JS & the DOM

Question 1

Why do we log things to the console?

Answer 1

So you can see what the data looks like; so you can see what’s going on

Question 2

What is a “model”?

Answer 2

Relation to DOM (document object model)

-> The DOM is a representation/model/replica of the HTML document, it is not the actual HTML document;

Question 3

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

Answer 3

The html document

Question 4

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

Answer 4

The DOM represents all the information in HTML document (elements, text, attributes, etc) as JavaScript objects

Question 5

What is a DOM Tree?

Answer 5

It is a model of the HTML document created by the browser and stored in memory. The DOM tree is a tree data structure of nodes

Question 6

Give two examples of document methods that retrieve a single element from the DOM.

Answer 6

1. document.querySelector( )

2. document.getElementById( )

Question 7

Give one example of a document method that retrieves multiple elements from the DOM at once.

Answer 7

document.querySelectorAll()

Question 8

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

Answer 8

Querying for an element using document.querySelector() method takes time/processing resources because you are traversing the DOM, looking for a node

If you have the node saved in a variable and want to do something to it, you don’t need to query the DOM for it again

Question 9

What console method allows you to inspect the properties of a DOM element object (and JavaScript objects in general)?

Answer 9

console.dir( )

Question 10

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

Answer 10

the DOM has to fully load before you can access / manipulate it with JavaScript

Question 11

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

Answer 11

It takes a CSS selector (as a string) as an argument and returns the first DOM node that matches the selector (or null if no matching node found)

Question 12

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

Answer 12

It takes a CSS selector (as a string) as argument and returns a static nodelist of nodes that match the selector

Question 13

What is the purpose of events and event handling?

Answer 13

They let your webpage respond/react to user actions - with events you can listen for stuff and then do stuff in response to it

Question 14

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

Answer 14

No. Just because a function is given parameter doesn’t mean it has to do anything with it.

Question 15

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

Answer 15

element.addEventListener()

Question 16

What is a callback function?

Answer 16

A function passed into another function as an argument. The outer function calls the callback function later when it needs it.

Question 17

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

Answer 17

The event object

Question 18

What is the event.target?

Answer 18

The element that received the event; the element that was interacted with.

Question 19

What is the className property of element objects?

Answer 19

It is the class attribute of that html element; access or update the class attribute using this property

Question 20

How do you update the CSS class attribute of an element using JavaScript?

Answer 20

element. className() method or

element. classList.add( ) / element.classList.remove()

Question 21

What is the textContent property of element objects?

Answer 21

The text content of the node and its descendents

Question 22

How do you update the text within an element using JavaScript?

Answer 22

element.textContent = newTextContent

Question 23

Is the event parameter of an event listener callback always useful?

Answer 23

No. Just because a function takes a parameter doesn’t mean it has to do anything to it.

Question 24

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

Answer 24

Don’t want to store information on DOM because it is unreliable; the DOM should not be your “source of truth’, the javascript code should be

Question 25

What event is fired when a user places their cursor in a form control?

Answer 25

focus

Question 26

What event is fired when a user's cursor leaves a form control?

Answer 26

blur

Question 27

What event is fired as a user changes the value of a form control?

Answer 27

input

Question 28

What event is fired when a user clicks the "submit" button within a ?

Answer 28

submit

Question 29

What does the event.preventDefault() method do?

Answer 29

Prevents the browser’s default response to the event

Question 30

What does submitting a form without event.preventDefault() do?

Answer 30

Causes page to reload with form values in url query string aka submit to server

Question 31

What property of a form element object contains all of the form's controls.

Answer 31

elements

Question 32

What property of form a control object gets and sets its value?

Answer 32

value

Question 33

Does the document.createElement() method insert a new element into the page?

Answer 33

No, before it appears on page must be appended to existing object on DOM

Question 34

How do you add an element as a child to another element?

Answer 34

element.appendChild() or element.append()

Question 35

What do you pass as the arguments to the element.setAttribute() method?

Answer 35

1. attributeName | 2. attributeValue

Question 36

What steps do you need to take in order to insert a new element into the page?

Answer 36

1. Create element 2. set attributes and text content 3. Attach it to the DOM by appending it as child of an existing node

Question 37

What is the textContent property of an element object for?

Answer 37

Used to set or access the text content of an element

Question 38

Name two ways to set the class attribute of a DOM element.

Answer 38

1. element.classList.add() | 2. element.className()

Question 39

What are two advantages of defining a function to create something (like the work of creating a DOM tree)?

Answer 39

1. Functions are reusable | 2. functions have name, so they make your code easier to read

Question 40

What is the event.target?

Answer 40

It is the element that an event originated from; the point of interaction

Question 41

Why is it possible to listen for events on one element that actually happen its descendent elements?

Answer 41

Because of event flow, which causes events to ‘bubble up’ the DOM

Question 42

What DOM element property tells you what type of element it is?

Answer 42

tagName

Question 43

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

Answer 43

Argument: css selector (as a string) Return value: first (ie. closest) ancestor element that matches selector;

Question 44

How can you remove an element from the DOM?

Answer 44

element.remove()

Question 45

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?

Answer 45

Add the event listener to the parent element (aka use event delegation)

Question 46

What is the event.target?

Answer 46

the element the user interacted with

Question 47

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

Answer 47

It hides the element (removes element from the element flow);

Question 48

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

Answer 48

Argument = a selector string Return value = a boolean value indicating if the element the method is called on matches the selector given as the argument

Question 49

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

Answer 49

Use the element.getAttribute() method