Recalculate the Rollup field using JS

Description - Dynamics CRM 365 has two types of calculated fields 

1- Calculated fields - This type of field is only available on the table.

2- Roll-up fields - This type of field is use to calcuate the sum from the child entity. These fields recalculated at a certain time or by manually clicking on recalculate button. 

Step 1 - Write a function below to calculate the Rollup Fields using JS

function funccalculateRollup(entityName, recordId, fieldName)
{
    // Define the entity set name, record ID, and field name
    // Create an XMLHttpRequest object
    var req = new XMLHttpRequest();
    req.open(this.method, this.url, true);
    // Set the request headers
    req.setRequestHeader("OData-MaxVersion", "4.0");
    req.setRequestHeader("OData-Version", "4.0");
    // Open the request with the GET method
    req.open("GET", Xrm.Page.context.getClientUrl() + "/api/data/v9.0/CalculateRollupField(Target=@p1,FieldName=@p2)?" +
        "@p1={'@odata.id':'" + entityName + "(" + recordId + ")'}&" +
        "@p2='" + fieldName + "'", true);
    // Define the callback function
    req.onreadystatechange = function ()
    {
        if (this.readyState === 4)
        {
            req.onreadystatechange = null;
            if (this.status === 200)
            {
                // Parse the response
                var result = JSON.parse(this.response);
                // Get the value of the rollup field
                var value = result[fieldName];
                // Do something with the value
                console.log(value);
            }
            else
            {
                // Handle the error
                console.error(this.statusText);
            }
        }
    };
    // Send the request
    req.send();
}

Step 2 
Call The above method 

function calculateRollupForBilledTime(executionContext)// Pass form Context
{
    var formContext = executionContext.getFormContext();
    var entityName = formContext.data.entity.getEntityName();
    var record = formContext.data.entity.getId();
    var entitySetName = "incidents";
    var recordId = record.replace("{", "").replace("}", "");
    var FieldName = "sbnfu_billabletime";
    funccalculateRollup(entitySetName, recordId, FieldName);
}