Why the JSON.parse returning [Object Object] instead of value?

here is my json:

[
    {
        "customer": "BigCo",
        "performances": [
            {
                "playID": "hamlet",
                "audience": 55
            },
            {
                "playID": "as-like",
                "audience": 35
            },
            {
                "playID": "othello",
                "audience": 40
            }
        ]
    }
]

here is my code(environment is node.js):

let readFile = require('fs').readFileSync

let invoices = JSON.parse(readFile('./invoices.json', 'utf8'))

console.log(invoices);

here is my result:

[
  { customer: 'BigCo', performances: [ [Object], [Object], [Object] ] }
]

I have read some questions related, and get some solutions like invoices = invoices[0] or JSON.parse(JSON.stringify()).
The thing I really hope to know is the reason.
Why do I get the result [ [Object], [Object], [Object] ] and Which part cause the problem?