Pretty-format toJSON maximum call stack error

[NickMalt]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
As I understand this code expect that toJson function return already stringified version of object, which is not right. And eventually this leads to Maximum call stack error since printing of serializable object is not handled after toJSON call. Or maybe I miss something?

If the current behavior is a bug, please provide the steps to reproduce and
either a repl.it demo through https://repl.it/languages/jest or a minimal
repository on GitHub that we can yarn install and yarn test.
Debugging gif

What is the expected behavior?
handle serializable object printing after a toJSON call

Please provide your exact Jest configuration and mention your Jest, node,
yarn/npm version and operating system.
jest: 21.2.1
node: 9

[SimenB]

From what I can understand in the docs, toJSON is expected to always return a string - just not necessarily something that can be fed to JSON.parse. Or am I misreading it?

/cc @pedrottimark

[cpojer]

JSON is a string representation of JavaScript objects. toJSON implies that you are creating that string representation. If you are returning something that is not a string it is by definition not JSON and means your API is wrongly named or doing the wrong thing.

[AntonPuko]

@cpojer toJSON() is kind of middlware for JSON.stringify() transformation, and it shouldn't always return string, but anything that can be passed directly to JSON.stringify().
just look at this example:
https://docs.microsoft.com/en-us/scripting/javascript/reference/json-stringify-function-javascript#example-2

[cpojer]

Yap, you are right, my mistake. cc @pedrottimark

[Qix-]

I think @AntonPuko outlined some simple reproduction steps in qix-/color#134 (mentioned above).

[pedrottimark]

@NickMalt @AntonPuko Hello, friends. pretty-format teaches me more and more JavaScript :)

There is a unit test in which pretty-format serializes an object returned by toJSON method: https://github.com/facebook/jest/blob/master/packages/pretty-format/src/__tests__/pretty_format.test.js#L663-L670

The problem is that pretty-format supports calling toJSON recursively EDIT CORRECT LINK: https://github.com/facebook/jest/blob/master/packages/pretty-format/src/__tests__/pretty_format.test.js#L690-L697

Because Color.prototype.toJSON returns an instance of Color which has a toJSON method, it’s colors all the way down to Maximum call stack size exceeded /cc @Qix- in case you are interested

By definition, JSON.stringify ignores prototype properties and own methods of an object.

Question for you and my fellow collaborators: can you think of a use case for pretty-format to call toJSON recursively? Or do you think the feature “fell out of” the implementation?

A tentative local change breaks the one unit test and formats the pretty Jest color as follows:

const Color = require('color');
const format = require('pretty-format');
format(Color.rgb(153, 66, 91));
/*
Object {
  "color": Array [
    153,
    66,
    91,
  ],
  "model": "rgb",
  "valpha": 1,
}
*/

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