get an array of arrays which hold a string and number of occurences

getWordOccurences.js

function getWordOccurences() {
  var textNodes;
  
  function getTextNodes() {
    var walker = document.createTreeWalker(
        document.body, 
        NodeFilter.SHOW_TEXT, 
        null, 
        false
    );
    
    var node;
    var textNodes = [];

    while(node = walker.nextNode()) {
        textNodes.push(node.nodeValue);
    }
    
    return textNodes;
  }
  
  var sortByOccurence = function(arr){
    var occurenceMap = {};
    
    for (var i = 0; i < arr.length; i++) {
      var el = arr[i];
      
      if (occurenceMap[el] == null) {
        occurenceMap[el] = 1;
      } else {
        occurenceMap[el]++;
      }
    }
    
    return sortByValue(occurenceMap);
  };
  
  function sortByValue(obj) {
    var sortable = [];
    
    for (var word in obj) {
      sortable.push([word, obj[word]]);
    }
    sortable.sort(function(a, b) {
      return b[1] - a[1];
    });
  
    return sortable;
  }
  
  textNodes = getTextNodes();
  return sortByOccurence(textNodes);
}