JSON.stringify returns a String. So, for example:
var data = [
{ id: 1, name: "bob" },
{ id: 2, name: "john" },
{ id: 3, name: "jake" },
];
JSON.stringify(data)
will return the equivalent of:
"[{\"id\":1,\"name\":\"bob\"},{\"id\":2,\"name\":\"john\"},{\"id\":3,\"name\":\"jake\"}]"
as a String value.
So when you have
<% for(var i=0; i<JSON.stringify(data).length; i++) {%>
what that ends up looking like is:
<% for(var i=0; i<"[{\"id\":1,\"name\":\"bob\"},{\"id\":2,\"name\":\"john\"},{\"id\":3,\"name\":\"jake\"}]".length; i++) {%>
which is probably not what you want. What you probably do want is something like this:
<table>
<% for(var i=0; i < data.length; i++) { %>
<tr>
<td><%= data[i].id %></td>
<td><%= data[i].name %></td>
</tr>
<% } %>
</table>
This will output the following table (using the example data from above):
<table>
<tr>
<td>1</td>
<td>bob</td>
</tr>
<tr>
<td>2</td>
<td>john</td>
</tr>
<tr>
<td>3</td>
<td>jake</td>
</tr>
</table>