System.Datetime.TryParse too forgiving in Javascript

[HLWeil]

Description

Please provide a succinct description of your issue.

Repro code

https://fable.io/repl/#?code=DYUwLgBARgNAbhAvBAygTwM5hAWwHQAiAhtgCoCWOIepATmgApG0YgAUARAIIBCAwhABsHAJQBYAFCSADrXIA7MADN5EDgGMi8niCYsQAEwBcEAKSwIcIsACuIRKYDyHaJaA&html=Q&css=Q

let b,v = System.DateTime.TryParse("ABC 6")

printfn "canBeParsed: %b, value=%O" b v

Expected and actual results

"ABC 6" should not be parsed as a date. In DotNet and Python, as expected, the bool returned is false. In Javascript though, the bool is true and the number after the "ABC" is parsed as the month.

Related information

• Fable version: 4.19.3
• Operating system: Windows 11

[HLWeil]

Would be happy to help out on this one myself. Seems like a good small starting issue.

[MangelMaxime]

Hello,

The place to investigate this is issue should be:

Fable/src/fable-library-ts/Date.ts

Lines 594 to 601 in 8f7ab69

	export function tryParse(v: string, defValue: FSharpRef<IDateTime>): boolean {
	try {
	defValue.contents = parse(v);
	return true;
	} catch (_err) {
	return false;
	}
	}

More specifically, I believe this problem is going to be in the parseRaw function:

Fable/src/fable-library-ts/Date.ts

Lines 509 to 582 in 8f7ab69

	export function parseRaw(input: string): [Date, Offset] {
	function fail() {
	throw new Error(`The string is not a valid Date: ${input}`);
	}
	if (input == null || input.trim() === "") {
	fail();
	}
	// ISO dates without TZ are parsed as UTC. Adding time without TZ keeps them local.
	if (input.length === 10 && input[4] === "-" && input[7] === "-") {
	input += "T00:00:00";
	}
	let date = new Date(input);
	let offset: Offset = null;
	if (isNaN(date.getTime())) {
	// Try to check strings JS Date cannot parse (see #1045, #1422)
	// tslint:disable-next-line:max-line-length
	const m = /^\s*(\d+[^\w\s:]\d+[^\w\s:]\d+)?\s*(\d+:\d+(?::\d+(?:\.\d+)?)?)?\s*([AaPp][Mm])?\s*(Z|[+-]([01]?\d):?([0-5]?\d)?)?\s*$/.exec(input);
	if (m != null) {
	let baseDate: Date;
	let timeInSeconds = 0;
	if (m[2] != null) {
	const timeParts = m[2].split(":");
	const hourPart = parseInt(timeParts[0], 10);
	timeInSeconds =
	hourPart * 3600 +
	parseInt(timeParts[1] || "0", 10) * 60 +
	parseFloat(timeParts[2] || "0");
	if (m[3] != null && m[3].toUpperCase() === "PM" && hourPart < 12) {
	timeInSeconds += 720;
	}
	}
	if (m[4] != null) { // There's an offset, parse as UTC
	if (m[1] != null) {
	baseDate = new Date(m[1] + " UTC");
	} else {
	const d = new Date();
	baseDate = new Date(d.getUTCFullYear() + "/" + (d.getUTCMonth() + 1) + "/" + d.getUTCDate());
	}
	if (m[4] === "Z") {
	offset = "Z";
	} else {
	let offsetInMinutes = parseInt(m[5], 10) * 60 + parseInt(m[6] || "0", 10);
	if (m[4][0] === "-") {
	offsetInMinutes *= -1;
	}
	offset = offsetInMinutes;
	timeInSeconds -= offsetInMinutes * 60;
	}
	} else {
	if (m[1] != null) {
	baseDate = new Date(m[1]);
	} else {
	const d = new Date();
	baseDate = new Date(d.getFullYear() + "/" + (d.getMonth() + 1) + "/" + d.getDate());
	}
	}
	date = new Date(baseDate.getTime() + timeInSeconds * 1000);
	// correct for daylight savings time
	date = new Date(date.getTime() + (date.getTimezoneOffset() - baseDate.getTimezoneOffset()) * 60000);
	} else {
	fail();
	}
	// Check again the date is valid after transformations, see #2229
	if (isNaN(date.getTime())) {
	fail();
	}
	}
	return [date, offset];
	}

The easiest way to work on this issue, is to copy your reproduction code in src/quicktest/QuickTest.fs and then run ./build.sh quicktest javascript.

Once, this is running you can edit the Date.js generated under the fable_modules in the src/quicktest folder. Like that you don't need to always recompile everything. Once this is fixed, you can copy your fixes from that temporary file to src/fable-library-ts/Date.ts.

Tip

If it helps in this F# amplifying session https://amplifyingfsharp.io/sessions/2023/12/15/ at around 27min I am showing how > it can be done for Python target. (similar logic applies for JavaScript).

Add a new test to capture this edges case, you can add it to JavaScript, Rust, Python, etc. so it can be checked for the most number of target possible.

[HLWeil]

Hey @MangelMaxime,

thank you very much for the indepth tips and explanations. I will look into it!