reimplement getElementsByTagName

getElementsByTagName.js

/*
List of elements of type/ reimplement getElementsByTagName

* Returns a list of all elements of given type which are children of provided parent.
* @param {! string} `tagName` The tag name of the element. eg. 'DIV'
* @param {Node} `parent` The root node under which the search must be performed.
* @return {!Array} A list of elements of given type that are children of provided parent. Empty if none could be found.

*/

var getElementsByTagName = function (tagName, parent) {
  var elements = {} // tag name : element
  var results = [] // array of elements
  var root = parent || document
  for (var i=0; i < root.all.length; i++) {
    elements[root.all[i].tagName] = root.all[i]
  }
  for (var tag in elements) {
    if (tag === tagName) {
      results.push(elements[tagName])
    }
  }
  return results
}

var div = getElementsByTagName('DIV')
console.log(div)

// can't use document.all b/c it's not supported in all browsers.
// and because parent.all wouldn't work.