StevenWuTG

const ballBoundaries = () => {
    // Bounce off Left Wall
    if (ballX < 0 && speedX < 0) {
      speedX = -speedX
    }
    // Bounce off Right Wall
    if (ballX > width && speedX > 0) {
      speedX = -speedX
    }
    // Bounce off player paddle (bottom)

StevenWuTG

/first you need to find a parent element node
const parent2 = document.querySelector("#parent-2")

const child1Ofparent2 = parent2.childNodes[0]
//this will return the child-3 node which is at the first index of parent2

const child2Ofparent2 = parent2.childNodes[1]
//this will return the child-4 node which is at the second index of parent2

StevenWuTG

//first you need to find a element node
const parent2 = document.querySelector("#parent-2")

const lastChildOfParent2 = parent2.lastElementChild
//will return the node with the id of child-4 which is the last child of parent-2

StevenWuTG

//first you need to find a element node
const parent2 = document.querySelector("#parent-2")

const firstChildOfParent2 = parent2.firstElementChild
//will return the node with the id of child-3 which is the first child of parent-2

StevenWuTG

//first you need to find a single element node
const parent1 = document.querySelector("#parent-1")

const parent2 = parent1.nextElementSibling
//this will return the node with the id of parent-2

StevenWuTG

//first you need to find a single element node
const parent2 = document.querySelector("#parent-2")

const parent1 = parent2.previousElementSibling
//this will return the node with the id of parent-1

StevenWuTG

//first you need to find a single element node
const parent1 = document.querySelector("#parent-1")

const grandparent = parent1.parentElement
//this will return the entire grandparent node containing the parents and the children

StevenWuTG

//find element nodes by a class using the "." css selector before the class
const parents = document.querySelectorAll(".parent")
//will return the array of parent-1 & parent-2

//or

const childrenOfParentIndex0 = parents[0].querySelectorAll(".child")
//will return the array of child-1 & child-2

StevenWuTG

//find a element node by a class using the "." css selector before the class
const grandparent = document.querySelector(".grandparent")

//or

//find a element node by a id using the "#" css selector before the id
const parent2 = grandparent.querySelector("#parent-2")

StevenWuTG

//selecting multiple classes
const parents = document.getElementsByClassName("parent")

//or if you want to target specific classes from specific parent nodes
const parent2 = document.getElementById("parent-2")
const childrenOfParent2 = parent2.getElementsByClassName("child")
//which will return an array of the divs of child-3 & child-4