Your object misses a comma as shown below:

name: "Blue Stilton",
    age: "13"//comma is missing here
    smelly: true

JSON.stringify works fine as shown below.

var cheese_array = [
  {
    name: "Chedder",
    age: "34",
    smelly: true
  },
  {
    name: "Brie",
    age: "4",
    smelly: false
  },
  {
    name: "Blue Stilton",
    age: "13",
    smelly: true
  }
 ];
console.log(JSON.stringify(cheese_array))

However I am not sure how you get a log [object Object], [object Object], [object Object] I am presuming you are console logging something else please check that in your code.

var data = { };
data.onlineUsers = getUsersInRoom();

var string = JSON.stringify(data);

Would this work for you?

edit

I just noticed your error is circular type, your user or room object is probably creating a circular reference.

User > Room > User > Room etc...

Because data is not just an Object - it's an Array. And when Arrays are json-stringified, all the non-numeric properties are discarded from result (as there's no way to represent them in [...] notation anyway).

If you still want to see that property in JSON, you have to use plain Objects. You can bite the bullet and just "plain-objectify" the original array with { ... } sugar:

const data = [{foo: "1", bar: "2"}, {foo: "2", bar:"3"}];
data.extra = ["a", "b"];
JSON.stringify( {...data} );
// "{"0":{"foo":"1","bar":"2"},"1":{"foo":"2","bar":"3"},"extra":["a","b"]}"

... but beware, all those "0", "1" props will be seen inside your JSON as is, and that's not the best of views. It's usually cleaner just to use additional property:

const data = [{foo: "1", bar: "2"}, {foo: "2", bar:"3"}];
const extra = ["a", "b"];
const output = {
  data,
  extra
};
JSON.stringify(output);
// "{"data":[{"foo":"1","bar":"2"},{"foo":"2","bar":"3"}],"extra":["a","b"]}"

JavaScript arrays are designed to hold data with numeric indexes. You can add named properties to them because an array is a type of object (and this can be useful when you want to store metadata about an array which holds normal, ordered, numerically indexed data), but that isn't what they are designed for.

The JSON array data type cannot have named keys on an array.

When you pass a JavaScript array to JSON.stringify the named properties will be ignored.

If you want named properties, use an Object, not an Array.

const test = {}; // Object
test.a = 'test';
test.b = []; // Array
test.b.push('item');
test.b.push('item2');
test.b.push('item3');
test.b.item4 = "A value"; // Ignored by JSON.stringify
const json = JSON.stringify(test);
console.log(json);