What happens if you set an Array with a JSON Object value?

I’m struggling with a big JSON Object and I got into a weird problem:
My Object has the following structure:

const bar = {
  arr: []
}

If I try to set its value in this way bar.arr.foo = 'ciao' it doesn’t give me any error. The result object is this

enter image description here

What is that strange “Array(0)” that the Chrome dev tools are pointing out? If I try to type typeof bar.arr it says “object”

But doing JSON.stringify(bar, null, 2) results in '{n "arr": []n}'