Javascript Flashcards
1
Q
What is the purpose of variables?
A
variables are used to store data that can be accessed later
2
Q
How do you declare a variable?
A
a keyword (like var) then the name of the variable
3
Q
How do you initialize (assign a value to) a variable?
A
variable name = (assignment operator) value
4
Q
What characters are allowed in variable names?
A
letters, numbers, $, _
5
Q
What does it mean to say that variable names are “case sensitive”?
A
variables spelled with the same characters but have differences in capitalization are considered different
6
Q
What is the purpose of a string?
A
stores data containing a series of characters
7
Q
What is the purpose of a number?
A
used for calculations, determining the size, movement, or time associated with something
8
Q
What is the purpose of a boolean?
A
defines whether something is true or false, for decisions
9
Q
What does the = operator mean in JavaScript?
A
assignment operator
10
Q
How do you update the value of a variable?
A
variableName = new value
11
Q
What is the difference between null and undefined?
A
null is the intentional absence of value
12
Q
Why is it a good habit to include “labels” when you log values to the browser console?
A
labels provide context to whatever you log in the console
13
Q
Give five examples of JavaScript primitives.
A
strings, numbers, booleans, null, undefined
14
Q
What data type is returned by an arithmetic operation?
A
number
15
Q
What is string concatenation?
A
combining a string with another value to create a new string
16
Q
What purpose(s) does the + plus operator serve in JavaScript?
A
adds numbers and concatenates strings
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 to the right of the operator to the value on the left and assigns the result to the variable on the left
19
Q
What are objects used for?
A
group together a set of related variables and functions
20
Q
What are object properties?
A
variables that are part of an object
21
Q
Describe object literal notation.
A
objectName = { property : value, … } ;
22
Q
How do you remove a property from an object?
A
delete keyword followed by the objectName.propertyName
23
Q
What are the two ways to get or update the value of a property?
A
dot or bracket notation
24
Q
What are arrays used for?
A
arrays store a list of values that are related to each other
25
Describe array literal notation.
arrayName = [ value, ... } ;
26
How are arrays different from "plain" objects?
the keys are index numbers, so there's a specific order to the values
27
What number represents the first index of an array?
[ 0 ]
28
What is the length property of an array?
the number of entries in an array
29
How do you calculate the last index of an array?
arrayName.length - 1
30
What is a function in JavaScript?
section of code that performs a task and can be reused
31
Describe the parts of a function definition.
function keyword, function name, ( ) with 0 or more parameters, { } code block, and and optional return statement
32
Describe the parts of a function call.
function name, ( ) containing the argument(s)
33
When comparing them side-by-side, what are the differences between a function call and a function definition?
function definition has a function keyword, parameters, { } containing a code block. function call only has the function name and arguments in ( )
34
What is the difference between a parameter and an argument?
parameters are for definition, arguments are for calls
35
Why are function parameters useful?
they allow different arguments to be passed through a function
36
What two effects does a return statement have on the behavior of a function?
return statements stop the code block from running and make the function produce a value
37
Why do we log things to the console?
to make sure our code is working properly and producing the expected results
38
What is a method?
a function that is a property of an object
39
How is a method different from any other function?
methods are a part of an object, they must be called with the object
40
How do you remove the last element from an array?
.pop( )
41
How do you round a number down to the nearest integer?
Math.floor( )
42
How do you generate a random number?
Math.random( ) * (range + 1) + start
43
How do you delete an element from an array?
.splice(index, # of items deleted)
44
How do you append an element to an array?
.push( )
45
How do you break a string up into an array?
.split( ' ' )
46
Do string methods change the original string? How would you check if you weren't sure?
no, console.log(string) to check
47
Roughly how many string methods are there according to the MDN Web docs?
about 36
48
Is the return value of a function or method useful in every situation?
no, sometimes you don't need a return value because it won't be useful
49
Roughly how many array methods are there according to the MDN Web docs?
about 36
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?
booleans
53
What is the purpose of an if statement?
to run a code block if a condition is met
54
Is else required in order to use an if statement?
no, sometimes nothing needs to happen if a condition isn't met
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?
put a pair of ( ) around each individual expression and use a logical operator in between
58
What is the purpose of a loop?
loops run a block of code a certain amount of times while a condition is met
59
What is the purpose of a condition expression in a loop?
conditions determine whether a code loop is run, otherwise infinite loops occur
60
What does "iteration" mean in the context of loops?
each time a loops runs through a code block
61
When does the condition expression of a while loop get evaluated?
before each iteration of a loop
62
When does the initialization expression of a for loop get evaluated?
once before the loop starts
63
When does the condition expression of a for loop get evaluated?
before each iteration of a loop
64
When does the final expression of a for loop get evaluated?
after each iteration of a loop
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 one to a value and either returns the updated value or returns the value then increments it
67
How do you iterate through the keys of an object?
for ... in loops
68
What is JSON?
a text-based format following JS object syntax, used for transmitting data across a network
69
What are serialization and deserialization?
serialization turns an object to be turned into a format that can be sent across a network. deserialization converts it back into an object
70
Why are serialization and deserialization useful?
they allow objects to be turned into transferrable data and converted back
71
How do you serialize a data structure into a JSON string using JavaScript?
JSON.stringify( )
72
How do you deserialize a JSON string into a data structure using JavaScript?
JSON.parse( )
73
How do you store data in localStorage?
.setItem(keyName, keyValue)
74
How to you retrieve data from localStorage?
.getItem(keyName)
75
What data type can localStorage save in the browser?
objects
76
When does the 'beforeunload' event fire on the window object?
fired when the window, document, and its resources are about to be unloaded
77
What is a method?
a function which is a property of an object
78
How can you tell the difference between a method definition and a method call?
a method definition has the function keyword, and a code block
79
Describe method definition syntax (structure).
function functionName(params) { code block};
80
Describe method call syntax (structure).
objectName.methodName(argument)
81
How is a method different from any other function?
methods are part of objects
82
What is the defining characteristic of Object-Oriented Programming?
objects can contain both data (as properties) and behavior (as methods)
83
What are the four "principles" of Object-Oriented Programming?
abstraction, encapsulation, inheritance, polymorphism
84
What is "abstraction"?
being able to work with complex things in simple ways
85
What does API stand for?
application programming interface
86
What is the purpose of an API?
it is an abstraction, a set of tools that people can use
87
What is this in JavaScript?
this is an implicit parameter in all JS functions
88
What does it mean to say that this is an "implicit parameter"?
it's available in a function's code block even though it was never included in the function's parameter list or declared with var
89
When is the value of this determined in a function; call time or definition time?
call time
90
```
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, the function hasn't been called yet
91
```
Given the above character object, what is the result of the following code snippet? Why?
var character = {
firstName: 'Mario',
greet: function () {
var message = 'It\'s-a-me, ' + this.firstName + '!';
console.log(message);
}
};
character.greet( );
```
it's-a-me, Mario! this.firstName is Mario in this context
92
```
Given the above character object, what is the result of the following code snippet? Why?
var character = {
firstName: 'Mario',
greet: function () {
var message = 'It\'s-a-me, ' + this.firstName + '!';
console.log(message);
}
};
var hello = character.greet;
hello( );
```
it's-a-me, undefined! this is the window object, which doesn't have a firstName property
93
How can you tell what the value of this will be for a particular function or method definition?
You can't tell until call time
94
How can you tell what the value of this is for a particular function or method call?
It's the object that the method is being called on
95
What kind of inheritance does the JavaScript programming language use?
prototype-based inheritance
96
What is a prototype in JavaScript?
an object that contains properties and methods that can be used by other objects
97
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?
they get the method from its prototype object
98
If an object does not have its own property or method by a given key, where does JavaScript look for it?
the prototype object
99
What does the new operator do?
creates a new empty JS object, sets the __proto__ property of the object as the constructor function's prototype, makes new object this, new returns object
100
What property of JavaScript functions can store shared behavior for instances created with new?
.prototype
101
What does the instanceof operator do?
it tests to see if the prototype property of a. constructor appears anywhere in the prototype chain of an object
102
What is a "callback" function?
a function passed into another function as an argument
103
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( )
104
How can you set up a function to be called repeatedly without using a loop?
setInterval( )
105
What is the default time delay if you omit the delay parameter from setTimeout() or setInterval( )?
0, which executes immediately, or the next event cycle
106
What do setTimeout( ) and setInterval( ) return?
an id number that can be passed to clearInterval( ) or clearTimeout( ) to cancel the timer
107
What must the return value of myFunction be if the following expression is possible?
myFunction( )( );
another function
108
```
What does this code do?
const wrap = value => ( ) => value;
```
returns a function that returns the value variable
109
In JavaScript, when is a function's scope determined; when it is called or when it is defined?
when it is defined
110
What allows JavaScript functions to "remember" values from their surroundings?
closures
