JSON.stringify

```js
function toJSON(data){

}

// test
toJSON(""); // -> ""
toJSON("abc"); // -> "abc"
toJSON(123); // -> 123
toJSON({a:1, b:2}); // -> {"a":1, "b":2}
toJSON(["1", 3, {name:"monica", age:18}]); //-> ["1", 3, {"name":"monica", "age":18}]
```