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LearningFuze > JavaScript > Flashcards

JavaScript Flashcards

1
Q

What is the purpose of variables?

A

to hold data (numbers, strings, booleans)

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2
Q

How do you declare a variable?

A

by creating a variable name

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3
Q

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

A

adding an equal sign next the variable name and then giving it a value/data

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4
Q

What characters are allowed in variable names?

A

letters, $, numbers but we shouldn’t use numbers

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5
Q

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

A

it means that the capitalization needs to be the same in order to call that variable (ex. var myName and var MyName is two different variables)

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6
Q

What is the purpose of a string?

A

it holds the data type of letters and other characters

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7
Q

What is the purpose of a number?

A

it holds the data type of numbers (integers, decimals, etc)

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8
Q

What is the purpose of a boolean?

A

it holds the data type of one of two values, true or false

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9
Q

What does the = operator mean in JavaScript?

A

a value is being assigned to a variable

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10
Q

How do you update the value of a variable?

A

by assigning it to a new value

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11
Q

What is the difference between null and undefined?

A

null is subject to change, there is nothing but it could change into something. while undefined is literally nothing

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12
Q

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

A

so that you won’t get confused to what is being logged

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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

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15
Q

What is string concatenation?

A

when two or more strings are added together

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16
Q

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

A

used to add one value to another

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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

it adds and re-assigns a new value to a variable

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19
Q

What are objects used for?

A

to hold a set of variables that all link to one thing/object

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20
Q

What are object properties?

A

it tells us about the object, stores info ab the object (ex. name:, age:,)

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21
Q

Describe object literal notation.

A

an array of keys and values

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22
Q

How do you remove a property from an object?

A

using the delete operator

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23
Q

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

A

using a dot notation or bracket notation (ex. example.name, example[‘name’])

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24
Q

What are arrays used for?

A

to store multiple values

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25
Q

Describe array literal notation.

A

values within brackets [ ]
(ex. var colors = [‘red’, ‘white’, ‘blue’]

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26
Q

How are arrays different from “plain” objects?

A

arrays store actual variables in an index starting from 0

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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 shows the amount of variables inside of an array

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29
Q

How do you calculate the last index of an array?

A

by subtracting the length of an array by 1
(ex. var lastIndex = example.length -1;)

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30
Q

What is a function in JavaScript?

A

it is a “power” tool, it is used to run a sequence of code when it is called

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31
Q

Describe the parts of a function definition.

A

a function definition is the name used for that function so you have something to call it, also contains parameters
(ex. function sayHello(name) {} )

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32
Q

Describe the parts of a function call.

A

you can call a function using the function definition and parenthesis (ex. sayHello(‘Tim’) )

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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

function call has the value inside(a parameter) the parenthesis while the function definition has a variable (argument) inside

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34
Q

What is the difference between a parameter and an argument?

A

a parameter is like a placeholder for the value like a variable, an argument has a value. when we define a function we declare a parameter, when we call a function we pass it arguments

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35
Q

Why are function parameters useful?

A

it’s a placeholder so that we know what will come out as an argument

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36
Q

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

A

it produces a value for us to use and it prevents any more code in the function’s code block to run

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37
Q

Why do we log things to the console?

A

so that we able to see what the code is producing

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38
Q

What is a method?

A

it is 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 objects while function just performs a task when called

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40
Q

How do you remove the last element from an array?

A

using the pop() method

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41
Q

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

A

using the Math.floor() method

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42
Q

How do you generate a random number?

A

using the Math.random() method

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43
Q

How do you delete an element from an array?

A

using the slice() , pop(), shift() method

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44
Q

How do you append an element to an array?

A

using the push() method

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45
Q

How do you break a string up into an array?

A

using the split() method

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46
Q

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

A

string methods doesn’t change the original string. to double check you can log it to the console

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47
Q

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

A

about 50

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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

about 40

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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

so that the code will only run if the requirements are met

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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 (x = y) {
return ‘yay;
}

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56
Q

What are the three logical operators?

A

&&, ||, !

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57
Q

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

A

by using a logical operator && or | |

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58
Q

What is the purpose of a loop?

A

to run code over and over until the outcome is false or when it is capped

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59
Q

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

A

to see if the expression is true or not so it knows when to stop

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60
Q

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

A

a sequence of code run repeatidly

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61
Q

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

A

after the initialization expression

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62
Q

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

A

after the incrementation in the final expression or at the beginning of the iteration if it is the first time

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63
Q

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

A

before starting a new iteration

64
Q

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

A

after the iteration of the loop

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;

66
Q

What does the ++ increment operator do?

A

goes up 1

67
Q

How do you iterate through the keys of an object?

A

using for in loops

68
Q

Why do we log things to the console?

A

it is a debugger, it helps inspect a certain element

69
Q

What is a “model”?

A

a model is the looks and design of a document that is being put on a page from memory. models are made with objects

70
Q

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

A

the document that is in the script element

71
Q

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

A

the nodes

72
Q

What is a DOM Tree?

A

a dom tree consists of multiple nodes each representing an text content, elements, attributes, etc.

73
Q

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

A

document.querySelector()
document.getElementById()

74
Q

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

A

document.querySelectorAll()

75
Q

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

A

it is easier to console log and console dir

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

because html loads top to bottom and you want everything to load before you can use dom elements

78
Q

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

A

a css selector and returns the first matching element

79
Q

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

A

a css selector and returns all matching elements

80
Q

What is the purpose of events and event handling?

A

it dictates an action that follow an event (user interaction)

81
Q

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

A

no

82
Q

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

A

element.addEventListener()

83
Q

What is a callback function?

A

a function passed into another function as an argument, which is then called 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

the event object

85
Q

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

A

a reference to the specific element in the document (whether its a class, id, input, etc). if we are not sure we can log it into the console, more info at mdn

86
Q

What is the difference between these two snippets of code?
element.addEventListener(‘click’, handleClick)
element.addEventListener(‘click’, handleClick())

A

the second one will return the result of the function

87
Q

What is the className property of element objects?

A

it updates the class for the element

88
Q

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

A

you use the .className next to the variable that has the element selected

89
Q

What is the textContent property of element objects?

A

updates the text content of the element being selected

90
Q

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

A

you use .textContent next to the variable that has the element selected

91
Q

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

A

no, sometimes this will limit what you can do

92
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

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 <form>?

A

‘submit’

97
Q

What does the event.preventDefault() method do?

A

prevents the default action

98
Q

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

A

dot notations of the properties

99
Q

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

A
100
Q

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

A

you could be making mistakes along the way without realizing then you have to change everything. wastes time

101
Q

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

A

no it does not

102
Q

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

A

appendChild()

103
Q

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

A

the attribute you want to set and the value

104
Q

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

A

createElement(), setAttribute(), appendChild()

105
Q

What is the textContent property of an element object for?

A

to update the text content of the object

106
Q

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

A

setAttribute() or className

107
Q

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

A

so that we can just call it multiple times easily

108
Q

What is the event.target?

A

it shows the event that is being targeted during the bubbling and capture phase

109
Q

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

A

because if we want to target a specific element within the parent element we can use event.target to do so (bubbling)

110
Q

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

A

event.target.tagName

111
Q

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

A

it takes a css selector and it returns the closest specified selector

112
Q

How can you remove an element from the DOM?

A

Element.remove()

113
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

create a div that holds everything inside and then select that div and add the event listener to it

114
Q

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

A

it removes the element from the page

115
Q

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

A

it takes a selector and it returns true or false

116
Q

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

A

getAttribute()

117
Q

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

A

after every step

118
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

querySelect each and every element?

119
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

a lot of if else statements

120
Q

What is JSON?

A

a text based data with object syntax

121
Q

What are serialization and deserialization?

A

converting the data into bytes so you can save it on a local drive. deserialization is the opposite

122
Q

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

A

JSON.stringify()

123
Q

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

A

JSON.parse()

124
Q

How do you store data in localStorage?

A

setItem()

125
Q

How do you retrieve data from localStorage?

A

getItem()

126
Q

What data type can localStorage save in the browser?

A

JSON strings

127
Q

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

A

when the window/document is about to be unloaded

128
Q

What is a method?

A

a method is a function which is the property of an object

129
Q

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

A

method definition is where you create your own custom methods, when you call a method you call it of the object
(object.method())

130
Q

Describe method definition syntax (structure).

A

var obj = {
property: function () {
return example;
}
}

131
Q

Describe method call syntax (structure).

A

object.method()

132
Q

How is a method different from any other function?

A

methods are associated with objects while functions are not

133
Q

What is the defining characteristic of Object-Oriented Programming?

A

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

134
Q

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

A

abstraction, encapsulation, inheritance, polymorphism

135
Q

What is “abstraction”?

A

being able to do complex things in simple ways

136
Q

What does API stand for?

A

application programming interface

137
Q

What is the purpose of an API?

A

an API simplifies programming by abstracting the underlying implementation and only exposing objects or actions the developer need

138
Q

What is this in JavaScript?

A

this is an implicit parameter of all Javascript functions

139
Q

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

A

that it is available in a function’s code block even though it was never included in the function’s parameter list or declared with var

140
Q

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

A

call time

141
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

it will be the window until we call it then it will be the variable character

142
Q

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

A

‘It’s a me, Mario!’

143
Q

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

A

undefined

144
Q

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

A

technically you can’t until you call it but we assume that it is the object that the this is in

145
Q

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

A

if it is in a function code block it is part of a function but if it is not then it is for a method call

146
Q

What kind of inheritance does the JavaScript programming language use?

A

prototype-base

147
Q

What is a prototype in JavaScript?

A

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

148
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

inheritance from the prototype chain

149
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 prototype

150
Q

What does the new operator do?

A

it creates a new instance for user-defined objects that has a constructor function

151
Q

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

A

prototype property

152
Q

What does the instanceof operator do?

A

it checks to see if the prototype property of the constructor matches with the prototype chain of an object and it returns a boolean

153
Q

What is a “callback” function?

A

a function that goings in another function as an argument

154
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()

155
Q

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

A

adding to the setInterval() as a callback function

156
Q

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

A

0 so it would execute immediately

157
Q

What do setTimeout() and setInterval() return?

A

it returns an id (timeoutID and intervalID) so that it can be identified and cleared using clearInterval() or clearTimeout()

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