Wrong result from `Number.prototype.toFixed`

[magic-akari]

Describe the bug
Wrong result from Number.prototype.toFixed

To Reproduce

const a = 1.25.toFixed(1);
console.log(a);

Expected behavior

Running this code, a should be set to "1.3" and printed, but a is instead set to "1.2". The expected behaviour can be found in the ECMAScript specification.

Build environment (please complete the following information):

• OS: macOS arm64
• Version: latest
• Target triple: aarch64-apple-darwin
• Rustc version: rustc 1.67.0 (fc594f156 2023-01-24)

Additional context
Add any other context about the problem here.

[vncsalencar]

As per my understanding this is because of

boa/boa_engine/src/builtins/number/mod.rs

Line 281 in 4750e8d

let this_fixed_num = format!("{this_num:.precision$}");

Which, as rust-lang/rust#102935 shows, Rust's format! rounds x.5 to whichever even number is closest.
So 1.25.toFixed(1) rounds down to 1.2 but 1.35.toFixed(1) rounds up to 1.4.

[magic-akari]

Is there any progress here?

Regarding the one failing test; I think we should merge this without that one.

But we may have an easier solution that to fix the issues in std. We already use ryu-js crate that converts numbers to strings. Conveniently the repository for the crate is in the boa-dev org: https://github.com/boa-dev/ryu-js ;)

Without looking further into it, my idea would be to implement the functionality of num_to_exponential and num_to_exponential_with_precision there. @HalidOdat I think you probably know the most about this, do you think we should try to do that, or is ryu-js not the right place?

Originally posted by @raskad in #1620 (comment)

I have found a recommended fix mentioned in this issue, and I believe it should be implemented in ryu-js.
Is there any plans to implement to_precision and to_fixed in ryu-js?

[HalidOdat]

I have found a recommended fix mentioned in this issue, and I believe it should be implemented in ryu-js.

I think putting it in the ryu-js crate is a good place. (maybe feature gated) :)

Is there any plans to implement to_precision and to_fixed in ryu-js?

I don't think there has been any work done on this, I created Issues to track the progress (boa-dev/ryu-js#33, boa-dev/ryu-js#32, boa-dev/ryu-js#31)

[HalidOdat]

I have started work on Number.prototype.toFixed() implementation in the ryu algorithm locally, Hopefully I will be able create a PR on ryu-js repo, when It passes all the tests, then create a PR for boa_engine with the changes here :)