How to accurately measure time taken by a sort function in JavaScript

Question

I am trying to measure the time taken by various sorting algorithms in JavaScript

I have tried various ways to measure the time taken but it always gives wrong time

var startTime, endTime, timeDiff;

startTime = new Date();
Bubble();
endTime = new Date();
timeDiff = endTime - startTime; //in ms

I have also tried returning the endTime from Bubble sort function but still the time calculated was wrong

Bubble Sort function

    function Bubble()
    {
        c_delay=0;
    
        for(var i=0;i<array_size-1;i++)
        {
            for(var j=0;j<array_size-i-1;j++)
            {
                div_update(divs[j],div_sizes[j],"yellow");//Color update
            
                if(div_sizes[j]>div_sizes[j+1])
                {
                    div_update(divs[j],div_sizes[j], "red");//Color update
                    div_update(divs[j+1],div_sizes[j+1], "red");//Color update
                
                    var temp=div_sizes[j];
                    div_sizes[j]=div_sizes[j+1];
                    div_sizes[j+1]=temp;
                
                    div_update(divs[j],div_sizes[j], "red");//Height update
                    div_update(divs[j+1],div_sizes[j+1], "red");//Height update
                }
                div_update(divs[j],div_sizes[j], "blue");//Color updat
            }
            div_update(divs[j],div_sizes[j], "green");//Color update
        }
        div_update(divs[0],div_sizes[0], "green");//Color update
        enable_buttons();
        return new Date();
    }

Calculating time after it has been returned by sort function:

        startTime = new Date();
        endTime = Bubble();
        timeDiff = endTime - startTime; //in ms

I have even tried with performance.now() but still not getting accurate time taken by the sort function