Bug 187650

Summary:	Number.toPrecision returns a wrong value
Product:	WebKit	Reporter:	isol2
Component:	JavaScriptCore	Assignee:	Nobody <webkit-unassigned>
Status:	RESOLVED WONTFIX
Severity:	Normal	CC:	ashvayka
Priority:	P2
Version:	Safari 11
Hardware:	Unspecified
OS:	Unspecified

Description isol2 2018-07-13 11:16:43 PDT

Version: JavaScriptCore v233804
OS: Ubuntu 16.04 x64

Hi everyone,
I found an issue on a test case using Number.toPrecision that I don’t know if it is considered as an incompatibility issue between engines or a real bug example, but according with ES6 specs some values are described as "approximately" maybe this is a case of it. 
V8, JSC and SpiderMonkey returns the wrong value if param > 17, only Chakra works as expected.

Steps to reproduce:
print(123.56.toPrecision(18) === "123.560000000000000");
print(123.56.toPrecision(18))

Actual results:
False
123.560000000000002

Expected results:
True
123.560000000000000

Comment 1 Alexey Shvayka 2021-05-06 17:46:26 PDT

(In reply to isol2 from comment #0)
> I found an issue on a test case using Number.toPrecision that I don’t know
> if it is considered as an incompatibility issue between engines or a real
> bug example, but according with ES6 specs some values are described as
> "approximately" maybe this is a case of it. 

Thank you for report!

This is most certainly a IEEE 754 precision issue like 0.1 + 0.2 != 0.3.
The spec prose is worded to highlight the inaccuracy: please see step 10.a of https://tc39.es/ecma262/#sec-number.prototype.toprecision.