How to use the heap.used function in heap

axios.get('/metrics').then(function (res) {
                    var metrics = res.data,
                        mem = metrics['mem'],
                        memFree = metrics['mem.free'],
                        heap = metrics['heap'],
                        heapUsed = metrics['heap.used'],
                        heapCommited = metrics['heap.committed'],
                        activeSession = metrics['httpsessions.active'];
                    // Memory
                    data.setValue(0, 1, 100 * memFree / mem);
                    // Heap (Used)
                    data.setValue(1, 1, 100 * heapUsed / heap);
                    // Heap (Commited)
                    data.setValue(2, 1, 100 * heapCommited / heap);
                    chart.draw(data, options);
                    // Active Session
                    document.getElementById('activeSession').innerText = activeSession;
                });
            }

ctrl.$onChanges = function () {
      ctrl.memory = {
        total: ctrl.metrics.mem,
        used: ctrl.metrics.mem - ctrl.metrics['mem.free'],
        unit: 'K'
      };
      ctrl.memory.percentUsed = $filter('number')(ctrl.memory.used / ctrl.memory.total * 100, 2);

      ctrl.heap = {
        total: ctrl.metrics['heap.committed'],
        used: ctrl.metrics['heap.used'],
        init: ctrl.metrics['heap.init'],
        max: ctrl.metrics['heap.max'] || ctrl.metrics.heap,
        unit: ctrl.metrics['heap.max'] ? 'B' : 'K'
      };
      ctrl.heap.percentUsed = $filter('number')(ctrl.heap.used / ctrl.heap.total * 100, 2);

      ctrl.nonheap = {
        total: ctrl.metrics['nonheap.committed'],
        used: ctrl.metrics['nonheap.used'],
        init: ctrl.metrics['nonheap.init'],
        max: ctrl.metrics.nonheap,
        unit: 'K'
      };
      ctrl.nonheap.percentUsed = $filter('number')(ctrl.nonheap.used / ctrl.nonheap.total * 100, 2);
    };