The first of the “traditional” JavaScript selectors, getElementsByTagName does exactly what it says, creating an HTMLCollection of elements from a web page. The result of the method is usually set to the value of a variable:
JavaScript Selectors: getElementsByClassName
Just as document.getElementById selects an element with a particular id value, getElementsByClassName gathers elements on a page that share a defined class value. As the plural name of the method suggests, getElementsByClassName collects an array of elements, never just one result… even if it matches only one element on the page.
JavaScript Selectors: getElementById
One of the earliest and most fundamental means of referencing an element in JavaScript, getElementById is very frequently used to “latch onto” an element for manipulation.