The Document method getElementById() returns an Element object representing the element whose id property matches the specified string. Since element IDs are required to be unique if specified, they're a useful way to get access to a specific element quickly.
If you need to get access to an element which doesn't have an ID, you can use querySelector() to find the element using any selector.
https://developer.mozilla.org/en-US/docs/Web/API/Document/getElementById
var element = document.getElementById(id);