JavaScript Querying elements efficiently, fork from Zepto.js

querySelectorPerf.js

// source/credits: https://github.com/madrobby/zepto/blob/master/src/zepto.js

// usage query(document.body, '.cls .childrens')
function query(element, selector){
    var found,
        maybeID = selector[0] == '#',
        maybeClass = !maybeID && selector[0] == '.',
        nameOnly = maybeID || maybeClass ? selector.slice(1) : selector, // Ensure that a 1 char tag name still gets checked
        isSimple = /^[\w-]*$/.test(nameOnly);
    return (element.getElementById && isSimple && maybeID) ? // Safari DocumentFragment doesn't have getElementById
      ( (found = element.getElementById(nameOnly)) ? [found] : [] ) :
      (element.nodeType !== 1 && element.nodeType !== 9 && element.nodeType !== 11) ? [] :
      [].slice.call(
        isSimple && !maybeID && element.getElementsByClassName ? // DocumentFragment doesn't have getElementsByClassName/TagName
          maybeClass ? element.getElementsByClassName(nameOnly) : // If it's simple, it could be a class
          element.getElementsByTagName(selector) : // Or a tag
          element.querySelectorAll(selector) // Or it's not simple, and we need to query all
      );
  }