Unless you have a custom object with custom .toString method returning JSON.stringify of that object, there is no obj that would give obj.toString() == JSON.stringify(obj).
When obj is an array like [1,2,3] then .toString() gives:
"1,2,3"
And JSON.stringify:
"[1,2,3]"
These are close but not quite the same, the JSON serialized one has no ambiguity with commas and directly runs as Javascript or can be parsed as JSON.
See:
["1,",2,3].toString();
//"1,,2,3" ... so you can't just split by comma and get original array
//it is in fact impossible to restore the original array from this result
JSON.stringify(["1,",2,3])
//'["1,",2,3]'
//original array can be restored exactly