Javascript Flashcards
1
Q
What is the purpose of variables?
A
used to store information to be referenced and manipulated in a computer program
2
Q
How do you declare a variable?
A
with a var, let or const
3
Q
How do you initialize (assign a value to) a variable?
A
using the assignment operator
4
Q
What characters are allowed in variable names?
A
letters, numbers, dollar signs, nonpunctuation characters
5
Q
What does it mean to say that variable names are “case sensitive”?
A
takes account lower or uppercase
6
Q
What is the purpose of a string?
A
holds text characters, literal constant
7
Q
What is the purpose of a number?
A
stores numbers…
8
Q
What is the purpose of a boolean?
A
determines true or false
9
Q
What does the = operator mean in JavaScript?
A
assignment operator
10
Q
How do you update the value of a variable?
A
reassignment
11
Q
What is the difference between null and undefined?
A
null means no value, undefined means just not defined at all
12
Q
Why is it a good habit to include “labels” when you log values to the browser console?
A
gives a good point of reference
13
Q
Give five examples of JavaScript primitives.
A
String, boolean, number, null and undefined
14
Q
What data type is returned by an arithmetic operation?
A
number
15
Q
What is string concatenation?
A
operation of joining character strings end-to-end
16
Q
What purpose(s) does the + plus operator serve in JavaScript?
A
addition and string concatenation
17
Q
What data type is returned by comparing two values (, ===, etc)?
A
boolean
18
Q
What does the += “plus-equals” operator do?
A
adds the value on the right, to the variable on the left, and then assigns that value back into the variable on the left
19
Q
What are objects used for?
A
stores its state in fields
20
Q
What are object properties?
A
Object properties are defined as a simple association between name and value
21
Q
Describe object literal notation.
A
an array of key:value pairs, with a colon separating the keys and values, and a comma after every key:value pair, except for the last
22
Q
How do you remove a property from an object?
A
delete operator
23
Q
What are the two ways to get or update the value of a property?
A
dot or bracket notation
object.property
object[‘property’]
24
Q
What are arrays used for?
A
ordered lists
25
Describe array literal notation.
square brackets [ ] with a comma between each item
26
How are arrays different from "plain" objects?
arrays are ordered, items are indexed, arrays will repair themselves if items are deleted, arrays have a set length
27
What number represents the first index of an array?
0
28
What is the length property of an array?
count and stores the number of items in the array
29
How do you calculate the last index of an array?
.length - 1;
30
What is a function in JavaScript?
a repeatable series of statements to accomplish an operation
31
Describe the parts of a function definition.
function keyword
function name
parameters
code block
32
Describe the parts of a function call.
name of function, (), arguments
33
When comparing them side-by-side, what are the differences between a function call and a function definition?
function call is shorter since it doesn't have to define the function in curly braces
34
What is the difference between a parameter and an argument?
para: placeholders, arguments are real data
35
Why are function parameters useful?
dont need to use a var, great placeholder
36
What two effects does a return statement have on the behavior of a function?
allows users to assign the result of the function and closes the function
37
Why do we log things to the console?
To keep track of our progress and to verify our output
38
What is a method?
function that is a property of an object
39
How is a method different from any other function
methods are called with dot notation since they're a property
40
How do you remove the last element from an array
pop method
41
How do you round a number down to the nearest integer?
floor method of math object
42
How do you generate a random number?
random method of math object
43
How do you delete an element from an array?
pop to remove at end, splice to remove at a point, shift to remove begining
44
How do you append an element to an array?
push method
45
How do you break a string up into an array?
split method
46
Do string methods change the original string? How would you check if you weren't sure?
strings are immutable, can check through console.log
47
Is the return value of a function or method useful in every situation?
no sometimes it's built in
48
Roughly how many array methods are there according to the MDN Web docs?
36
49
What three-letter acronym should you always include in your Google search about a JavaScript method or CSS property?
mdn
50
Give 6 examples of comparison operators.
> , < , >= , <= , === , and !== .
51
What data type do comparison expressions evaluate to?
boolean
52
What is the purpose of an if statement?
decision-making statement that guides a program to make decisions based on specified criteria
53
Is else required in order to use an if statement?
no
54
Describe the syntax (structure) of an if statement.
if (condition) inside code-block
| statement1
55
What are the three logical operators?
|| (OR), && (AND), ! (NOT).
56
How do you compare two different expressions in the same condition?
logical and or logical or
57
What is the purpose of a loop?
repeats a sequence of instructions until a specific condition is met.
58
What is the purpose of a condition expression in a loop?
Condition is an expression that is tested each time the loop repeats. As long as condition is true, the loop keeps running.
59
What does "iteration" mean in the context of loops?
Iteration is the repetition of a function or process in a computer program.
60
When does the condition expression of a while loop get evaluated?
Before the code-block executes
61
When does the initialization expression of a for loop get evaluated?
The very first action, before anything
62
When does the condition expression of a for loop get evaluated?
before code block, and after the code block until the condition = false.
63
When does the final expression of a for loop get evaluated?
after the code block after the final condition has happened.
64
Besides a return statement, which exits its entire function block, which keyword exits a loop before its condition expression evaluates to false?
break
65
What does the ++ increment operator do?
(adds one to) its operand and returns a value
66
How do you iterate through the keys of an object?
through a for in loops
67
How do you iterate through the keys of an object?
through a for in loops
68
Why do we log things to the console?
To see if our expected output to match
69
What is a "model"?
a representation of the page
70
Which "document" is being referred to in the phrase Document Object Model?
html
71
What is the word "object" referring to in the phrase Document Object Model?
each object represents a different part of the page loaded in the browser window. data type
72
What is a DOM Tree?
the dom specifies the way in which the browser should structure the model. an element and its child elements with all the relevant information.
73
Give two examples of document methods that retrieve a single element from the DOM.
```
getElementsByTagName()
or querySelector('> tagname')
```
74
Give one example of a document method that retrieves multiple elements from the DOM at once.
getElementsByTagName()
| querySelector("h1")
75
Why might you want to assign the return value of a DOM query to a variable?
so you can reuse it or modify it
76
What console method allows you to inspect the properties of a DOM element object?
console.dir
77
Why would a tag need to be placed at the bottom of the HTML content instead of at the top?
so that way it can load all the html elements first
78
What does document.querySelector() take as its argument and what does it return?
the first Element within the document that matches the specified selector, or group of selectors.
79
What does document.querySelectorAll() take as its argument and what does it return?
a static (not live) NodeList representing a list of the document's elements that match the specified group of selectors.
80
Why do we log things to the console?
To make we see our expected output, and to check it's functional
81
What is the purpose of events and event handling?
used to handle and verify user input, user actions, and browser actions:
82
Are all possible parameters required to use a JavaScript method or function?
no
83
What method of element objects lets you set up a function to be called when a specific type of event occurs?
addEventListener() method
84
What is a callback function
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.
85
What object is passed into an event listener callback when the event fires?
after an asynchronous operation has completed
86
What is the event.target? If you weren't sure, how would you check? Where could you get more information about it?
console log it if u arent sure, The read-only target property of the Event interface is a reference to the object onto which the event was dispatched.
87
What is the difference between these two snippets of code?
The first one is a callback function
88
What is the className property of element objects?
sets or returns the class name of an element
89
How do you update the CSS class attribute of an element using JavaScript?
elm.setAttribute('class'
90
What is the textContent property of element objects?
sets or returns the text content of the specified node, and all its descendants
91
How do you update the text within an element using JavaScript?
HTML DOM textContent Property
92
Is the event parameter of an event listener callback always useful?
no
93
Would this assignment be simpler or more complicated if we didn't use a variable to keep track of the number of clicks?
harder
94
Why is storing information about a program in variables better than only storing it in the DOM?
when you mix and match things get complicated
95
What event is fired when a user places their cursor in a form control?
focus event
96
What event is fired when a user's cursor leaves a form control?
blur event
97
What event is fired as a user changes the value of a form control?
input event
98
What event is fired when a user clicks the "submit" button within a ?
submit event
99
What does the event.preventDefault() method do?
Prevents browser from reloading the page
100
What does submitting a form without event.preventDefault() do?
Reloads/refreshes the page
101
What property of a form element object contains all of the form's controls.
Elements property
102
What property of a form control object gets and sets its value?
Value property
103
What is one risk of writing a lot of code without checking to see if it works so far?
It’s hard to see where the code went wrong.
104
What is an advantage of having your console open when writing a JavaScript program?
To see if the code is working, or to check variable values, etc.
105
Does the document.createElement() method insert a new element into the page?
just creates an Element Node with the specified name
106
How do you add an element as a child to another element?
Node.appendChild()
107
What do you pass as the arguments to the element.setAttribute() method?
name and value
108
What steps do you need to take in order to insert a new element into the page?
The first step is to create the node (element) you wish to append, the next is to find where you wish to append it within the document, and the final step is to actually do the appending.
109
What is the textContent property of an element object for?
The textContent property sets or returns the text content of the specified element, and all its descendants
110
Name two ways to set the class attribute of a DOM element.
setAttribute, element.className
111
What are two advantages of defining a function to do create something (like the work of creating a DOM tree)?
You can create applications that update the data of the page without needing a refresh. Also, you can create applications that are customizable by the user and then change the layout of the page without a refresh.
112
What is the event.target?
event.target returns the where element thats being dispatched, so you can retrieve any property/ attribute that has a value
113
Why is it possible to listen for events on one element that actually happen its descendent elements?
Because of event delegation and That event listener analyzes bubbled events to find a match on child elements.
114
What DOM element property tells you what type of element it is?
event.target.tagName
115
What does the element.closest() method take as its argument and what does it return?
method searches up the DOM tree for the closest element which matches a specified element type. whatever css selector you used.
Will return itself or the matching ancestor. If no such element exists, it returns null
116
How can you remove an element from the DOM?
remove method
117
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?
event delegation and utilize the closest method
118
What is event target
The read-only target property of the Event interface is a reference to the object onto which the event was dispatched.
119
What is the affect of setting an element to display: none?
the affected element will disappear.
120
What does the element.matches() method take as an argument and what does it return?
The Element. matches() method returns true if the element would be selected by the specified selector string; returns a boolean
121
How can you retrieve the value of an element's attribute?
The getAttribute() method of the Element interface returns the value of a specified attribute on the element.
122
At what steps of the solution would it be helpful to log things to the console?
declaring, and changing a var
123
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?
spaghetti
124
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?
spaghetti
125
What is JSON?
JSON stands for JavaScript Object Notation
JSON is a lightweight format for storing and transporting data
JSON is often used when data is sent from a server to a web page
126
What are serialization and deserialization?
Serialization is a mechanism of converting the state of an object into a byte stream. Deserialization is the reverse process where the byte stream is used to recreate the actual Java object in memory.
127
Why are serialization and deserialization useful?
can be serialized on one platform and deserialized on an entirely different platform. Serialization allows the developer to save the state of an object and re-create it as needed, providing storage of objects as well as data exchange. T
128
How do you serialize a data structure into a JSON string using JavaScript?
JSON.stringify()
129
How do you deserialize a JSON string into a data structure using JavaScript?
JSON.parse
130
How to you store data in localStorage?
Window.localStorage
131
How to you retrieve data from localStorage?
getItem method of the localStorage object
132
What data type can localStorage save in the browser?
JSON / strings
133
When does the 'beforeunload' event fire on the window object?
The beforeunload event is fired when the window, the document and its resources are about to be unloaded. The document is still visible and the event is still cancelable at this point.
134
What is a method?
procedure associated with a message and an object.
135
How to declare a method
create an object, each property will declare a function with parameters, within the function, return in what you wanna do.
136
How can you tell the difference between a method definition and a method call?
A method is a function which is a property of an object. There are two kind of methods: Instance Methods which are built-in tasks performed by an object instance, or Static Methods which are tasks that are called directly on an object constructor.
137
Describe method call syntax (structure).
assign a function to a object property.
138
How is a method different from any other function?
A method, like a function, is a set of instructions that perform a task. The difference is that a method is associated with an object, while a function is not.
139
What is the defining characteristic of Object-Oriented Programming?
Object-oriented methodology relies on three characteristics that define object-oriented languages: encapsulation, polymorphism, and inheritance.
140
What are the four "principles" of Object-Oriented Programming?
Encapsulation, Abstraction, Inheritance, and Polymorphism.
141
What is "abstraction"?
is the process of taking away or removing characteristics from something in order to reduce it to a set of essential characteristics. the turn off light example. takes something complex, make it easy.
142
What does API stand for?
Application programming interface
143
What is the purpose of an API?
allows apps to send information between each other. While there are numerous protocols and technologies involved, the underlying purpose of APIs is always the same: to let one piece of software communicate with another.
144
What is this in JavaScript?
it's an implicit parameter, meaning 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.
145
What does it mean to say that this is an "implicit parameter"?
meaning 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.
146
When is the value of this determined in a function; call time or definition time?
call time
147
How can you tell what the value of this will be for a particular function or method definition?
if you cannot see the function being called, then you do not know what the value of this will be.
148
How can you tell what the value of this is for a particular function or method call?
the value of this can be recognized as "the object to the left of the dot" when the function is called (as a method). if not, the value of this can be recognized as "the object to the left of the dot" when the function is called (as a method).
149
What kind of inheritance does the JavaScript programming language use?
prototypal inheritance
150
What is a prototype in JavaScript?
Prototypes are the mechanism by which JavaScript objects inherit features from one another. to provide inheritance, objects can have a prototype object, which acts as a template object that it inherits methods and properties from.
151
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?
those methods are defined on a "prototype" object and arrays simply borrow those methods when they're needed.
152
If an object does not have its own property or method by a given key, where does JavaScript look for it?
looks through the in the prototype chain, and borrows it from the object.
153
What does the new operator do?
The new operator 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.
154
What property of JavaScript functions can store shared behavior for instances created with new?
prototype property
155
What does the instanceof operator do?
used to test if an object is of a given type. useful for testing
156
What is a "callback" function?
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.
157
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?
using the setTimeout()
158
How can you set up a function to be called repeatedly without using a loop?
by using setInterval and calling a function every second
159
What is the default time delay if you omit the delay parameter from setTimeout() or setInterval()?
0 seconds after code has been read and run
160
What do setTimeout() and setInterval() return?
The returned intervalID is a numeric, non-zero value which identifies the timer created by the call to setInterval()
161
What is Array.prototype.filter useful for?
creates a new array that's been trimmed down
162
What is Array.prototype.map useful for?
creates a new array populated with the results of calling a provided function on every element in the calling array.
163
What is Array.prototype.reduce useful for?
Combining the elements of an array into a single value.
164
What is "syntactic sugar"?
A cleaner way of writing something
165
What is the typeof an ES6 class?
function
166
Describe ES6 class syntax.
use the keyword class, give it a name which is typically uppercased, (cause it's a pattern for something like XML), add in an optional constructor within the code block with optional parameters, the constructor is a function that gets executed immediately whenever a new thing get instantiated.
inside the constructor function, have whatever the parameter is being assigned to the parameter of this object.
then inside the class code block: you can throw in whatever methods you want, which is shorthand syntax.
167
Describe ES6 class syntax
use the keyword class, give it a name which is typically uppercased, (cause it's a pattern for something like XML), add in a constructor within the code block with relevant parameters, the constructor is a function that gets executed immediately whenever a new thing get instantiated.
inside the constructor function, have whatever the parameter is being assigned to the parameter of this object.
then inside the class code block: you can throw in whatever methods you want, which is shorthand syntax. in there you can do whatever.
168
What is "refactoring"?
changing code, without changing it's external behavior. intended to improve the design, structure, and/or implementation of the software
169
What does fetch() return?
returns the data of the format JSON, also it lets you know if your promised failed if you had invalid input
returns a promise, then use then or catch
170
What is the default request method used by fetch()?
GET
171
How do you specify the request method (GET, POST, etc.) when calling fetch?
use object literal, method property within that code block
172
What must the return value of myFunction be if the following expression is possible?
myFunction()();
the return of the inner function.
173
What allows JavaScript functions to "remember" values from their surroundings?
closures
174
In JavaScript, when is a function's scope determined; when it is called or when it is defined?
when it is defined
