Tiffany B. Brown

Meet the Selectors API

DOM Scripting is sometimes clunky. Consider the retrieval of elements with a particular class name. The code might look like this:

var p = document.getElementsByTagName('p');
var newsitems = [];
var i;
for(i = 0; i < p.length; i++){
  if( p[i].className == 'newsitem' ){
     newsitems.push( p[i] );
  }
}

In the example above, we’ve retrieved all elements in the document using document.getElementsByTagName(). Then we’ve iterated over them, checking for the presence of the desired class name.

It’s ugly, right? That᾿s even more true when you look at the elegant, CSS-style selectors of libraries like jQuery and MooTools.

Code like that above is why the W3C developed a Selectors API. It consists of two methods on the document object: querySelector() and querySelectorAll().

querySelector() and querySelectorAll() both accept any CSS selector that the browser supports. All recent browser releases support both methods, though Internet Explorer 8 is limited to CSS2 selectors.

querySelectorAll(), as its name suggests, returns a node list of elements matching the given selector. querySelector() only returns the first matching element. You can pass any CSS selector as an argument, including ones like the adjacent sibling combinator and nth-child() if the browser supports it.

Let's re-write the above code using querySelectorAll().

var newsitems = document.querySelectorAll('.newsitem');

That’s much more concise. The newsitems variable contains only those elements with the newsitem class. Using the Selectors API greatly simplifies the code we need to use when interacting with the DOM.

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