You need to provide initial value to reduce
console.log('total', tempvalue.reduce(getSum,0));first time when you use reduce you have an empty array and which is causing reduce to produce an error. you can pass initial value and it should work as expected.
Note: If initialValue isn't provided, reduce() will execute the callback function starting at index 1, skipping the first index. If initialValue is provided, it will start at index 0.
$(document).ready(function() { function getSum(total, num) { return total + num; } var adultCalc = function(values, data, calcParams) { var tempvalue = []; data.forEach(function(data) { var count = data.age * data.qty; tempvalue.push(count); }); console.log('array', tempvalue); console.log('total', tempvalue.reduce(getSum,0)); /*return tempvalue;*/ } var tabledata = [{ id: 1, name: "Oli Bob", age: "12", qty: "1", dob: "" }, { id: 3, name: "Christine Lobowski", age: "42", qty: "1", dob: "22/05/1982" }, { id: 4, name: "Brendon Philips", age: "35", qty: "2", dob: "01/08/1980" }, { id: 5, name: "Margret Marmajuke", age: "16", qty: "0", dob: "31/01/1999" }, { id: 5, name: "Marmajuke", age: "17", qty: "0", dob: "31/01/1999" }, { id: 4, name: "Philips", age: "27", qty: "0", dob: "01/08/1980" } ]; var table = new Tabulator("#example-table", { height: 205, data: tabledata, layout: "fitColumns", columns: [{ title: "Name", field: "name", width: 150 }, { title: "Age", field: "age", bottomCalc: adultCalc }, { title: "Qty", field: "qty" }, { title: "Date Of Birth", field: "dob", sorter: "date", align: "center" } ] });});<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script><link href="https://unpkg.com/tabulator-tables@4.1.4/dist/css/tabulator.min.css" rel="stylesheet"><script type="text/javascript" src="https://unpkg.com/tabulator-tables@4.1.4/dist/js/tabulator.min.js"></script><div id="example-table"></div>