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Web Development > Javascript > Flashcards

Javascript Flashcards

1
Q

What is the purpose of variables?

A

variables are used to store data that can be accessed later

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2
Q

How do you declare a variable?

A

a keyword (like var) then the name of the variable

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3
Q

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

A

variable name = (assignment operator) value

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4
Q

What characters are allowed in variable names?

A

letters, numbers, $, _

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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

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6
Q

What is the purpose of a string?

A

stores data containing a series of characters

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7
Q

What is the purpose of a number?

A

used for calculations, determining the size, movement, or time associated with something

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8
Q

What is the purpose of a boolean?

A

defines whether something is true or false, for decisions

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9
Q

What does the = operator mean in JavaScript?

A

assignment operator

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10
Q

How do you update the value of a variable?

A

variableName = new value

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11
Q

What is the difference between null and undefined?

A

null is the intentional absence of value

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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

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13
Q

Give five examples of JavaScript primitives.

A

strings, numbers, booleans, null, undefined

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14
Q

What data type is returned by an arithmetic operation?

A

number

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15
Q

What is string concatenation?

A

combining a string with another value to create a new string

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16
Q

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

A

adds numbers and concatenates 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 of the operator to the value on the left and assigns the result to the variable on the left

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19
Q

What are objects used for?

A

group together a set of related variables and functions

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20
Q

What are object properties?

A

variables that are part of an object

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21
Q

Describe object literal notation.

A

objectName = { property : value, … } ;

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22
Q

How do you remove a property from an object?

A

delete keyword followed by the objectName.propertyName

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23
Q

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

A

dot or bracket notation

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24
Q

What are arrays used for?

A

arrays store a list of values that are related to each other

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25
Q

Describe array literal notation.

A

arrayName = [ value, … } ;

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26
Q

How are arrays different from “plain” objects?

A

the keys are index numbers, so there’s a specific order to the values

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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 entries in an array

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29
Q

How do you calculate the last index of an array?

A

arrayName.length - 1

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30
Q

What is a function in JavaScript?

A

section of code that performs a task and can be reused

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31
Q

Describe the parts of a function definition.

A

function keyword, function name, ( ) with 0 or more parameters, { } code block, and and optional return statement

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32
Q

Describe the parts of a function call.

A

function name, ( ) containing the argument(s)

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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 has a function keyword, parameters, { } containing a code block. function call only has the function name and arguments in ( )

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34
Q

What is the difference between a parameter and an argument?

A

parameters are for definition, arguments are for calls

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35
Q

Why are function parameters useful?

A

they allow different arguments to be passed through a function

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36
Q

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

A

return statements stop the code block from running and make the function produce a value

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37
Q

Why do we log things to the console?

A

to make sure our code is working properly and producing the expected results

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38
Q

What is a method?

A

a function that is a property of an object

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39
Q

How is a method different from any other function?

A

methods are a part of an object, they must be called with the object

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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( ) * (range + 1) + start

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43
Q

How do you delete an element from an array?

A

.splice(index, # of items deleted)

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44
Q

How do you append an element to an array?

A

.push( )

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45
Q

How do you break a string up into an array?

A

.split( ‘ ‘ )

46
Q

Do string methods change the original string? How would you check if you weren’t sure?

A

no, console.log(string) to check

47
Q

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

A

about 36

48
Q

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

A

no, sometimes you don’t need a return value because it won’t be useful

49
Q

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

A

about 36

50
Q

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

A

mdn

51
Q

Give 6 examples of comparison operators.

A

< , > , <=, >=, ===, !==

52
Q

What data type do comparison expressions evaluate to?

A

booleans

53
Q

What is the purpose of an if statement?

A

to run a code block if a condition is met

54
Q

Is else required in order to use an if statement?

A

no, sometimes nothing needs to happen if a condition isn’t met

55
Q

Describe the syntax (structure) of an if statement.

A

if (condition) { code block;}

56
Q

What are the three logical operators?

A

&&, ||, !

57
Q

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

A

put a pair of ( ) around each individual expression and use a logical operator in between

58
Q

What is the purpose of a loop?

A

loops run a block of code a certain amount of times while a condition is met

59
Q

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

A

conditions determine whether a code loop is run, otherwise infinite loops occur

60
Q

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

A

each time a loops runs through a code block

61
Q

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

A

before each iteration of a loop

62
Q

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

A

once before the loop starts

63
Q

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

A

before each iteration of a loop

64
Q

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

A

after each iteration of a loop

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

66
Q

What does the ++ increment operator do?

A

adds one to a value and either returns the updated value or returns the value then increments it

67
Q

How do you iterate through the keys of an object?

A

for … in loops

68
Q

What is JSON?

A

a text-based format following JS object syntax, used for transmitting data across a network

69
Q

What are serialization and deserialization?

A

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
Q

Why are serialization and deserialization useful?

A

they allow objects to be turned into transferrable data and converted back

71
Q

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

A

JSON.stringify( )

72
Q

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

A

JSON.parse( )

73
Q

How do you store data in localStorage?

A

.setItem(keyName, keyValue)

74
Q

How to you retrieve data from localStorage?

A

.getItem(keyName)

75
Q

What data type can localStorage save in the browser?

A

objects

76
Q

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

A

fired when the window, document, and its resources are about to be unloaded

77
Q

What is a method?

A

a function which is a property of an object

78
Q

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

A

a method definition has the function keyword, and a code block

79
Q

Describe method definition syntax (structure).

A

function functionName(params) { code block};

80
Q

Describe method call syntax (structure).

A

objectName.methodName(argument)

81
Q

How is a method different from any other function?

A

methods are part of objects

82
Q

What is the defining characteristic of Object-Oriented Programming?

A

objects can contain both data (as properties) and behavior (as methods)

83
Q

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

A

abstraction, encapsulation, inheritance, polymorphism

84
Q

What is “abstraction”?

A

being able to work with complex things in simple ways

85
Q

What does API stand for?

A

application programming interface

86
Q

What is the purpose of an API?

A

it is an abstraction, a set of tools that people can use

87
Q

What is this in JavaScript?

A

this is an implicit parameter in all JS functions

88
Q

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

A

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
Q

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

A

call time

90
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

nothing, the function hasn’t been called yet

91
Q 
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( );
A

it’s-a-me, Mario! this.firstName is Mario in this context

92
Q 
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( );
A

it’s-a-me, undefined! this is the window object, which doesn’t have a firstName property

93
Q

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

A

You can’t tell until call time

94
Q

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

A

It’s the object that the method is being called on

95
Q

What kind of inheritance does the JavaScript programming language use?

A

prototype-based inheritance

96
Q

What is a prototype in JavaScript?

A

an object that contains properties and methods that can be used by other objects

97
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

they get the method from its prototype object

98
Q

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

A

the prototype object

99
Q

What does the new operator do?

A

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
Q

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

A

.prototype

101
Q

What does the instanceof operator do?

A

it tests to see if the prototype property of a. constructor appears anywhere in the prototype chain of an object

102
Q

What is a “callback” function?

A

a function passed into another function as an argument

103
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( )

104
Q

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

A

setInterval( )

105
Q

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

A

0, which executes immediately, or the next event cycle

106
Q

What do setTimeout( ) and setInterval( ) return?

A

an id number that can be passed to clearInterval( ) or clearTimeout( ) to cancel the timer

107
Q

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

A

another function

108
Q 
What does this code do?
const wrap = value => ( ) => value;
A

returns a function that returns the value variable

109
Q

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

A

when it is defined

110
Q

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

A

closures

Web Development (14 decks)

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