ag-Grid javaScript, TypeError: rowData is undefined - ag-grid

Question

ag-Grid, following the official demo of javascript but using API like real world over hard-coded data. Note: no jQuery, just use the primitive plain XMLHttpRequest() for ajax.
F12 verified API returns data in the same structure as demo, has children node inside, and gripOptions.rowData is assigned with the returned data.
Tried instantiating rowData inside of gripOptions as
rowData: [], got the same error
Or
rowData: {}, got ReferenceError: rowData is not defined.
HTML:
<script src="/scripts/agGrid/ag-grid.js"></script>
<script src="/scripts/agGrid/myAG.js"></script>
<br />JavaScript ag-Grid
<div id="myGrid" style="height: 200px;" class="ag-fresh"></div>
myAG.js:
var httpApi = new XMLHttpRequest();
var columnDefs = [
{ headerName: "Client Name", field: "ClientName", unSortIcon: true, cellRenderer: "group" },
{ headerName: "Division", field: "Division" },
{ headerName: "Others", field: "Others" }
];
var gridOptions = {
columnDefs: columnDefs,
getNodeChildDetails: getNodeChildDetails
};
function getNodeChildDetails(rowItem) {
if (rowItem.ClientName) {
return {
group: true,
// provide ag-Grid with the children of this group
children: rowItem.children,
// the key is used by the default group cellRenderer
key: rowItem.ClientName
};
} else {
return null;
}
}
// wait for the document to be loaded, otherwise
// ag-Grid will not find the div in the document.
document.addEventListener("DOMContentLoaded", function () {
$.ajax({
type: "GET",
url: "/api/myAG/Tree",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function (data) {
gridOptions.rowData = data;
var eGridDiv = document.querySelector('#myGrid');
new agGrid.Grid(eGridDiv, gridOptions);
},
error: function (XMLHttpRequest, textStatus, errorThrown) {
alert(errorThrown);
}
})
});
Version:
ag-grid = v8.1.0
FireFox = 50.1.0
Error message:
F12 confirms data exists and assigned:
inside of ag-grid.js, the line it complains about but rowData has data:

Answer 1

See this answered post, basically an additional check is needed for the tree.
ag-Grid, try to make Tree Demo work using own data