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Additional Info

About query selectors

JavaScript has many different query selectors, some seemingly interchangeable, for example:

  • document.head and document.body
  • document.getElementById, document.getElementsByClassName and document.getElementsByTagName
  • document.querySelector and document.querySelectorAll

Knowing which to use for optimal performance depends on the situation, but a good rule of thumb is:

  • to access the head of the document, use document.head over document.getElementsByTagName('head')
  • to access the body of the document, use document.body over document.getElementsByTagName('body')
  • to access an element by id use document.getElementById('id') over document.querySelector('#id')
  • to access one element by class name use document.getElementsByClassName('className')[0] over document.querySelector('.className')
  • to access multiple elements by class name use document.getElementsByClassName('className') over document.querySelectorAll('.className')
  • to access one element by tag name use document.getElementsByTagName('tagName')[0] over document.querySelector('tagName')
  • to access multiple elements by tag name use document.getElementsByTagName('tagName') over document.querySelectorAll('tagName')

In most cases querySelector and querySelectorAll should be used only on selectors more sophisticated than a simple id, class or tag name.