Doing vanilla Javascript selecting or setting elements with their id or class name.
Selecting
Selecting a HTML element with its id with getElementById():
document.getElementById("theId");
Selecting an element based on the class name with getElementByClassName()
document.getElementByClassName("class-name");
This returns an array that you can use indices with.
Selecting an element that has class orange inside an element with the id of square:
document.getElementById("square").getElementsByClassName("orange");
Checking if has class
How to check if an element has a class name. This example is checking the element with the id statusText for the class blue:
document.getElementsByClassName("statusText").classList.contains("blue");
This returns true on a result that does have the class and false for when the element does not contain the class.
Checking if id found
Doing a check to see if an id is found or exists:
if(document.getElementById("theId")){ //Element exists } else { //Element does not exist }