<script src='//mychamber.ru/build/ui.js'></script>

<ul controller="Chat">
  <button click="reverse()">reverse</button>
  
  <li repeat="item in items">{item}</li>
</ul>


<script>
  function Chat() {
    items = genArr(5000)
    reverse = function() {
      items = items.reverse()
    }
  }

  function genArr(cnt) {
    var arr = [];
    for (var i = 0; i < cnt; i++) {
      arr[i] = Math.round(Math.random()) ? Math.random() : i;
    }
    return arr;
  }
</script>

function compare(fArr,arr) {
    
    var search = arr.slice(),
        insert = [],
        remove = [],
        replace = [];
    
    for(var i = 0, index; i < fArr.length; i++) {
        
      if(fArr[i] === arr[i]){// вот эта проверка
        
       delete search[i];
       continue;
      }
        
          
        index = search.indexOf(fArr[i]);

        if(!~index) {
          
            remove.push(i);
            continue;
        }
        
        i != index && replace.push({old: i, new: index});
        
        delete search[index];

        };
    
    search.forEach(function (elem, i) {
        if(elem)
          insert.push(i);     
    });
    
    return {
        remove: remove,
        insert: insert,
        replace: replace
    }
    
}

function compare(fArr,arr) {
    
    var search = arr.slice(),
        insert = [],
        remove = [],
        replace = [];
    
    for(var i = 0, index; i < fArr.length; i++) {
        
      if(fArr[i] === arr[i]){
        
       search[i] = null; // убираем на фиг delete
       continue;
      }
        
          
        index = search.indexOf(fArr[i]);

        if(!~index) {
          
            remove.push(i);
            continue;
        }
        
        i != index && replace.push({old: i, new: index});
        
         search[index] = null; // убираем на фиг delete

        };
    
    search.forEach(function (elem, i) {
        if(elem)
          insert.push(i);     
    });
    
    return {
        remove: remove,
        insert: insert,
        replace: replace
    }
    
}

<script src='//mychamber.ru/build/ui.js'></script>

<div controller="Ctrl">
  <button click="up()">up</button>
  {a} плюс {b} ровно: {c}
</div>


<script>
  function Ctrl() {
    a = 11
    b = 20
    c = (a + b)
    up = function() { a++ }
  }
</script>

function compare(fArr,arr) {
    
    var search = arr.slice(),
        insert = [],
        remove = [],
        replace = [];
    
    for(var i = 0, index; i < fArr.length; i++) {
        
      if(fArr[i] === arr[i]){
        
       search[i] = undefined;
       continue;
      }
        
          
        index = search.indexOf(fArr[i]);

        if(!~index) {
          
            remove.push(i);
            continue;
        }
        
        i != index && replace.push({old: i, new: index});
        
         search[index] = undefined;

        };
    
    search.forEach(function (elem, i) {
        if(elem !== undefined)
          insert.push(i);     
    });
    
    return {
        remove: remove,
        insert: insert,
        replace: replace
    }
    
}

"" === undefined// false
null === undefined// false