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LearningFuze > Module 1 Javascript > Flashcards

Module 1 Javascript Flashcards

1
Q

What is the purpose of variables?

A

It tells the script to store information to be used again later.

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2
Q

How do you declare a variable?

A

declare var, let, or const. Can be declared undefined or can have value assigned initially.

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3
Q

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

A

Use equal sign =

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4
Q

What characters are allowed in variable names?

A

Must start with any letter, dollar sign $, or underscore _, numbers as long as it is not the first letter, no - or .

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5
Q

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

A

Capitalization matters in variable name

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6
Q

What is the purpose of a string?

A

String stores a series of letters or other characters, used for adding text.

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7
Q

What is the purpose of a number?

A

Store numerical values.

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8
Q

What is the purpose of a boolean?

A

Booleans store a true or false value.

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9
Q

What does the = operator mean in JavaScript?

A

single equal sign = is for assigning values.

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10
Q

How do you update the value of a variable?

A

state variableName = newValue; Reassign, not redeclare.

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11
Q

What is the difference between null and undefined?

A

Null represents an nonexistend or invalid object. Undefined means nothing has been declared or assigned, so there is nothing, not even an object.

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12
Q

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

A

Helps you debug so you know at which step and for what reason the console is 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

Combining two or more strings into one string. Numbers concatenated with strings also become a string.

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16
Q

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

A

It can both add numbers as an operator and concatenate strings.

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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

Adds the value to the right and assigns it to the variable on the left.

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19
Q

What are objects used for?

A

Grouped set of property variables to create a model of a real world object.

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20
Q

What are object properties?

A

The variables within the object

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21
Q

Describe object literal notation.

A

the object name is being declared and assigned with opening curly brace, within are key values and property pairs separated by commas, and closing curly brace.

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22
Q

How do you remove a property from an object?

A

Use delete keyword and call out the object property using either dot or bracket notation.

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23
Q

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

A

Name the object property using either dot or bracket notation, and assign a new value if desired.

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24
Q

What are arrays used for?

A

To have an ordered list of items

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25
Q

Describe array literal notation.

A

The array name being declared and assigned with an opening bracket, a list of values separated by commas, and a closing bracket.

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26
Q

How are arrays different from “plain” objects?

A

Array values are numerically indexed.

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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

The number of values the array contains

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29
Q

How do you calculate the last index of an array?

A

Take the array.length - 1

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30
Q

What is a function in JavaScript?

A

Pack up a dynamic code that can be reused to do something within javascript. Or- set of instructions that we can call again at any time.

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31
Q

Describe the parts of a function definition.

A

function name, function parameters, function codeblock with opening and closing curly braces, return statement within codeblock.

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32
Q

Describe the parts of a function call.

A

Function name with the parentheses and parameters within if applicable.

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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 definition sets up the code for the function to run when it is called, but does not run it. Function call runs the code within the definition.

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34
Q

What is the difference between a parameter and an argument?

A

Parameters are placeholders for arguments to be used, arguments are what the code actually uses.

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35
Q

Why are function parameters useful?

A

They let you hold the place of future unknown arguments to be used.

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36
Q

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

A

It tells the function to produce a value, prevents anything else after in the function code from being run, and exits the function codeblock.

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37
Q

Why do we log things to the console?

A

To help keep track of what is going on in the code and help with debugging

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38
Q

What is a method?

A

Functions that are properties of an object

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39
Q

How is a method different from any other function?

A

They are stored within and called from an object. They are attached to an object.

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40
Q

How do you remove the last element from an array?

A

use array.prototype.pop() method.

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41
Q

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

A

use Math.floor(#) method.

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42
Q

How do you generate a random number?

A

use Math.random() method. Only generates between 0 and 1, 0 and 1 not inclusive.

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43
Q

How do you delete an element from an array?

A

use array.prototype.splice(index, deleteCount)

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44
Q

How do you append an element to an array?

A

Use array.prototype.push() method to end, unshift to push to front.

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45
Q

How do you break a string up into an array?

A

Use string.prototype.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

No, and you can check with console.log(). Note strings are immutable, they cannot be changed.

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47
Q

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

A

38 (Just note that there are more than 30 and it can be looked up)

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48
Q

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

A

No. Note keyword ‘every’, not always have to be returned. For example, pop method just used to remove something from array but you don’t use what is popped.

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49
Q

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

A

36 (Just note that there are more than 30 and it can be looked up)

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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 value (true or false)

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53
Q

What is the purpose of an if statement?

A

Sets up conditional statement and tells it run if true.

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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 (condition) {code}

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56
Q

What are the three logical operators?

A

and &&, or ||, not !

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57
Q

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

A

Use logical operator to use both expressions

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58
Q

What is the purpose of a loop?

A

To repeat a code multiple times as needed

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59
Q

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

A

It tells the loop whether to run or stop

60
Q

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

A

Each time the loop is run once

61
Q

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

A

Before start of loop every loop

62
Q

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

A

Once at start of loop

63
Q

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

A

After initialization, before each loop iteration, and after final expression.

64
Q

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

A

After code block is completed.

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. Note break ends a loop, but does not end the function. Bonus: continue tells loop don’t do anything for the iteration, go to next.

66
Q

What does the ++ increment operator do?

A

Adds 1 to the variable and assigns it to the variable.

67
Q

How do you iterate through the keys of an object?

A

with for…in loop

68
Q

What is a “model”?

A

A representation of something

69
Q

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

A

HTML document

70
Q

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

A

The objects representing the parts document

71
Q

What is a DOM Tree?

A

The object model made up of objects representing parts of the html document.

72
Q

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

A

getElementById() or querySelector()

73
Q

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

A

querySelectorAll(), alternatively getElementsByClassName() or getElementsByTagName()

74
Q

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

A

So you can easily save and reference the location of the elements to be used again later. Why it is useful: saves time to keep querying the every time an element needs to be pulled up. Saving it in variable no longer re-queries the whole document.

75
Q

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

A

console.dir()

76
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 parse all elements of a html document before javascript can access them.

77
Q

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

A

It takes css selectors as arguments and returns the first matching element.

78
Q

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

A

It takes css selectors as arguments and returns a nodelist of matching elements.

79
Q

What does the transform property do?

A

Lets you modify an element on the screen. Rotate, scale, skew, or translate (move) an element.

80
Q

Give four examples of CSS transform functions.

A

rotate, scale, skew, translate

81
Q

What is the purpose of events and event handling?

A

To create an interactive website, set up functionality in response to user interactions.

82
Q

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

A

No

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 method

84
Q

What is a callback function?

A

A function used as an argument for another function.

85
Q

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

A

Event object.

Note: event is an object. addEventListener passes in the event object.

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 returns the target that triggered the event. You can console log or look it up on MDN.

87
Q

What is the difference between these two snippets of code?

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

One calls a callback function. The other sets up an incomplete definition that does nothing.

88
Q

What is the className property of element objects?

A

It queries the class name of the element

89
Q

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

A

Query the class location and reassign className to a new string

90
Q

What is the textContent property of element objects?

A

It is the text content contained within the node

91
Q

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

A

Query the text location and reassign textContent to a new string

92
Q

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

A

No

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

DOM is always being modified, but javascript maintains variables within it.

95
Q

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

A

focus event

96
Q

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

A

blur event

97
Q

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

A

input event

98
Q

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

A

submit event

99
Q

What does the event.preventDefault() method do?

A

Prevents page from refreshing after submittal

100
Q

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

A

Page refreshes upon submittal

101
Q

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

A

HTMLFormElement.elements

102
Q

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

A

(HTMLFormElement.elements[index#] or HTMLFormElement[‘name’])

103
Q

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

A

You don’t know where or why some part of the code fails or does something unexpected

104
Q

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

A

To see how the code is working at each step

105
Q

The transition property is shorthand for which four CSS properties?

A

transition-delay, transition-duration, transition-property, transition-timing-function

106
Q

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

A

No, it only creates a node

107
Q

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

A

node.appendChild()

108
Q

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

A

the name of the attribute as a string and the attribute value as a string.

109
Q

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

A

Create an element and append it to an existing element in the document

110
Q

What is the textContent property of an element object for?

A

The text to be displayed within the text node.

111
Q

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

A

node.className = ‘classname’ or element.setAttribute(‘class’, ‘classname’)
Bonus 3rd: classList.add

112
Q

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

A

It can repeatedly do the same thing as needed and keeps the code organized.

113
Q

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

A

Height and width, see mdn for full list of features

114
Q

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

A

viewport meta tag, aka meta tag with name=”viewport”

115
Q

What is the event.target?

A

the target element of that event

116
Q

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

A

Bubbling, where the event starts from most specific and flows out to least specific.

117
Q

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

A

event.target.tagname. Note: always uppercase. Trivia: all HTML used to be written in uppercase, which is why this happens.

118
Q

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

A

It takes a selector as argument and returns the closest ancestor to that selector, or the element itself. Or null if nothing can be found.

119
Q

How can you remove an element from the DOM?

A

element.remove();

120
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

Add event listener to the parent element.

121
Q

What is a breakpoint in responsive Web design?

A

Any points at which css changes relative to media query. Not just screen width. Example: print.

122
Q

What is the advantage of using a percentage (e.g. 50%) width instead of a fixed (e.g. px) width for a “column” class in a responsive layout?

A

They stay at a percent relative to the total content width

123
Q

If you introduce CSS rules for a smaller min-width after the styles for a larger min-width in your style sheet, the CSS rules for the smaller min-width will “win”. Why is that?

A

Css is read from top to bottom. Last one read is on top. So smaller min width should be placed earlier.

124
Q

What is the event.target?

A

The target element of an event triggered

125
Q

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

A

It is not visibly displayed on a document and removes that element from document flow. Note this is different from visibility: none. Visibility none will maintain its space. Opacity none will still be interactable.

126
Q

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

A

It takes a selector and returns true or false.

127
Q

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

A

the element.getAttribute(attributeName) method.

128
Q

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

A

All steps such as checking event target, event target matches, event target attribute, whether the loops are properly looping through the right elements, etc.

129
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 to set up listener for every tab

130
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 would have to manually check each nodelist with long list of if-statements to see if it is the right node to show or hide.

131
Q

What is JSON?

A

A string that represents an object which can be passed as information to other computers and programs

132
Q

What are serialization and deserialization

A

Taking an object and turning it into a string which is saved as different data, and turning a string back into an object.

133
Q

Why are serialization and deserialization useful?

A

It lets you transfer data between computers and different programs

134
Q

How do you serialize data into JSON using JavaScript?

A

stringify

135
Q

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

A

parse

136
Q

How do you store data in localStorage?

A

Use the localStorage.setItem(keyname, keyvalue) method.

137
Q

How do you retrieve data from localStorage?

A

Use the localStorage.getItem(keyname) method.

138
Q

What data type can localStorage save in the browser?

A

JSON data.

139
Q

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

A

Before a page is unloaded, such as if it is refreshed or closed.

140
Q

What is a code block? What are some examples of a code block?

A

Declarations of code within curly braces { } such as in function, if, for, while.

141
Q

What does block scope mean?

A

When something exists only within a code block.

142
Q

What is the scope of a variable declared with const or let?

A

Both are block scope

143
Q

What is the difference between let and const?

A

let can declare new variables in loops so each iteration is tracked. Const ‘constant’ creates a read-only variable that cannot be changed, though the contents of the variable (if it is object, array, or number) can be changed.

144
Q

Why is it possible to .push() a new value into a const variable that points to an Array?

A

The variable cannot be reassigned so the reference to the array is immutable. But you can change the array contents within. It is only immutable reference. You can’t change a string for example.

145
Q

How should you decide on which type of declaration to use?

A

Whether or not you want it to be global scope and if it can be changed.

146
Q

What is the syntax for writing a template literal?

A

Use backticks `

147
Q

What is “string interpolation”?

A

Turning a variable value into a string.

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