JSON.parse on a number as a string doesn't throw error

Question

I believed that this is one of the ways to correctly distinguish an invalid JSON object as string from a valid one

 function isValidJson(s) { try { JSON.parse(s); return true; } catch (e) { return false; } } console.log(isValidJson(1234)); // true console.log(isValidJson('1234')); // true console.log(isValidJson('Is life a valid JSON?')); // false :( console.log(isValidJson('{"a" : { "b": [1, 2, 3] }}')); // true 

This is the most common answer I see, in regards to achieving the distinction. I don't seem to quite understand what is happening here. This snippet actually recognizes numbers and numbers as strings as valid JSON objects. Could anyone throw light on why/how this is happening?

Also, on reading the MDN docs for JSON.parse , I found the reviver method. I wrote a function which seems to work for all scenarios. Would this function actually be the right way?

 function isValidJson(s) { try { JSON.parse(s, function(k, v) { if (k === "" && typeof v === "number") { throw "Invalid JSON"; } return v; }); return true; } catch (e) { return false; } } console.log(isValidJson(1234)); // false console.log(isValidJson('1234')); // false console.log(isValidJson('Is life a valid JSON?')); // false :( console.log(isValidJson('{"a" : { "b": [1, 2, 3] }}')); // true 

[EDIT] I may have wrongly implied that I'm looking to distinguish valid JSON strings. I meant, valid JSON objects as strings . Edited the question with regards to this.

Answer 1

A JSON text can be any of the JSON data types.

1234

… is a valid JSON text consisting of a Number. (Passing 1234 as a number to JSON.parse works because it gets converted to a string by JavaScript's normal typecasting rules).

"Is life a valid JSON?"

… is a valid JSON text consisting of a String.

Is life a valid JSON?

… is not a valid JSON text. It is just a bunch of characters which don't match the syntax of any JSON data type.

So no. Your second function would not be right. It claims that 1234 is not valid JSON.

Answer 2

I think the case where isValidJsonString(1234) returns true is just Javascript being its usual eager self at turning things into strings at the first opportunity. If you add a check to the function that the argument is actually a string, everything is fine:

function isValidJsonString(s) {
  if (typeof s !== 'string') {
    return false;
  }
  try {
    JSON.parse(s);
    return true;
  } catch (e) {
    return false;
  }
}

The others already work as they should.

This snippet actually recognizes numbers and numbers as strings as valid JSON strings. Could anyone throw light on why/how this is happening?

It is happening because numbers as strings are valid JSON strings.

[EDIT] I may have wrongly implied that I'm looking to distinguish valid JSON strings. I meant, valid JSON objects as strings. Edited the question with regards to this.

Your current method also doesn't do that, it only excludes numbers.

JSON values can be seven different things: object (in {} ), array (in [] ), string (in "" ), number, null , false or true . Of these, you exclude only the number.

If you use typeof v !== "object" , then accept only objects, arrays and null . Maybe that is what you want, maybe not.

If you only want JSON objects, just check that the first non-whitespace character in your JSON string is a { .