JSON.parse() returns any, but should return Object

[inad9300]

TypeScript Version: 2.3.0

Expected behavior: JSON.parse() should return Object, as stated in MDN.

Actual behavior: JSON.parse() returns any, essentially getting rid of the useful compile-time type checks.

I realized about this while working on a project with a person new to TypeScript. She made a copy/paste mistake and ended up with a code equivalent to this:

function doSomething(data: string): Promise<any> {
    return JSON.parse(data)
}

For which TypeScript didn't show any error.

[mhegazy]

Object in JavaScript is similar to any. JSON.parse can return number | string| boolean | null | object | Array.

The type any here is intentional. it is meant to ease the use of the function (otherwise you will need to cast on every use). The assumption here is you will validate the input value against some criteria and that you will cast to the correct type.

[inad9300]

If that's the policy, fine then. IMHO if one wants ease of use, will go with JavaScript; TypeScript sounds to me like the option to chose when you want precisely type correctness.

[mohsen1]

JSON.parse("1") === 1 is valid JS code

[inad9300]

I know. I understand I was wrong on my initial suggestion, Object as the return type is not a valid solution. @mhegazy ideas were closer to a correct answer, although I don't quite get the role of object (lowercase "o") on his attempt.

After giving it some thought, I think it is not possible to correctly represent JSON.parse()'s return type using TypeScript, because it may well return an Array of Arrays recursively. Nevertheless, anything more accurate than any would be very welcomed, and a reasonable number (say, 16) of recursion levels may be represented manually, e.g.

type JsonObject =
    (number | string | boolean | null | Object) | Array<
        (number | string | boolean | null | Object) | Array<
            (number | string | boolean | null | Object) | Array<any>>>

function parseOne(): JsonObject {
    return JSON.parse('1')
}

const n: number = <number> parseOne()

I wonder what is the approach taken by TypeScript in similar recursive cases (if they exist).

[gcanti]

I think it is not possible to correctly represent JSON.parse()'s return type using TypeScript

Maybe

export type JSONObject = { [key: string]: JSON }
export interface JSONArray extends Array<JSON> {}
export type JSON = null | string | number | boolean | JSONArray | JSONObject

EDIT: TypeScript 2.2.2

[inad9300]

I'm amazed that works, but with JSONArray as a type complains about a circular reference.

[aluanhaddad]

I realized about this while working on a project with a person new to TypeScript. She made a copy/paste mistake and ended up with a code equivalent to this:

function doSomething(data: string): Promise<any> {
    return JSON.parse(data)
}

For which TypeScript didn't show any error.

Indeed, but if you are OK with explicitly returning Promise<any> it is an indication and errors are not far off anyway.

[inad9300]

I don't get your point, sorry.

[mhegazy]

This issue seems to have been already addressed or is unactionable at the moment. I am auto-closing it for now.

[rsazima]

Hi, if I understood this issue correctly, it would be the cause for the behaviour below. Is that correct?

> interface I { id: number }
> let i: I = { id: 'a' }
⨯ Unable to compile TypeScript
[eval].ts (1,5): Type '{ id: string; }' is not assignable to type 'I'.
  Types of property 'id' are incompatible.
    Type 'string' is not assignable to type 'number'. (2322)
> let i: I = JSON.parse(JSON.stringify({ id: 'a' }))
undefined
> i
{ id: 'a' }

[inad9300]

Yes.

[varshildoshi]

is it possible to create a typescript function in json??
if possible plz send me some source or link for that..