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Web Development > JavaScript > Flashcards

JavaScript Flashcards

1
Q

What are objects used for?

A

Objects allow us to put data that makes sense together in one group/container.

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2
Q

What are object properties?

A

Data stored in an object.

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3
Q

Describe object literal notation.

A

var objectName = {
property: value,
otherProperty: otherValue
}

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4
Q

How do you remove a property from an object?

A

Using the delete operator.

delete object.propertyName

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5
Q

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

A

Dot Notation:
object.propertyName = “new value”

Bracket Notation:
object[‘properyName’] = “new value”

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6
Q

What is the purpose of variables?

A

Variables allow us to store data in memory to use later.

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7
Q

How do you declare a variable?

A

var variableName;

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8
Q

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

A

var variableName = value;

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9
Q

What characters are allowed in variable names?

A

letters, numbers (non-starting), $, _

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10
Q

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

A

When calling a variable, it must be exactly the name of that variable with exact casing.
variable and Variable are different variables.

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11
Q

What is the purpose of a string?
What is the purpose of a number?
What is the purpose of a boolean?

A

Strings store text data
Numbers store numbers, used for measurements, etc.
Booleans store true or false, used for yes and no situations

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12
Q

What does the = operator mean in JavaScript?

A

Assignment

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13
Q

How do you update the value of a variable?

A

varName = updatedValue;

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14
Q

What is the difference between null and undefined?

A

Null is a value for purposely blank, undefined means a variable has not been assigned a value

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15
Q

Why is it a good habit to include “labels” when you log values to the browser console?

A

Labels give us context to what is being logged.

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16
Q

What data type is returned by an arithmetic operation?

A

number

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17
Q

What is string concatenation?

A

The joining of strings

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18
Q

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

A

Addition, concatenation of strings

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19
Q

What data type is returned by comparing two values (, ===, etc)?

A

Boolean, with values true or false

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20
Q

What does the += “plus-equals” operator do?

A

Adds the right side value to the left side variable and reassigns that new value to the left side variable

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21
Q

What are arrays used for?

A

Arrays are used for storing lists of similar data.

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22
Q

Describe array literal notation.

A

[firstItem, secondItem]

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23
Q

How are arrays different from “plain” objects?

A

Arrays have order, item keys are indices.

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24
Q

What number represents the first index of an array?

A

0, arrays are 0 indexed

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25
Q

What is the length property of an array?

A

The length property is the number of items in the array

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26
Q

How do you calculate the last index of an array?

A

length-1

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27
Q

Why are function parameters useful?

A

Allows us to pass data in dynamically

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28
Q

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

A

1) Cause the function to produce a value

2) Exits the function, code in the code black after will not run

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29
Q

What is a function in JavaScript?

A

A callable object that contains reusable code

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30
Q

Describe the parts of a function definition.

A
  • function keyword
  • name
  • (parameters)
  • { code block
    }
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31
Q

Describe the parts of a function call.

A

functionName(arguments);

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32
Q

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

A

The definition has function keyword, code block, and paramters compared to arguments in the function call which has no function keyword and no code block

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33
Q

What is the difference between a parameter and an argument?

A

Parameters are placeholders in the function definition.

Arguments are actual values passed into a function call.

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34
Q

Why do we log things to the console?

A

For inspection, testing and spotting errors in code

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35
Q

What is a method?

How is a method different from any other function?

A

A function that is a property of an object.

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36
Q

How do you remove the last element from an array?

A

With the pop() method

array.pop()

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37
Q

How do you round a number down to the nearest integer?

A

Math.floor()

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38
Q

How do you generate a random number?

A

Math.floor(Math.random( ) * (end - start) + 1 ) + start
(end-start) - gives range
+1 - because Math.random( ) is 1 (non-inclusive)

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39
Q

How do you delete an element from an array?

A

array.splice(start, deleteCount)

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40
Q

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

A

No, strings are immutable

Alter a string, then console.log the original string

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41
Q

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

A

Not always, functions may just be used to change a variable but not have to return anything.

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42
Q

Give 6 examples of comparison operators.

A 
>
<
>=
<=
==
===
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43
Q

What data type do comparison expressions evaluate to?

A

boolean, true or false

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44
Q

What is the purpose of an if statement?

A

To execute a block of code if the condition is truthy

45
Q

Is else required in order to use an if statement?

A

No

46
Q

Describe the syntax (structure) of an if statement.

A

if (condition) {
code block
}

47
Q

What are the three logical operators?

A

&&, ||, !

48
Q

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

A

Parenthesis around each expression and joined with a logical operator.

49
Q

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

A

At the beginning of each iteration, before the code block runs.

50
Q

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

A

Before anything.

Once at the start of the loop.

51
Q

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

A

Before each.

Before each iteration.

52
Q

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

A

After each.

After the code block and before the next iteration’s condition is evaluated.

53
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;

54
Q

What does the ++ increment operator do?

A

Increments the variable after an expression.

55
Q

How do you iterate through the keys of an object?

A

Using a for in loop.

56
Q

What is a “model”?

A

A representation of the actual thing

57
Q

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

A

The HTML document

58
Q

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

A

JavaScript objects

59
Q

What is the DOM?

A

Javascript object as a model of an HTML document.

Contains all the information about the HTML document but is NOT the HTML document

60
Q

What is a DOM Tree?

A

A representation of all HTML elements as their own DOM nodes.

61
Q

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

A

To be able to reuse the queried value later on.

62
Q

What console method allows you to inspect the properties of a DOM element object?

A

console.dir()

63
Q

Why would a script tag need to be placed at the bottom of the HTML content instead of at the top?

A

So all the HTML content prior can load first

64
Q

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

A

Takes a CSS selector and returns the first element that matches that selector

65
Q

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

A

Takes a CSS selector and returns a NodeList of all matching element with that selector

66
Q

What is the purpose of events and event handling?

A

To allow creation of interactive and dynamic websites

67
Q

Are all possible parameters required to use a JavaScript method or function?

A

No,

68
Q

What method of element objects lets you set up a function to be called when a specific type of event occurs?

A

addEventListener() method

69
Q

What is a callback function?

A

a function passed into another function that is then invoked inside that function

70
Q

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

A

The event object containing all information relevant to that specific event

71
Q

What is the event.target? If you weren’t sure, how would you check?

A

Target is the element we are dealing with currently, where it originated from.

72
Q

What is the textContent property of element objects?

A

The text content of a node and all descendent nodes

73
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.

74
Q

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

A

It’s better to have data stored in JavaScript if will be used again in JavaScript rather than querying for the element every time.

75
Q 
What is the className property of element objects?
How do you update the CSS class attribute of an element using JavaScript?
A 
The element's class attribute. 
targetElement.className = "class"
76
Q

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

A

Visually hides the elements, but programatically still there

77
Q

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

A

returns true or false if the string CSS selector would be valid to match that element

78
Q

How can you retrieve the value of an element’s attribute?

A

getAttribute() method

79
Q

At what steps of the solution would it be helpful to log things to the console?

A

every step

80
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

Would have to create a new event listener for every view.

81
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

Would have a long list of list of else if statements.

82
Q

What is JSON?

A

It is a format design to replicate the format of JavaScript objects.

83
Q

What are serialization and deserialization?

A

Serialization - process of turning an object in memory into stream of bytes (characters in a string)
Deserialization - turns stream of bytes into an object in memory

84
Q

Why are serialization and deserialization useful?

A

Makes it possible to transfer the data within the contents of an object or array.

85
Q

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

A

JSON.stringify( ) method

86
Q

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

A

JSON.parse( ) method

87
Q

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

A

The method definition contains the keyword function and the code block.

The method call has the object followed by a dot and the method name with parenthesis and any args.

88
Q

Describe method definition syntax (structure).

A

Assigning a function to a property in an object or adding to an object.

89
Q

Describe method call syntax (structure).

A

object.method()

90
Q

What is the defining characteristic of Object-Oriented Programming?

A

Objects can contain both data (variables) and behavior (methods).

91
Q

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

A

Abstraction
Encapsulation
Inheritance
Polymorphism

92
Q

What is “abstraction”?

A

Simplifying complex processes to into simple ways to interact with those complex things.

93
Q

What does API stand for?

What is the purpose of an API?

A

Application Programming Interface.

To give programmers a way to interact with a system in a simplified way.

94
Q

What is this in JavaScript?

A

Keyword referring to the object that it belongs to.

95
Q

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

A

Means it is present in any function code block despite it not being in the parameter or declared with var keyword.

96
Q

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

A

During the call time.

Within the function definition, “this” does not have a value yet, it is nothing until call time.

97
Q

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

A

You cannot tell the value of ‘this’ because it is created when the function is invoked.

98
Q

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

A

When the function is invoked, ‘this’ is the object left of the dot. Otherwise ‘this’ does not exist yet and cannot have a value.

99
Q

What kind of inheritance does the JavaScript programming language use?

A

Prototypal inheritance

100
Q

What is a prototype in JavaScript?

A

An object that contains properties and methods that other objects can use.

101
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

The prototype object has those methods that other objects, arrays, and numbers can delegate to.

102
Q

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

A

Looks in the object prototype.

103
Q

What does the new operator do?

A

Creates a new instance of an object, sets the prototype of the object.
The ‘this’ becomes the new instance of the object
Runs the code.
Returns the object.

104
Q

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

A

prototype

105
Q

What does the instanceof operator do?

A

Checks to see if the prototype of a an object matches the prototype specified.

pet instanceOf petConstructor, returns true

106
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

Can use setTimeout() function.

107
Q

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

A

Can use setInterval( ) function.

108
Q

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

A

0 milliseconds.

109
Q

What do setTimeout() and setInterval() return?

A

They return timerID or intervalID

Web Development (7 decks)

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