From Javascript, both the methods can be used to return multiple elements in the document but the difference is in what it returns.

• .querySelectorAll : Non-Live node list - Returns all the elements that matches the specified CSS selector(s) as a static NodeList object.
• .getElementsByClassName : Live node list - Returns all the elements that matches the spcified CSS class name as a HTMLCollection.

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1var elements1 = document.querySelectorAll(".clickable"); 
2var elements2 = document.getElementsByClassName("clickable");

getElementsByClassName() is useful when working with dynamic elements as its updated when the DOM changes.