orderPlaced.js

document.cookie = "orderId="+0 +",counter="+0

let httpRequest = new XMLHttpRequest(),
jsonArray,
method = "GET",
jsonRequestURL = "https://5d76bf96515d1a0014085cf9.mockapi.io/order";

httpRequest.open(method, jsonRequestURL, true);
httpRequest.onreadystatechange = function()
{
    if(httpRequest.readyState == 4 && httpRequest.status == 200)
    {
        // convert JSON into JavaScript object
        jsonArray = JSON.parse(httpRequest.responseText)
        console.log(jsonArray)    
        jsonArray.push(
            {
                "id": (jsonArray.length)+1, "amount": 200,"product":["userOrder"]
            })

        // send with new request the updated JSON file to the server:
        httpRequest.open("POST", jsonRequestURL, true)
        httpRequest.setRequestHeader("Content-Type", "application/x-www-form-urlencoded")
        httpRequest.send(jsonArray)
    }
}
httpRequest.send(null);