It used to be not so trivial to access elements in pure JavaScript using the DOM. This to two new methods, querySelector & querySelectorAll, that are now supported in all modern browsers, this task is now much easier.

querySelector

querySelector returns the first element that match the provided CSS query. Use it on the full document:

let myElem = document.querySelector('#myElem');

Or use it on an element to get an element within the element:

let elem = document.querySelector('p');
let myElem = elem.querySelector('#myElem');

querySelectorAll

querySelector returns all the elements that match the provided selector:

let elems = document.querySelectorAll('p');

And here you can do the same and narrow your selection. Let’s select span elements that are in the first p element:

let firstP = document.querySelector('p');
let spanElems = firstP.querySelectorAll('span');

You can find more data on support for the query selector feature across the major browsers from caniuse.com.