JavaScript Flashcards
1
Q
What is the purpose of variables?
A
To store values to use later on
2
Q
How do you declare a variable?
A
var name
3
Q
How do you initialize (assign a value to) a variable?
A
var name = “xxxx”
4
Q
What characters are allowed in variable names?
A
letters (uppercase, lowercase), $, underscore _ , numbers (can’t start with numbers)
5
Q
What does it mean to say that variable names are “case sensitive”?
A
lower case and upper case are not exchangeable in variable name
6
Q
What is the purpose of a string?
A
to store text
7
Q
What is the purpose of a number?
A
to store numeric value
8
Q
What is the purpose of a boolean?
A
to store binary values and make decisions
9
Q
What does the = operator mean in JavaScript?
A
assignment operator (to put value in something)
10
Q
How do you update the value of a variable?
A
name = new value (do not need to redeclare var after it’s been declared first time)
11
Q
What is the difference between null and undefined?
A
null is an assigned value (intentional). undefined = empty
12
Q
Why is it a good habit to include “labels” when you log values to the browser console?
A
to make it easier to debug
13
Q
Give five examples of JavaScript primitives.
A
string, number, boolean, null, undefined,
14
Q
What data type is returned by an arithmetic operation?
A
number
15
Q
What is string concatenation?
A
adding strings together
16
Q
What purpose(s) does the + plus operator serve in JavaScript?
A
add numerical values or concatenate strings
17
Q
What data type is returned by comparing two values (, ===, etc)?
A
Boolean - true or false
18
Q
What does the += “plus-equals” operator do?
A
add onto current variable value
19
Q
What are objects used for?
A
to store relevant data together (grouping)
20
Q
What are object properties?
A
to store additional data relevant to objects
21
Q
Describe object literal notation.
A
{
property : value
}
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 notation and bracket notation
24
Q
What are arrays used for?
A
store multiple values
25
Describe array literal notation.
list of zero or more expressions, each of which represents an array element, enclosed in square brackets
26
How are arrays different from "plain" objects?
Arrays have an order & numeric indexes, and [ ]
27
What number represents the first index of an array?
0
28
What is the length property of an array?
true count of values stored in an array
29
How do you calculate the last index of an array?
length - 1
30
What is a function in JavaScript?
repeatable, reusable block of code with the potential to receive and return different values.
31
Describe the parts of a function definition.
keyword function, name of the function, parameter, opening curly brace
```
function functionName (parameter) {
}
```
32
Describe the parts of a function call.
name of the function followed by arguments
getFunction(2,2);
33
When comparing them side-by-side, what are the differences between a function call and a function definition?
```
function definition has keyword and code block
function call has arguments and values
```
34
What is the difference between a parameter and an argument?
parameter is a placeholder
| argument is data provided for the function call and returns a value
35
Why are function parameters useful?
they're a placeholder
36
What two effects does a return statement have on the behavior of a function?
returns the value and
| exits out of the function code block
37
Why do we log things to the console?
to display and debug
38
What is a method?
function that stores data about the object
39
How is a method different from any other function?
40
How do you remove the last element from an array?
"top" method - take off last item
41
How do you round a number down to the nearest integer?
floor method
42
How do you generate a random number?
random method
43
How do you delete an element from an array?
splice method
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?
47
Roughly how many string methods are there according to the MDN Web docs?
48
Is the return value of a function or method useful in every situation?
49
Roughly how many array methods are there according to the MDN Web docs?30
30
50
What three-letter acronym should you always include in your Google search about a JavaScript method or CSS property?
MDN
51
Give 6 examples of comparison operators.
===, , <=, >=, !=
52
What data type do comparison expressions evaluate to?
Boolean
53
What is the purpose of an if statement?
to determine whether to perform a function or not
54
Is else required in order to use an if statement?
no
55
Describe the syntax (structure) of an if statement.
if (condition) { code block }
56
What are the three logical operators?
&&, ||, !
57
How do you compare two different expressions in the same condition?
&& or || operator
58
What is the purpose of a loop?
Repeated block of code happening under some condition
59
What is the purpose of a condition expression in a loop?
tell the function when to stop running
60
What does "iteration" mean in the context of loops?
one repetition of a loop code block
61
When does the condition expression of a while loop get evaluated?
before each iteration (before it runs)
62
When does the initialization expression of a for loop get evaluated?
in the very beginning (first step)
63
When does the condition expression of a for loop get evaluated?
after initialization and before the code block runs
64
When does the final expression of a for loop get evaluated?
after each iteration
65
Besides a return statement, which exits its entire function block, which keyword exits a loop before its condition expression evaluates to false?
break
66
What does the ++ increment operator do?
adds 1 to the variable and assigns it to the var
67
How do you iterate through the keys of an object?
for-in loop
68
Why do we log things to the console?
to test the output
69
What is a "model"?
representation (not the actual thing, but the recreation of it, usually in a smaller scale)
70
Which "document" is being referred to in the phrase Document Object Model?
HTML document
71
What is the word "object" referring to in the phrase Document Object Model?
Object = data type, object, in javascript
72
What is a DOM Tree?
73
Give two examples of document methods that retrieve a single element from the DOM.
getElementById, querySelector
74
Give one example of a document method that retrieves multiple elements from the DOM at once.
querySelectorAll
75
Why might you want to assign the return value of a DOM query to a variable?
to be able to re-use it without having to call it over and over again
76
What console method allows you to inspect the properties of a DOM element object?
dir (short for directory) method
77
Why would a tag need to be placed at the bottom of the HTML content instead of at the top?
so that html can run before javascript does
78
What does document.querySelector() take as its argument and what does it return?
css selector, returns first element that matches
79
What does document.querySelectorAll() take as its argument and what does it return?
css selector, and returns nodelist (houses list of DOM elements)
80
What is the purpose of events and event handling?
perform an action when an event is called.
81
Are all possible parameters required to use a JavaScript method or function?
no (e.g. third (boolean) value for addEventListener)
82
What method of element objects lets you set up a function to be called when a specific type of event occurs?
addEventListener
83
What is a callback function?
function definition being passed as an argument
84
What object is passed into an event listener callback when the event fires?
event object
85
What is the event.target? If you weren't sure, how would you check? Where could you get more information about it?
target is a property in the event object that tells you where the event originally occurred.
86
What is the difference between these two snippets of code?
element. addEventListener('click', handleClick)
element. addEventListener('click', handleClick())
handleClick is being called as the event executes
| handleClick() is being called (immediately)
87
What is the className property of element objects?
get or set value of a className attribute
88
What is the className property of element objects?
89
How do you update the CSS class attribute of an element using JavaScript?
90
What is the textContent property of element objects?
To get or set the text content
91
How do you update the text within an element using JavaScript?
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?
94
Why is storing information about a program in variables better than only storing it in the DOM?
95
What event is fired when a user places their cursor in a form control?
focus
96
What event is fired when a user's cursor leaves a form control?
blur
97
What event is fired as a user changes the value of a form control?
input
98
What event is fired when a user clicks the "submit" button within a ?
submit
99
What does the event.preventDefault() method do?
prevents default behavior from the event
100
What does submitting a form without event.preventDefault() do?
refresh the entire 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?
hard to figure out where it went wrong
104
What is an advantage of having your console open when writing a JavaScript program?
you can see what's going on as you progress
105
Does the document.createElement() method insert a new element into the page?
no
106
How do you add an element as a child to another element?
appendChild method
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?
createElement
| parent.appendChild(child)
109
What is the textContent property of an element object for?
assign new text to an element or see what the text is
110
Name two ways to set the class attribute of a DOM element.
setAttribute
| className
111
What are two advantages of defining a function to do create something (like the work of creating a DOM tree)?
you can re-use it and
| give it a name
112
What is the event.target?
113
Why is it possible to listen for events on one element that actually happen its descendent elements?
114
What DOM element property tells you what type of element it is?
tagName (element name in all caps)
115
What does the element.closest() method take as its argument and what does it return?
takes a selector as an argument, and returns the ancestor
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?
add it to the ancestor element
118
What is the event.target?
DOM element object representing the element where the event originally occured
119
What is the affect of setting an element to display: none?
does not display/gets hidden
120
What does the element.matches() method take as an argument and what does it return?
takes CSS selector and returns boolean
121
How can you retrieve the value of an element's attribute?
getAttribute method
122
At what steps of the solution would it be helpful to log things to the console?
every step
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?
you'd need to write a code for every single tab
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?
separate conditional block for each view
125
What is JSON?
Javascript Object Notion. data-interchange format
126
What are serialization and deserialization?
Serialization is converting object into string aka bytes aka data
Deserialization is the opposite
127
Why are serialization and deserialization useful?
seriliazation is good for storing a data
| deserialization is good for using the data
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?
setItem () method
131
How to you retrieve data from localStorage?
getItem () method
132
What data type can localStorage save in the browser?
string
133
When does the 'beforeunload' event fire on the window object?
before the page unloads
134
What is a method?
A method is a function which is a property of an object
135
How can you tell the difference between a method definition and a method call?
definition = property, assignment, function definition
| method call = object.method()
136
Describe method definition syntax (structure).
property: function {
code block
},
137
Describe method call syntax (structure).
object.method(arguments)
138
How is a method different from any other function?
methods needs to have name of the object and dot before it
139
What is the defining characteristic of Object-Oriented Programming?
objects can contain both data (as properties) and behavior (as methods).
140
What are the four "principles" of Object-Oriented Programming?
Abstraction
Encapsulation
Inheritance
Polymorphism
141
What is "abstraction"?
simplifying complex process. modeling.
142
What does API stand for?
application programming interface
143
What is the purpose of an API?
connection between computers or between computer programs
144
What is this in JavaScript?
this is an implicit parameter of all JavaScript functions
145
What does it mean to say that this is an "implicit parameter"?
parameter that we don't need to specify, but is present
146
When is the value of this determined in a function; call time or definition time?
when it is called
147
```
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);
}
};
```
nothing. it's not being called
148
Given the above character object, what is the result of the following code snippet? Why?
character.greet();
it's-a-me Mario !
149
```
Given the above character object, what is the result of the following code snippet? Why?
var hello = character.greet;
hello();
```
it's-a-me undefined !
150
How can you tell what the value of this will be for a particular function or method definition?
Nothing, since it's not being called yet
151
How can you tell what the value of this is for a particular function or method call?
object to the left of the dot.
| If no dot, then window
152
What kind of inheritance does the JavaScript programming language use?
protoype
153
What is a prototype in JavaScript?
154
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?
155
If an object does not have it's own property or method by a given key, where does JavaScript look for it?
prototype
156
What does the new operator do?
1. Creates a blank, plain JavaScript object.
2. Adds a property to the new object (__proto__) that links to the constructor function's prototype object
3. Binds the newly created object instance as the this context (i.e. all references to this in the constructor function now refer to the object created in the first step).
4. Returns this if the function doesn't return an object.
157
What property of JavaScript functions can store shared behavior for instances created with new?
prototype
158
What does the instanceof operator do?
Checks if the prototype property of the right side of the operator is present in the prototype chain of the left side
159
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.
160
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?
setTimeout
161
How can you set up a function to be called repeatedly without using a loop?
setInterval
162
What is the default time delay if you omit the delay parameter from setTimeout() or setInterval()?
delays it by 0 seconds (useful for having it occur after something else)
163
What do setTimeout() and setInterval() return?
timeoutID which is a positive integer
164
What is a code block? What are some examples of a code block?
Group of code to be executed together. eg. function code block
165
What does block scope mean?
A block scoped variable means that the variable defined within a block will not be accessible from outside the block
166
What is the scope of a variable declared with const or let?
Block-scoped
167
What is the difference between let and const?
let can be reassigned, cont cannot
168
Why is it possible to .push() a new value into a const variable that points to an Array?
You can change const but not reassign
169
How should you decide on which type of declaration to use?
If the variable needs to be reassigned, use let. If not, use const.
170
What is the syntax for writing a template literal?
backticks ( ` ) in lieu of single/double quotes
text content
$ { } for javascript expression
/n for new line
171
What is "string interpolation"?
Substituting expression results or variables into a string
172
What is destructuring, conceptually?
Taking pieces of data out of a variable and assigning it to a new variable
173
What is the syntax for Object destructuring?
Const { property names, property name2, ....} = cont name
174
What is the syntax for Array destructuring?
const [element, element1, element2] = array
175
How can you tell the difference between destructuring and creating Object/Array literals?
If { } or [ ] is on the left side of the assignment operator, then it's destructuring
If on the right side, creating
176
What is the syntax for defining an arrow function?
const functionName = ( ) => {
}
177
When an arrow function's body is left without curly braces, what changes in its functionality?
178
How is the value of this determined within an arrow function?
In an arrow function, value of this is determined when it's being defined (vs. when it's called)
