queryselector function in javascript

/*Do this for classes*/
document.querySelector(".example"); 
/*Do this for IDs*/
document.querySelector("#example");
/*This selects all of the elements in those groups*/

/*The querySelector() method returns the first element that matches a specified 
CSS selector(s) in the document. Note: The querySelector() method only returns 
the first element that matches the specified selectors. To return all the 
matches, use the querySelectorAll() method instead.*/

// for example 

//for class
document.querySelector(".ClassName")

// for id 
document.querySelector("#ID")

// etc.