Date objects serialized by toRecords fail instanceof Date check

[davidgovea]

Have you tried latest version of polars?

• yes

What version of polars are you using?

Observed on 0.10, still present on main branch

What operating system are you using polars on?

Mac / Linux

What node version are you using

Node 20

Describe your bug.

When .toRecords() returns a Date object, it fails instanceof Date checks

What are the steps to reproduce the behavior?

Extended the existing test with a demonstration:
https://github.com/davidgovea/nodejs-polars/pull/3/files

What is the actual behavior?

See test run here: https://github.com/davidgovea/nodejs-polars/actions/runs/9680675801/job/26709383256

serializedDate.constructor.name
=> 'Date' ✅

serializedDate instanceof Date
=> false ❌

What is the expected behavior?

x instanceof Date should be true.

---

Have not taken a look under the hood yet, but I am happy to do so when I get a free moment. For now, I'm just paving over the issue with an additional new Date(polarsToRecordsDate) wrapper in my code

[Bidek56]

toRecords returns a JSON array: [{"date":"2024-07-07T00:00:00.000Z"}]
Why would you expect actual[0].date to be an instance of a Date?
I think you need to convert it to Date to be a Date else it will be a string/object just like everything else in JSON.

[davidgovea]

Hey @Bidek56, thanks for taking a look!

I'd be fine with that being the case (everything serialized to ISO date string), but today it seems like we're in a strange middleground.

If it were as you suggest, then the constructor.name would be String, not Date.
Additionally, the object has all of the Date methods available.

If it quacks like a Date, I'd assume it should pass instanceof Date 😆

But yeah making toRecords return standard JSON would make sense too.

[Bidek56]

If you are looking for schema, I would suggest using df.toDataResource() which starts with df.toRecords() then it adds field type to be:
{"data":[{"date":"2024-07-08T00:00:00.000Z"}],"schema":{"fields":[{"name":"date","type":"string"}]}}

[universalmind303]

@davidgovea this seems like a bug in jest

Using alternative test runners, it all passes

> node example.js

const pl = require("nodejs-polars");
const { test } = require("node:test");
const assert = require('node:assert/strict');
test("toRecords:date", () => {
	const df = pl
		.DataFrame({
			date: [new Date()],
		})
		.withColumn(pl.col("date").cast(pl.Date).alias("date"));

	const serializedDate = df.toRecords()[0].date;
	assert.strictEqual(serializedDate instanceof Date, true);
});

> bun test example.js

const pl = require("nodejs-polars");

test("toRecords:date", () => {
	const df = pl
		.DataFrame({
			date: [new Date()],
		})
		.withColumn(pl.col("date").cast(pl.Date).alias("date"));

	const serializedDate = df.toRecords()[0].date;
  console.log(serializedDate)
	expect(serializedDate instanceof Date).toEqual(true);
});

additionally, using various versions of node & bun directly return the same result

Welcome to Node.js v22.1.0.
Type ".help" for more information.
> const pl = require("nodejs-polars");
undefined
> const df = pl.DataFrame({
...     date: [new Date("2020-01-01"), new Date("2020-01-02")],
... });
undefined
> df.toRecords()[0].date instanceof Date

[davidgovea]

Wow that's a weird one! 🤦 Thanks @universalmind303

Realizing that I only observed this in jest tests as well, not in production. What is it doing under the hood?!

Again really appreciate the response here