How to create JSON object Node.js

What I believe you're looking for is a way to work with arrays as object values:

var o = {} // empty Object
var key = 'Orientation Sensor';
o[key] = []; // empty Array, which you can push() values into


var data = {
    sampleTime: '1450632410296',
    data: '76.36731:3.4651554:0.5665419'
};
var data2 = {
    sampleTime: '1450632410296',
    data: '78.15431:0.5247617:-0.20050584'
};
o[key].push(data);
o[key].push(data2);

This is standard JavaScript and not something NodeJS specific. In order to serialize it to a JSON string you can use the native JSON.stringify:

JSON.stringify(o);
//> '{"Orientation Sensor":[{"sampleTime":"1450632410296","data":"76.36731:3.4651554:0.5665419"},{"sampleTime":"1450632410296","data":"78.15431:0.5247617:-0.20050584"}]}'

The JavaScript Object() constructor makes an Object that you can assign members to.

myObj = new Object()
myObj.key = value;
myObj[key2] = value2;   // Alternative