You can define how an object stringifies with String() and toString(), and how it represents itself when converted to JSON

to-string-to-json.js

({
  value: 'hello world',
  toString() { return `stringified that's: ${this.value}` }
}).toString()

// "stringified that's: hello world"

JSON.stringify({
  value: 'hello world',
  toJSON() { return [{JSONified_thats: this.value}] }
})

// "[{"JSONified_thats":"hello world"}]"