simple-lru.js
function () {
      var tail = this._tail
        , head = this._head;
      for (var i = head - 1; i >= tail; i--) {
        var headEntry = this._byOrder[i];
        if (headEntry) {
          this._head = i + 1;
          return headEntry;
        }
      }
    }
function () {
      var tail = this._tail
        , head = this._head;
      for (var i = head - 1; i >= tail; i--) {
        var headEntry = this._byOrder[i];
        if (headEntry) {
          this._head = i + 1;
          return headEntry;
        }
      }
    }
function ( ) { var tail = this . _tail , head = this . _head ; for ( var i = head - 1 ; i >= tail ; i -- ) { var headEntry = this . _byOrder [ i ] ; if ( headEntry ) { this . _head = i + 1 ; return headEntry ; } } }
update head index
@return {Entry|undefined}
@api private
update head index