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LFZ > JavaScript > Flashcards

JavaScript Flashcards

1
Q

What is the purpose of variables?

A

Store bits of data.

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2
Q

How do you declare a variable?

A

create a variable using

var(variable keyword) then providing it a name quantity(variable name): var quantity;

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3
Q

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

A

By using the =(assignment operator) sign followed by the data.

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4
Q

What characters are allowed in variable names?

A

a-z, 0-9, $ and _

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5
Q

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

A

You can have two variables with different cases that refer to different variables but use the same word.

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6
Q

What is the purpose of a string?

A

To store written text often times information about a user.

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7
Q

What is the purpose of a number?

A

Used to count the items of things or doing mathematical equations.

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8
Q

What is the purpose of a boolean?

A

Gives us the ability to make decisions.

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9
Q

What does the = operator mean in JavaScript?

A

It is the assignment operator.

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10
Q

How do you update the value of a variable?

A

use the variable name followed by = and the new value:

fullname = Kalen Cobb

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11
Q

What is the difference between null and undefined?

A

Null = used by a developer to denote future use

undefined is usually accidental and the computer is telling you it 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

So you can keep track of your output.

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13
Q

Give some examples of JavaScript primitives.

A

strings, numbers, booleans, null, undefined.

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14
Q

What datatype is returned by an arithmetic operation?

A

Number

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15
Q

What is string concatenation?

A

The conjoining of two or more strings together using the + operator.

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16
Q

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

A

Addition and Concatenation

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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

Addition Assignment

adds whatever is on the right side to the value of the variable and then the result of that expression is assigned to the variable.

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19
Q

What are objects used for?

A

A box to keep related stuff together

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20
Q

What are object Properties?

A

Another way to store data, exclusive to the specific variable

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21
Q

Describe object literal notation.

A

Properties and values nested inside curly braces.

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22
Q

How do you remove a property from an object?

A

delete operator (although this is rare)

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23
Q

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

A

dot notation or bracket notation

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24
Q

What are arrays used for?

A

Storing lists of data

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25
Q

Describe array literal notation.

A

values nested inside square brackets.

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26
Q

How are arrays different from “plain” objects?

A

Arrays are an ordered list with numeric indexes

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27
Q

What is the length property of an array?

A

A property that holds the number of items in an Array

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28
Q

How do you calculate the last index of an array?

A

array.length -1

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29
Q

Describe the parts of a function definition.

A
  1. Function keyword
  2. Function name (optional)
  3. Comma separated parameters surrounded by parenthesis ( ).
  4. Function code block starting with an opening curly brace {
  5. Optional return statement
  6. end of the functions code block indicated by a closing curly brace }
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30
Q

What is a function in JavaScript?

A

A group of actions that are repeatable

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31
Q

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

A

The Definition is followed by the code block which holds the tasks, while the Call is followed by parentheses only.

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32
Q

What is the difference between a parameter and an argument?

A

A parameter is a placeholder for the argument that is passed to it.

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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

The function keyword will not be present when being called.

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34
Q

What is the difference between a parameter and an argument?

A

A parameter is a variable placeholder for the argument that is passed to it.

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35
Q

Why are function parameters useful?

A

We can input data we didn’t previously have when writing the function.

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36
Q

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

A
  1. Causes the function to produce a value we can use in our program.
  2. Prevents any more code in the function’s code block from being run.
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37
Q

Why do we log things to the console?

A

To debug our work during the development phase.

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38
Q

What is a method?

A

A method 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

It is being used ON an object (object.method())

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40
Q

How do you remove the last element from an array?

A

.pop()

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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

Use .splice() method

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44
Q

How do you append an element to an array?

A

.push() method

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45
Q

How do you break a string up into an array?

A

.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 look at the documentation.

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47
Q

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

A

Look at the Docs!

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48
Q

What three-letter acronym should you alwasy include in your google search about a JavaScript method or CSS property?

A

MDN

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49
Q

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

A

Not really.

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50
Q

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

A

look at the docs!

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51
Q

What data type do comparison expressions evaluate to?

A

Boolean

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52
Q

Is else required in order to use an if statement?

A

No

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53
Q

Describe the syntax (structure) of an if statement.

A

Keyword if (condition in parenthesis) opening curly brace
code
closing curly brace

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54
Q

What are three logical operators?

A

&&(logical and), ||(logical or), ! (logical not)

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55
Q

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

A

Using && and/or ||

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56
Q

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

A

Using && and/or ||

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57
Q

What is the purpose of a loop?

A

Allow us to do a repeatable action.

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58
Q

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

A

To check whether or not the loop should stop.

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59
Q

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

A

The number of times we run the action.

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60
Q

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

A

Immediately and before every iteration.

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61
Q

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

A

Before anything else but only once.

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62
Q

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

A

After the initialization and with every iteration.

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63
Q

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

A

At the end of each iteration.

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64
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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65
Q

What does the ++ increment operator do?

A

Increases by an increment of one after it is used.

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66
Q

How do you iterate through the keys of an object?

A

Use a for…in loop.

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67
Q

Why do we log things to the console?

A

To check our work and inspect our data.

Verification and inspecting data.

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68
Q

What is a “model”?

A

A system or thing used as an example to follow or imitate.

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69
Q

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

A

The HTML document

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70
Q

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

A

JavaScript Objects

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71
Q

What is a DOM Tree?

A

A model of a webpage consisting of all of the elements and their nodes including the parent and any child and other descendents.

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72
Q

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

A

getElementById()

querySelector() (use this primarily)

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73
Q

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

A

getElementsByClassName()
getElementsByTagName()
querySelectorAll() (use this primarily)

74
Q

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

A

For future use to help with browser search speed.

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

HTML needs to load completely before we access it through JavaScript

77
Q

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

A

A css selector in the form of a string.

It returns the first HTML element object.

78
Q

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

A

A css selector in the form of a string.

It returns a node list of all HTML elements.

79
Q

Why do we log things to the console?

A

To confirm our assumptions about our data.

80
Q

What is the purpose of events and event handling?

A

To make the webpage more interactive.

81
Q

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

A

That the content in that portion is optional.

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

A function to be used at some point in the future.

84
Q

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

A

event

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

The property which holds a reference to the HTML element that triggered the event.

Look up the documents on MDN.

86
Q

What is the difference between these two snippets of code?

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

A

The handleClick() is calling the function whereas handleClick is being passed to the method.

87
Q

What is the className property of element objects?

A

A string containing the value of a class attribute on that element.

88
Q

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

A

.className

89
Q

What is the textContent property of element objects?

A

A string containing the value of the text content of an element.

90
Q

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

A

.textContent = value

91
Q

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

A

No.

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

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

A

Easier access for later use.

94
Q

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

A

focus event

95
Q

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

A

blur event

96
Q

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

A

submit event

97
Q

What does the event.preventDefault() method do?

A

prevents the browser from reloading the page

98
Q

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

A

the elements property

99
Q

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

A

the value

100
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 things are failing.

101
Q

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

A

Ability to check if things are working.

102
Q

what event is fired when a user changes the value of a form control?

A

input event

103
Q

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

A

The page will be reloaded.

104
Q

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

A

No

105
Q

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

A

.appendChild()

106
Q

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

A

name of the attribute you want to set

and the value you want that attribute to have

107
Q

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

A

create element then append to the element you want it to be attached to

108
Q

What is the textContent property of an element object for?

A

setting or getting text of an element

109
Q

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

A

.setAttribute

.className

110
Q

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

A

You don’t have to write the same code over and over.

Easier to test because you’re putting data in and getting data out.

111
Q

What is the event.target?

A

The element most directly interacted with by an event.

112
Q

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

A

event bubbling

113
Q

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

A

.tagName

114
Q

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

A

A selector containing a selector list, and it returns the element which is the closest ancestor of the element.

115
Q

How can you remove an element from the DOM?

A

.remove()

116
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

Add the event listener to the parent item that will contain the new item(s).

117
Q

What is the event.target?

A

The element that originated the event

118
Q

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

A

Turns off the display of an element so that it has no effect on layout.

119
Q

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

A

It takes a selector in the form of 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

Every.

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

Add new event listeners, queries, etc to make up for the new tab/view.

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

It would have more code to check every tab.

124
Q

What is a Method?

A

A function which is a property of an object.

Instance Methods: built in tasks performed by an object instance.

Static Methods: Tasks that are called directly on an object constructor.

125
Q

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

A

Method definition starts with a function keyword and is followed by the code block.

Method call is being called using parentheses and dot notation.

126
Q

Describe method definition syntax (structure).

A

Property: function (parameters) {
code block
};

127
Q

Describe method call syntax (structure).

A

object.methodName()

128
Q

How is a method different from any other function?

A

Dot or bracket notation must be used to be called.

129
Q

What is the defining characteristic of Object-Oriented Programming?

A

Objects can contain both data and properties as methods.

130
Q

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

A

Abstraction
Encapsulation
Inheritance
Polymorphism

131
Q

What is “abstraction”?

A

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

132
Q

What does API stand for?

A

Application Programming Interface

133
Q

What is the purpose of an API?

A

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

134
Q

What is This in JavaScript?

A

An implicit parameter that allows us to see the object that ‘this’ is being invoked upon.

135
Q

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

A

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

136
Q

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

A

Function is called.

137
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

There is no ‘this’ yet.

138
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 this is referring to the object character as defined in the calling of greet().

139
Q

Given the above character object, what is the result of the following code snippet? why?

var hello = character.greet;
hello();
A

undefined because hello is not a method of the character function?

140
Q

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

A

You cannot know what it will be.

141
Q

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

A

This is defined when the function/method is called. If it is part being called as a property of an object it will return from that object, otherwise it is the global window object.

142
Q

What kind of inheritance does the JavaScript programming language use?

A

Prototype-based (or prototypal)

143
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

Prototypal Inheritance

144
Q

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

A

in the prototypes?

145
Q

What does the NEW operator do?

A

Creates a new instance of an object.

146
Q

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

A

prototype

147
Q

What does the INSTANCEOF operator do?

A

Check whether the instance we are checking is an instance of something else and it returns a boolean.

148
Q

What is a “callback” function?

A

A callback function is 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.

149
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()

150
Q

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

A

setInterval()

151
Q

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

A

0

152
Q

What do setTimeout() and setInterval() return?

A

A timeout ID and an interval ID.

153
Q

What is AJAX?

A

A technique for loading data into part of a page without having to refresh the entire page. The data is often sent in a format called JavaScript Object Notation (JSON).

154
Q

What does AJAX acronym stand for?

A

Asynchronous JavaScript And XML.

155
Q

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

A

XMLHttpRequest.

156
Q

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

A

Load

157
Q

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

A

They share the proto event target.

158
Q

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

A

A piece of code inside curly braces
Function code block
conditional code block
loops

159
Q

What does block scope mean?

A

The variable is only used within the code block in which it is initialized.

160
Q

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

A

Block-Scope

161
Q

What is the difference between let and const?

A

Let is mutable while const is immutable, mostly.

162
Q

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

A

You are not assigning the variable to a new value, you are assigning it A new property.

163
Q

How should you decide on which type of declaration (const or let) to use?

A

If you need a variable to be modifiable you should use let otherwise use const.

164
Q

What is destructuring, conceptually?

A

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables

165
Q

What is the syntax for Object destructuring?

A

Let/const keyword

Opening { for property list

Comma separated property keys

Closing } for property list

Assignment operator followed by object name.

166
Q

What is the syntax for Array destructuring?

A

Let/const keyword

Opening square bracket

Comma separated variables.

Closing square bracket

Assignment operator followed by name of array.

167
Q

How can you tell the difference between destructuring and creating Object/Array literals?

A

The syntax.

168
Q

What is the syntax for writing a template literal

A

Surround the string in backticks instead of quotes. Within the string you can use JavaScript Expressions (${}).

169
Q

What is “string interpolation”?

A

String formatting: The ability to substitute part of the string for the values of variables or expressions.

170
Q

What is this: ${data}?

A

A JavaScript expression.

171
Q

What is the syntax for defining an arrow function?

A

let/const keyword

parameters (a, b) if theres more than one you need parenthesis.

arrow =>

code

(If you use the block syntax you need to use the return keyword {return a + b}

If you use an expression you do not need curly braces, but if you use a statement curly braces are required.

172
Q

When an arrow function’s body is left without curly braces, what changes in its functionality?

A

It expects an expression and it will return a value.

173
Q

How is the value of this determined within an arrow function?

A

It captures the “this” value of the enclosing context instead of creating its own “this” context.

174
Q

What is Array.prototype.filter useful for?

A

Finding a specific variety of data in an array.

175
Q

What is Array.prototype.map useful for?

A

Manipulating each piece of data from an array individually.

176
Q

What is Array.prototype.reduce useful for?

A

Manipulating the data from an array using the other data in the array.

177
Q

What must the return value of myFunction be if the following expression is possible?

myFunction()();

A

It returns a function.

178
Q

What does this code do?

const wrap = value =>()=>value;

A

Creates a variable named wrap that returns a function that takes no arguments and returns a value.

179
Q

In JavaScript, when is a function’s scope determined; when it is called or when it is defined?

A

When it is defined.

180
Q

What allows JavaScript functions to “remember” values from their surroundings?

A

Lexical Scope

LFZ (14 decks)

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