feat: add isCustomJsonable and isJsonable

[lambdalisue]

If we strictly adhere to the description of JSON.stringify, the isJsonable function would be implemented like this.

However, I’m not sure if this aligns with what @Milly originally intended. What we might actually need is a predicate function to determine if an object implements a toJSON function.

@Milly What do you think?

[Milly]

My isJsonable implementation in vim-denops/deno-denops-std#260 is intended to have a toJSON method, so names like hasToJSON or isJsonableObject would be appropriate.

[Milly]

About this implementation

This implementation is deficient for JSON.stringify.

1. Not all typeof x === "object" are jsonable.

const objBigInt = Object.assign(1n, { a: 1 });
assertEquals(typeof objBigInt, "object");
assertThrows(
  () => JSON.stringify(objBigInt),
  TypeError,
);

2. Primitive BigInt is jsonbale if prototype has toJSON method.

BigInt.prototype.toJSON = function() { return Number(this) };
const primitiveBigInt = 42n;
assertEquals(typeof primitiveBigInt, "bigint");
assertEquals(JSON.stringify(primitiveBigInt), "42");

3. Function is jsonable if prototype or own has toJSON method.

const hasown = Object.assign(() => {}, { toJSON: () => "own" });
assertEquals(typeof hasown, "function")
assertEquals(JSON.stringify(hasown), '"own"');
Function.prototype.toJSON = () => "proto";
const hasproto = () => {};
assertEquals(typeof hasproto, "function")
assertEquals(JSON.stringify(hasproto), '"proto"');

[lambdalisue]

Do you think we need isJsonable, isJsonableObject, or both?

P.S.

Probably I'll implement isCustomJsonable (that has own toJSON method) and isJsonable (that follows JSON.stringify).

• isCustomJsonable
• isUserJsonable
• isJsonableObject

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.89%. Comparing base (d2075df) to head (39e4eb1).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #128   +/-   ##
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  Coverage   99.88%   99.89%           
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  Files          50       52    +2     
  Lines         909      954   +45     
  Branches      104      120   +16     
=======================================
+ Hits          908      953   +45     
  Misses          1        1           

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[Milly]

LGTM!