Question

Why we use root node. In below given function they use root node function. I want to understand this function also what is meaning of ‘!rootNode’.

 function getElementsByClassName(cn, rootNode) {
  if (!rootNode) {
    rootNode = document;
  } 
  for (var r=[], e=rootNode.getElementsByTagName('*'), i=e.length; i--;) {
    if ((' '+e[i].className+' ').indexOf(' '+cn+' ')>-1) {
      r.push(e[i]); 
    }
  }
  return r;  
}

Answer 1

In your example, rootNode is the HTML element from which to start recursively searching the hierarchical DOM tree (which is how the web page is represented in JavaScript) for elements with class name cn.

This allows the caller of your function to specify from where they want to search for elements with class name cn. If the caller does not specify rootNode the function just returns every element in the entire web page with class name cn.

The array r=[] is initialised as an empty array into which the elements that are found get added (using push).

i-- is used because the for loop starts at i=e.length – the function goes through the elements returned by getElementsByTagName from the end of the array through to the start. The for loop evaluates the result of i-- as a boolean, exitting when i hits -1 (because on this iteration the boolean evaluation will have been done against the falue 0, because i-- post-decrements the value of i).