issue with converting complex javascript object to string to write to file [duplicate]

I have this object in a .js file being run with NodeJS on the command line. I am trying to save the "print-friendly" version of it to a variable, and then to write the variable to a file:

var obj = {}

obj['keys'] = ['a', 'b', 'c', 'd',]
obj['animals'] = [
    {name: 'giraffe'}, 
    {name: 'zebra'}, 
    {name: 'bird'}, 
    ]
    
obj['arrs'] = {
    'sky': {color: 'blue'},
    'grass': {color: 'green'},
    'brick': {color: 'brown'},
}

obj['arrs_2'] = []
obj['arrs_2']['sky'] = {color: 'blue'}
obj['arrs_2']['grass'] = {color: 'green'}
obj['arrs_2']['brick'] = {color: 'brown'}

When I use console.log(obj), I get:

{
  keys: [ 'a', 'b', 'c', 'd' ],
  animals: [ { name: 'giraffe' }, { name: 'zebra' }, { name: 'bird' } ],
  arrs: {
    sky: { color: 'blue' },
    grass: { color: 'green' },
    brick: { color: 'brown' }
  },
  arrs_2: [
    sky: { color: 'blue' },
    grass: { color: 'green' },
    brick: { color: 'brown' }
  ]
}

Everything prints out correctly. But if I try to console.log(JSON.stringify(obj, null, 4)), I get:

{
    "keys": [
        "a",
        "b",
        "c",
        "d"
    ],
    "animals": [
        {
            "name": "giraffe"
        },
        {
            "name": "zebra"
        },
        {
            "name": "bird"
        }
    ],
    "arrs": {
        "sky": {
            "color": "blue"
        },
        "grass": {
            "color": "green"
        },
        "brick": {
            "color": "brown"
        }
    },
    "arrs_2": []
}

I was trying:

var out = JSON.stringify(obj, null, 4)
fs.writeFileSync('out.txt', out, (err) => {
    if (err) {
        throw err;
    }}
)

to write it, but the JSON.stringify() is unable to print the contents of arrs_2 (and displays no errors). Is there a way to capture the console.log() output to a variable without actually printing it to the screen? Or maybe a different function than JSON.stringify() that can convert the full object into a string like console.log() can?