Implement Symbol.hasInstance for MockDate

[runjak]

Hey 👋

so I'm enjoying use of both: ladle and date-fns. And this leads me to a situation where I'm using date-fns isValid and isDate to check some data. In particular we've got a component that does date based calculations based on a MockDate.

My understanding is that date-fns depends on the global Date object in code like this:

// node_modules/date-fns/isDate.js
export function isDate(value) {
  return (
    value instanceof Date ||
    (typeof value === "object" &&
      Object.prototype.toString.call(value) === "[object Date]")
  );
}

So when calculating new dates we get instances of RealDate, and check whether a RealDate is instanceof a MockDate, because MockDate rightly replaces the global date when mocking with ladle.

For inheritance this is usually the wrong way: a parent is not an instance of the child class. My suggestions would be to treat it as that though in case of MockDate with the following snippet:

static [Symbol.hasInstance](instance: unknown): boolean {
    return instance instanceof RealDate;
  }

I've tested it by test-wise appending (but not comitting) this code to packages/ladle/lib/app/src/mock-date.ts:

let d = new RealDate();
let m = new MockDate();

console.log({ l: m, d });

console.table([
  ["d instanceof RealDate", d instanceof RealDate],
  ["m instanceof RealDate", m instanceof RealDate],
  ["m instanceof MockDate", m instanceof MockDate],
  ["d instanceof MockDate", d instanceof MockDate],
]);

It leads to output like this:

❯ node packages/ladle/lib/app/src/mock-date.ts
{ l: 2025-05-26T15:36:10.887Z, d: 2025-05-26T15:36:10.887Z }
┌─────────┬─────────────────────────┬──────┐
│ (index) │ 0                       │ 1    │
├─────────┼─────────────────────────┼──────┤
│ 0       │ 'd instanceof RealDate' │ true │
│ 1       │ 'm instanceof RealDate' │ true │
│ 2       │ 'm instanceof MockDate' │ true │
│ 3       │ 'd instanceof MockDate' │ true │
└─────────┴─────────────────────────┴──────┘

[changeset-bot]

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[runjak]

@tajo would you like me to add a Changeset to this PR?

[runjak]

Note that the original MockDate as implemented in https://github.com/boblauer/MockDate/blob/master/src/mockdate.ts#L41 sets the prototype like this:

MockDate.prototype = RealDate.prototype;

With modern JS this leads to the following TypeError:

MockDate.prototype = RealDate.prototype;
                   ^
TypeError: Cannot assign to read only property 'prototype' of function 'class Date extends RealDate {
                         ...<omitted>...
}'

I believe this to be the reason why the prototype line has been omitted from ladles mock date, and it is also what causes the issue in date-fns isDate and when using instanceof between MockDate and Date objects.

That is why I'm asking to adjust the implementation of Symbol.hasInstance.

[tajo]

@runjak makes sense. Can you please add some unit test and changeset? Thanks!