MuJS Bug: Error.toString() Incorrectly Appends Stack Trace

[Lydia-66]

BUG Description
Suspected bug in MuJS: When converting an Error object to a string using toString(), MuJS incorrectly appends the stack trace to the resulting string by default. According to the ECMAScript 5.1 specification, the default behavior of Error.prototype.toString should only return a formatted string containing the error’s name and message. Automatically appending the stack trace is a deviation from this standard behavior and may lead to inconsistencies or unexpected results in applications that rely on strict error formatting.

Version
MuJS 1.3.7

Command

/home/engines/mujs-1.3.7/mujs-1.3.7/build/debug/mujs

Testcase

print("Error: test" === String.prototype.trim.call(new Error("test")));
print("Error: test");
print(String.prototype.trim.call(new Error("test")));

Actual Output

false
Error: test
Error: test
          at abug.js:3

Expected Output

true
Error: test
Error: test

Specification Reference
According to ECMAScript 5.1, Section 15.11.4.4, Error.prototype.toString should return a string in the form:

"<error name>: <error message>"

https://262.ecma-international.org/5.1/#sec-15.11.4.4
The specification makes no mention of including stack trace information in the toString() output. Therefore, MuJS’s behavior appears to be non-compliant.

[avih]

I believe this behavior is specific to the mujs binary which is generally intended to be used as REPL, where it's useful to show the stack traces by default. It adds this code https://github.com/ccxvii/mujs/blob/master/main.c#L224-L231 here https://github.com/ccxvii/mujs/blob/master/main.c#L347 and this changes the Error.prototype.toString method to include the trace.

The actual library does not have this behavior.

That being said, I think few things can be done about it at the mujs binary:

1. Disable this behavior if we're not in REPL mode.
2. Maybe show the stack trace at the REPL without modifying Error.prototype.toString.
3. And/or restore Error.prototype.toString to be the normal one, and add Error.prototype.stack to be the error+stack, which is the common property/method in other JS engines, like node.js and V8, though on its own it won't be enough to show the stack trace in REPL by default. (in mujs Error.prototype.stackTrace is the trace alone, while elsewhere .stack is the error+trace).

For rererence, in mpv we do this:
https://github.com/mpv-player/mpv/blob/cc660f8609fd2f8a2a753c4fc1c3e18e6713847c/player/javascript.c#L409-L421