Mobx: Observable array does not display correctly

Question

I am trying to understand how to use observable array with Mobx.

I have a hard time to figure out why this:

let entities = observable([]);
entities[0] = "foo";
autorun(() =>{
  console.log(entities);
});

writes:

[$mobx: Object]
0: (...)
1: (...)
2: (...)
3: (...)
4: (...)
5: (...)
6: (...)
7: (...)
8: (...)
9: (...)
10: (...)
11: (...)
12: (...)
13: (...)
14: (...)
15: (...)
16: (...)
17: (...)
...
999: (...)

Instead of a classic array?

Answer 1

Figure out!

As stated in the docs

Bear in mind that Array.isArray(observable([])) will yield false, so whenever you need to pass an observable array to an external library, it is a good idea to create a shallow copy before passing it to other libraries or built-in functions (which is good practice anyway) by using array.slice() or array.peek(). So Array.isArray(observable([]).slice()) will yield true.

The doc exemple show us a todos.filter() which could lead to confusion because todos looks like a real JS Array. But it is not.

So for my exemple to work I just have to console.log(entities.slice()) which will display a real JS array.