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Web Development > Javascript > Flashcards

Javascript Flashcards

1
Q

What is the purpose of variables?

A

To give us a location to store data to use in the future

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2
Q

How do you declare a variable?

A

By using var, setting a variable name, then using the = operator to set it to a value

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3
Q

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

A

Using the = operator

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4
Q

What characters are allowed in variable names?

A

Letters, numbers, dollar sign, underscore

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5
Q

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

A

It is sensitive to capitalizations

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6
Q

What is the purpose of a string?

A

To store text

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7
Q

What is the purpose of a number?

A

To store a numerical or mathematical value

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8
Q

What is the purpose of a boolean?

A

To store a true or false value

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9
Q

What does the = operator mean in JavaScript?

A

It’s used to set a variable to a value (assignment operator)

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10
Q

How do you update the value of a variable?

A

State the variable name, assignment operator, set it to new value

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11
Q

What is the difference between null and undefined?

A

Null is an assigned value and explicitly means nothing. Undefined means a variable has been declared, but the value has not yet been defined

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12
Q

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

A

It adds readability and gives more context

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13
Q

Give five examples of JavaScript primitives.

A

String, number, boolean, null, undefined

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14
Q

What data type is returned by an arithmetic operation?

A

A number data type

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15
Q

What is string concatenation?

A

When you combine multiple strings using the + operator

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16
Q

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

A

To perform addition or concatenate multiple strings together to create one value

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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-equals” operator do?

A

A shorthand operator that allows you to add the value from the right operand to the left operand, then assigns the new value to the left operand

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19
Q

What are objects used for?

A

To store multiple data points or properties within a variable

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20
Q

What are object properties?

A

A collection of values attached to an object

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21
Q

Describe object literal notation.

A

A variable assigned to multiple properties using curly brackets, then a list of properties and their values separated by a colon, comma to separate multiple properties.

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22
Q

How do you remove a property from an object?

A

Keyword “delete” then the object.property or object[‘property’]

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23
Q

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

A

getAttribute, setAttribute

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24
Q

What are arrays used for?

A

To store a list of elements that we can access by a single variable. To-do lists, lists of users, etc are all great ways to leverage arrays

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25
Q

Describe array literal notation.

A

Declare var, variable name, assignment operator, square brackets, and put each value in the brackets, separated by commas

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26
Q

How are arrays different from “plain” objects?

A

Objects use properties and values, while arrays have values only. Objects don’t have an order, while arrays do. Objects use dot notation while arrays use bracket notation.

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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

It tells you how many items are in your array

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29
Q

How do you calculate the last index of an array?

A

Array.length minus 1

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30
Q

What is a function in JavaScript?

A

A set of statements that performs a task or calculates a value

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31
Q

Describe the parts of a function definition.

A

Function keyword, function name, parenthesis for parameters, curly bracket for function code block, return keyword, then expression, closed curly bracket to close declaration

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32
Q

Describe the parts of a function call.

A

Function name, parenthesis, 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

Parameters vs arguments. Function definition has function key work and code block. Function call contains the function name and parenthesis to call it.

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34
Q

What is the difference between a parameter and an argument?

A

Parameters are properties of a function definition. Arguments are properties of a function call.

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35
Q

Why are function parameters useful?

A

Parameters allow us to pass information or instructions into functions

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36
Q

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

A

A return statement ends the execution of a function, and returns control to the calling function

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37
Q

Why do we log things to the console?

A

To ensure the code is working properly and to debug code

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38
Q

What is a method?

A

a property of an object that contains a function definition

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39
Q

How is a method different from any other function?

A

A method has to be attached to 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

Floor method

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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 would you check if you weren’t sure?

A

They do NOT change the original string. Do it then console log it.

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47
Q

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

A

40 or so

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48
Q

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

A

no

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49
Q

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

A

30 or so

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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

> , =, <=, >=, ===, !==

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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 guide a program to make decisions based on specified criteria

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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

If keyword, parenthesis for condition, curly brace for condition code block, return

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56
Q

What are the three logical operators?

A

&&, ||, and !

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57
Q

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

A

Logical operators

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58
Q

What is the purpose of a loop?

A

Loops allow you to repeat a process over and over without having to write the same (potentially long) instructions each time you want your program to perform a task.

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59
Q

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

A

an expression that is tested each time the loop repeats. As long as condition is true, the loop keeps running.

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60
Q

What does “iteration” mean in the context of loops?

A

Is the single time a loop code block executes

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61
Q

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

A

Before each iteration

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62
Q

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

A

Beginning of the loop

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63
Q

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

A

Before each iteration takes place

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64
Q

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

A

After each iteration takes place

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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

Increases the value of a variable by 1, then substitutes the new value

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67
Q

How do you iterate through the keys of an object?

A

For in loop

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68
Q

Why do we log things to the console?

A

To see what the code is doing / debugging

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69
Q

What is a “model”?

A

Contains all the logic in Javascript

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70
Q

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

A

The HTML document

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71
Q

What is the word “object” referring to in the phrase Document Object Model?

A

A model of the document being represented by javascript objects

72
Q

What is a DOM Tree?

A

A representative chunk of the page built as javascript objects

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

To use again or reference later

76
Q

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

A

Console.dir (directory method)

77
Q

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

A

The HTML document loads from top to bottom. It needs to come after the HTML elements so the DOM content is there before the script loads.

78
Q

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

A

It takes a string as an argument (CSS selector) and returns the first instance of that element that matches in the HTML document

79
Q

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

A

It takes a string as an argument (CSS selector) and returns a node list. A node list is not a DOM element.

80
Q

Why do we log things to the console?

A

To see what the code is doing. Great debugging tool.

81
Q

What is the purpose of events and event handling?

A

To react to something the user does. Event handling is the setup of the response of what happens when the user does something.

82
Q

Are all possible parameters required to use a JavaScript method or function?

A

No. You can use just the one or ones that are relevant for the specific task

83
Q

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

A

addEventListener()

84
Q

What is a callback function?

A

A function definition being passed as an argument to another function

85
Q

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

A

The event object, or the object with all the information about the event that just occurred

86
Q

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

A

It stores the DOM element where the event originated from (the start point of the event). Console log it.

87
Q

What is the difference between these two snippets of code?

element. addEventListener(‘click’, handleClick)
element. addEventListener(‘click’, handleClick())

A

The first is setting up an event listener that calls the function handleClick when a user clicks. The second is executing the function handleClick right this second. There is no return so it will be undefined.

88
Q

What is the className property of element objects?

A

used to set or return the value of an element’s class attribute

89
Q

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

A

Use className or setAttribute

90
Q

What is the textContent property of element objects?

A

Represents the text content of the node and it’s descendants

91
Q

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

A

textContent

92
Q

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

A

No. If you know what you want to do you probably don’t need it.

93
Q

Would this assignment be simpler or more complicated if we didn’t use a variable to keep track of the number of clicks?

A

More complicated

94
Q

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

A

Organization, simplicity, and to refer to it later

95
Q

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

A

Focus

96
Q

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

A

Blur

97
Q

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

A

Input

98
Q

What event is fired when a user clicks the “submit” button within a form?

A

Submit

99
Q

What does the event.preventDefault() method do?

A

Prevents the default behavior from happening

100
Q

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

A

It would refresh the page because it doesn’t stop the default action

101
Q

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

A

HTMLFormElements

102
Q

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

A

Value

103
Q

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

A

If you get an error, you won’t know where or why it’s not working

104
Q

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

A

You can see errors in real time which allows you to know exactly where in the code it’s happening

105
Q

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

A

No. It creates a new element node, but it’s not attached to the DOM yet

106
Q

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

A

appendChild method

107
Q

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

A

Two strings. The first being the name of the attribute, the second is the value of the attribute

108
Q

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

A

Query the parent element, create a new element, append new element to parent element

109
Q

What is the textContent property of an element object for?

A

to set text content for the HTML element or get the text content written inside that element

110
Q

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

A

setAttribute method or className

111
Q

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

A

Makes it reusable. Debugging.

112
Q

What is the event.target?

A

The target event property returns the element that triggered the event

113
Q

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

A

Event bubbling

114
Q

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

A

tagName property

115
Q

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

A

It takes a selector as an argument and returns the closest ancestor Element or itself, which matches the selectors

116
Q

How can you remove an element from the DOM?

A

The remove method on the element we want to remove

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

Dom event delegation. Put eventlistener on the parent.

118
Q

What is the event.target?

A

The target event property returns the element that triggered the event.

119
Q

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

A

It takes a CSS selector as a string and returns a boolean

120
Q

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

A

getAttribute method

121
Q

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

A

At every step

122
Q

If you were to add another tab and view to your HTML, but you didn’t use event delegation, how would your JavaScript code be written instead?

A

You would have go create event listeners for each element

123
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

You’d have to write a bunch of conditionals

124
Q

What is JSON?

A

JSON stands for JavaScript Object Notation. Converts objects and arrays into a string for data transfer.

125
Q

What are serialization and deserialization?

A

Serialization is the process of turning an object in memory into a stream of bytes so you can do stuff like store it on disk or send it over the network. Deserialization is the reverse process: turning a stream of bytes into an object in memory.

126
Q

Why are serialization and deserialization useful?

A

It allows us to transfer data in a uniform way

127
Q

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

A

Use the stringify method of the JSON object. Pass in an argument that will be an object or array and the return of the method will be a string.

128
Q

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

A

Use the parse method of the JSON object. Pass in a string as an argument and it will return an object or an array.

129
Q

How do you store data in localStorage?

A

setItem method of the local storage object. Arguments are a string of the name of the key, and the string of the value.

130
Q

How do you retrieve data from localStorage?

A

getItem method of the localStorage object. The argument is the key of the value you want to retrieve.

131
Q

What data type can localStorage save in the browser?

A

Strings

132
Q

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

A

The ‘beforeunload’ event is fired when the window, the document and its resources are about to be unloaded.

133
Q

What is a method?

A

A method is a function stored in a property of an object

134
Q

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

A

A method definition contains the word ‘function’ to define it with a code block that contains the logic for the method, while the method call is the name of the function followed by ()

135
Q

Describe method definition syntax (structure).

A

Object name, property, colon, function definition

136
Q

Describe method call syntax (structure).

A

object.Method name with ()

137
Q

How is a method different from any other function?

A

It’s stored on properties of objects only

138
Q

What is the defining characteristic of Object-Oriented Programming?

A

In OOP, objects can contain both data and behavior. Methods can use both within the object.

139
Q

What are the four “principles” of Object-Oriented Programming?

A

encapsulation, abstraction, inheritance and polymorphism

140
Q

What is “abstraction”?

A

hide the implementation details and highlight an object’s essential features to the users

141
Q

What does API stand for?

A

Application programming interface

142
Q

What is the purpose of an API?

A

to give programmers a way to interact with a system in a simplified, consistent fashion.

143
Q

What is this in JavaScript?

A

‘This’ refers to the object it’s being invoked on

144
Q

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

A

The object on which the method is called. It is called implicit because we don’t declare it when we declare the method.

145
Q

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

A

Call time

146
Q 
What does this refer to in the following code snippet?
var character = {
  firstName: 'Mario',
  greet: function () {
    var message = 'It\'s-a-me, ' + this.firstName + '!';
    console.log(message);
  }
};
A

‘This’ is nothing because the greet function was never called

147
Q

Given the above character object, what is the result of the following code snippet? Why?
character.greet();

A

“It’s-a-me Mario!” Because the method is being called on the character object.

148
Q 
Given the above character object, what is the result of the following code snippet? Why?
var hello = character.greet;
hello();
A

It will return undefined because hello() is not attached to an object

149
Q

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

A

You don’t know

150
Q

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

A

If the method is attached to an object on the left using dot notation, ‘this’ will return values from the object

151
Q

What kind of inheritance does the JavaScript programming language use?

A

Prototypal inheritance

152
Q

What is a prototype in JavaScript?

A

Prototypes are the mechanism by which JavaScript objects inherit features from one another

153
Q

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

A

setprototypeof() on the object

154
Q

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

A

It will look for it in the prototype

155
Q

What does the new operator do?

A

lets developers create an instance of a user-defined object type or of one of the built-in object types that has a constructor function.In short, Creates a new blank object, sets the prototype, makes the new object this, then returns the object at the end of the object.

156
Q

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

A

Prototype property which stores an object

157
Q

What does the instanceof operator do?

A

check the type of an object at the run time

158
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 or action.

159
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()

160
Q

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

A

setInterval()

161
Q

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

A

There is no time delay

162
Q

What do setTimeout() and setInterval() return?

A

Returns an ID that is a positive integer value which identifies the timer created by the call

163
Q

What is a client?

A

A program that is requesting data

164
Q

What is a server?

A

A program that provides data

165
Q

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

A

GET

166
Q

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

A

HTTP method (GET, PUT, POST), request target, and HTTP version

167
Q

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

A

Protocol version, status code, and status text

168
Q

What are HTTP headers?

A

a field of an HTTP request or response that passes additional context and metadata about the request or response

169
Q

Where would you go if you wanted to learn more about a specific HTTP Header?

A

MDN

170
Q

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

A

No

171
Q

What is AJAX?

A

Allows you to request data from a server and load it without having to refresh the entire page

172
Q

What does the AJAX acronym stand for?

A

Asynchronous JavaScript and XML

173
Q

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

A

XMLHttpRequest

174
Q

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

A

Load

175
Q

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

A

Because of prototypal inheritance

Web Development (4 decks)

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