDN web docs?

A

3 static methods

34 instance methods

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48
Q

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

A

no - sometimes return isn’t needed

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49
Q

Roughly how many array methods are there according to the MDN web docs?

A

3 static methods

31 instance methods

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50
Q

What three-letter acronym should you always include in your Google search about a Javascript method or CSS property?

A

MDN

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51
Q

Give 6 examples of comparison operators

A
  1. ==
  2. !=
  3. ===
  4. !==
  5. > , >=
    6.
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52
Q

What data type do comparison expressions evaluate to?

A

boolean

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53
Q

What is the purpose of an if statement?

A

to evaluate a condition and if true, to execute the code block

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54
Q

Is else required in order to use an if statement?

A

no

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55
Q

Describe the syntax (structure) of an if statement

A
  1. if keyword
  2. condition in parentheses ( )
  3. code block to execute within curly brace { }
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56
Q

What are three logical operators?

A

&& logical and
|| logical or
! logical not

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57
Q

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

A

using logical operators and (&&) or (||)

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58
Q

What is the purpose of a loop?

A

to repeat an action or a code to check a condition

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59
Q

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

A

if returns true a code block will run, if false it will stop

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60
Q

What does ‘iteration’ mean in the context of loops?

A

each time the loop runs through

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61
Q

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

A

before each pass through the loop

- if evaluates true, statement is executed

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62
Q

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

A

once before the loop begins

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63
Q

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

A

before each loop iteration

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64
Q

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

A

at the end of each loop iteration but before the condition runs a second time

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65
Q

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

A

break

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66
Q

What does the ++ increment operator do?

A

adds one to its operand and returns a value

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67
Q

How do you iterate through the keys of an object?

A

for..in loops

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68
Q

Why do we log things to the console?

A

to check for errors/debugging/double checking your values

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69
Q

What is a model?

A

a recreation or representation of something

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70
Q

Which document is being referred to in the phrase Document Object Model?

A

the model of the webpage created by the browser

71
Q

What is the word ‘object’ referring to in the phrase Document Object Model?

A

the objects that make up the DOM - each object represents a different part of the page loaded in the browser window

72
Q

What is a DOM tree?

A

the model of a webpage created by the browser
- has four main types of nodes
a structure with all the child elements inside it

73
Q

Give two examples of document methods that retrieve a single element from the DOM.

A

getElementById( )

querySelector ( )

74
Q

Give one example of a document method that retrieves multiple elements from the DOM at once

A

getElementsByClassName ( )

querySelectorAll ( )

75
Q

Why might you want to assign the return value of a DOM query to a variable?

A

allows you to access those elements faster in the future

76
Q

What console method allows you to inspect the properties of a DOM element object?

A

console.dir ( )

77
Q

Why would a tag need to be placed at the bottom of the HTML content instead of at the top?

A

the browser needs to go through all the elements in the HTML page before the javascript code can access them

78
Q

What does document.querySelector() take as its argument and what does it return?

A

CSS selectors

returns the first element with the document that matches the specified selector or group of selectors

79
Q

What does document.querySelectorAll() take as its argument and what does it return?

A

one or more selectors to match against

returns a NodeList (that is static) of the documents elements that match the specified group of selectors

80
Q

What is the purpose of events and event handling?

A

to notify code of interesting things that have taken place

  • handling is the steps involved in triggering javascript code when a user interacts with the HTML on a webpage
  • to respond to the actions of users
81
Q

What do [ ] square brackets mean in function and method syntax documentation

A

square brackets in syntax documentation indicates an optional section

82
Q

what method of element objects lets you set up a function to be called when a specific type of event occurs?

A

addEventListener ( )

83
Q

What is a callback function?

A

function that is passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action

84
Q

What object is passed into an event listener callback when the event fires?

A

object built by the javascript engine that is describing all the relative info that occurred.
- the event object which contains all the data

85
Q

What is the event.target ? If you weren’t sure how would you check? Where could you get more info about it?

A
  • the element object from which the event originated from
  • a reference to the object onto which the event was dispatched
  • console.log(event.target)
  • MDN
86
Q

What is the difference between element.addEventListener(‘click’, handleClick)& element.addEventListener(‘click’, handleClick())?

A

when the parentheses are after a function call, the code will run straight away rather than waiting until the event triggers it

87
Q

What is the className property of element objects?

A

sets or returns the class name of an element

88
Q

How do you update the CSS class attribute of an element using Javascript?

A

element.className

89
Q

What is the textContent property of element objects?

A

represents the text content of the node and its descendants

90
Q

How do you update the text within an element using Javascript?

A

node.textContent

91
Q

Is the event parameter of an event listener callback always useful?

A

not always necessary

92
Q

Why is story information about a program in variables better than only storing it in the DOM?

A

easier to keep track of and cleaner code

93
Q

What event is fired when a user places their cursor in a form control?

A

focus

94
Q

What event is fired when a user’s cursor leaves a form control?

A

blur

95
Q

What event is fired as a user changes the value of a form control?

A

input

96
Q

What event is fired when a user clicks the ‘submit’ button within a ?

A

submit

97
Q

What does event.preventDefault() method do?

A

cancels the event if it is cancelable

98
Q

What does submitting a form without event.preventDefault() do?

A

submits the form and disappears

99
Q

What property of a form element object contains all of the form’s controls?

A

elements property

100
Q

What property of a form control object gets and sets its value?

A

value property

101
Q

What is one risk of writing a lot of code without checking to see if it works so far?

A

you wont know if the code isnt working

102
Q

What is an advantage of having your console open when writing a Javascript program?

A

always helps check for error in code

103
Q

Does the document.createElement() method insert a new element into the page?

A

no

104
Q

How do you add an element as a child to another element?

A

appendchild()

105
Q

What do you pass as the arguments to the element.setAttribute() method?

A

name of the attribute and its value

106
Q

What steps do you need to take in order to insert a new element into the page?

A 
1. create the element node
optionally give it content
2. grab the spot you want to append it to
using querySelector
3. add the element to the DOM tree
107
Q

What is the textContent property of an element object for?

A

reading and writing textContent

108
Q

Name two ways to set the class attribute of a DOM element

A

element. setAttribute(“class”)

element. className

109
Q

What are two advantages of defining a function to create something (like the work of creating a DOM tree)?

A

can define function once and repeat it as much as you want throughout the code
can manipulate based on user response

110
Q

Give two examples of media features you can query in an @media rule

A
  1. width

2. aspect-ratio

111
Q

Which HTML meta tag is used in mobile-responsive web pages?

A

viewport meta tag

- < meta name=”viewport” content=”width=device-width, initial-scale=1” >

112
Q

What is the event.target?

A

the target property of the Event interface.

a reference to the object onto which the event was dispatched

113
Q

Why is it possible to listen for events on one element that actually happen to its descendent elements?

A

Event bubbling

114
Q

What DOM element property tells you what type of element it is?

A

element.tagName property

115
Q

What does element.closest( ) method take as its argument and what does it return?

A

takes selectors as its argument - a DOMString containing a selector list
- will return itself or the matching ancestor

116
Q

How can you remove an element from the DOM?

A

remove ( ) method

- removes objects from the tree it belongs to

117
Q

If you wanted to insert new clickable DOM elements into the page using Javascript, how could you avoid adding an event listener to every new element individually?

A

event delegation

- adds the event listener to the one parent and then analyzes the bubbled events to find a match on child elements

118
Q

What is the event.target?

A
  • the thing on which the event was dispatched

the target property of the Event interface
- a reference to the object onto which the event was dispatched

119
Q

What is the effect of setting an element to display: none?

A

hides the entire document and removes it from the document flow

120
Q

What does the element.matches( ) method take as an argument and what does it return?

A

a string representing the CSS selector it wants to test

returns a boolean

121
Q

How can you retrieve the value of an element’s attribute?

A

getAttribute ( ) method

122
Q

At what steps of the solution would it be helpful to log things to the console?

A

after you declare a variable or any time a variable changes

after an if statement or conditional to see if code is working

123
Q

If you were to add another tab and view to your html, but you didnt use event delegation, how would your javascript code be written instead?

A

would have to use a new addEventListener for each tab and view you wanted to add

124
Q

If you didn’t use a loop to conditionally show or hide the views in the page, how would your javascript code be written instead?

A

lots more conditionals

lots more code - would have to change each className and attribute individually

125
Q

What is JSON?

A

A standard text-based format for representing structured data based on Javascript Object Syntax

126
Q

What are serialization and deserialization?

A

Serialization is the process of converting an object into a stream of bytes so that it can be transferred over a network or stored in persistent storage
Deserialization is the reverse: when you get a stream of bytes from network or storage and convert it back into an object

127
Q

Why are serialization and deserialization useful?

A

allows you to transfer data between different platforms
network transmission
persistence

128
Q

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

A

stringify method of the JSON object

129
Q

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

A

parse method of the JSON object

130
Q

How do you store data in localStorage?

A

localStorage.setItem

- takes a key name and value name as arguments

131
Q

How do you retrieve data from localStorage?

A

localStorage.getItem

132
Q

What data type can localStorage save in the browser?

A

only strings

133
Q

When does the ‘beforeunload’ event fire on the window object?

A

when the window, the document and its resources are about to be unloaded

134
Q

What is a method?

A

A function that is the property of an object

135
Q

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

A

A call will have real values in its arguments whereas a definition will placeholders as parameters

136
Q

Describe method definition syntax (structure).

A

Variable declared with the name of the function as its property
the function has a function keyword as its value along with the function codeblock and return statement

137
Q

Describe method call syntax (structure).

A

object.method(argument)

138
Q

How is a method different from any other function?

A

method is a function, but it is a function attached to an object which has access to the data stored on that object
it is a property of an object

139
Q

What is the defining characteristic of Object-Oriented Programming?

A

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

140
Q

What are the four principles of object-oriented programming?

A
  1. abstraction
  2. encapsulation
  3. inheritance
  4. polymorphism
141
Q

What is abstraction?

A

being able to work with possibly complex things in simple ways

142
Q

What does API stand for?

A

application programming interface

143
Q

What is the purpose of an API?

A

simplifies programming by abstracting the underlying implementation and only exposing objects or actions the developer needs

144
Q

What is this in javascript?

A

an implicit parameter that refers to the object it belongs to

145
Q

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

A

its available in a functions code block even though it was never included in the functions parameter list or declared with var

146
Q

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

A

call time

147
Q

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

A

you can’t determine until it has been called

148
Q

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

A

its either given the value of the global window object OR

you find where the function is called and look for an object to the left of the dot

149
Q

What kind of inheritance does the Javascript programming language use?

A

a prototype based (or prototypal) inheritance

150
Q

What is a prototype in javascript?

A

an object that contains properties and (predominantly) methods that can be used by other objects

151
Q

How is it possible to call methods on strings, arrays, and numbers even though those methods don’t actually exist on objects, arrays, and numbers?

A

by defining on a prototype object and calling the datatypes with those methods

152
Q

If an object does not have it’s own property or method by a given key, where does javascript look for it?

A

the proto object

153
Q

What does the new operator do?

A

creates an instance of a user-defined object type or of one of the built in object types that has a constructor function

154
Q

What property of Javascript functions can store shared behavior for instances created with new?

A

function.prototype property

155
Q

What does the instanceof operator do?

A

tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object
return value is a boolean value

156
Q

What is a callback function?

A

a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine action

157
Q

Besides adding an event listener callback function to an element or the document, what is one way to delay the execution of a javascript function until some point in the future?

A

setTimeout( )

158
Q

How can you set up a function to be called repeatedly without using a loop?

A

setInterval( )

159
Q

What is the default time delay if you omit the delay parameter from setTimeout( ) or setInterval( )?

A

0 - executes immediately

160
Q

What do setTimeout( ) and setInterval( ) return?

A

a non-zero number that identifes the timer created by the call to setInterval - value can be passed to clearInterval to cancel

161
Q

What is a client?

A

service requester

162
Q

What is a server?

A

provider of a resource or service

163
Q

Which HTTP method does a browser issue to a web server when you visit a URL?

A

GET

164
Q

What three things are on the start-line of an HTTP request message?

A

HTTP method
Request Target
HTTP version

165
Q

What three things are on the start-line of an HTTP response message?

A

protocol version

usually 1.1

166
Q

What are HTTP headers?

A

the metadata of your requests
let the client and the server pass additional information with an HTTP request or response
consists of its case-insensitive name followed by a colon then by its value
whole header consist of one single line

167
Q

Is a body required for a valid HTTP request or response message?

A

no it is optional

168
Q

What is AJAX?

A

a technique for loading data into part of a page without having to refresh the entire page

169
Q

What does the AJAX acronym stand for?

A

Asynchronous Javascript and XML

170
Q

Which object is built into the browser for making HTTP requests in JavaScript?

A

XMLHttpRequest

171
Q

What event is fired by XMLHttpRequest objects when they are finished loading the data from the server

A

load

172
Q

An XMLHttpRequest object has an addEventListener( ) method just like DOM elements. How is it possible that they both share this functionality?

A

prototypal inheritance

173
Q

What is the JavaScript Event Loop?

A

checks if the call stack is empty - if it is then new functions are added from the event queue, if it’s not then the current function is processed

174
Q

What is different between ‘blocking’ and ‘non-blocking’ with respect to how code is executed?

A

blocking executes in a series - blocks executing of a statement until it completes the current statement
non-blocking executes in parallel or concurrently
blocking refers to operations that block further execution until that operation finishes while non-blocking refers to code that doesn’t block execution

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