MuJS Bug: Error.toString() Appends Stack Trace

Hey everyone, let's dive into a quirky bug we've spotted in MuJS. It's all about how MuJS handles the toString() method for Error objects, and it's a bit off the ECMAScript 5.1 standard. Let's break it down, make it super clear, and see why this matters.

BUG Description

So, the main issue we've stumbled upon is that MuJS seems to be adding the stack trace to the string output when you use toString() on an Error object. Now, according to the ECMAScript 5.1 spec, this isn't how it's supposed to work. The Error.prototype.toString method should only give you a neatly formatted string that includes the error's name and message. Think of it like a clean, concise error summary.

The problem arises when MuJS automatically tacks on the stack trace. This can lead to some unexpected behavior and inconsistencies, especially in applications that are relying on the error formatting to be just so. Imagine you've got some code that's parsing error messages, and suddenly, there's a stack trace hanging off the end – that could throw a wrench in the works.

To really understand why this is a big deal, let's think about the broader picture. JavaScript is designed to be a predictable language. When we follow standards like ECMAScript, we're all on the same page. This ensures that code behaves consistently across different environments. If an engine like MuJS deviates from these standards, it can create headaches for developers who expect certain behaviors. It's like ordering a pizza and finding out they've added pineapple – it might not be what you wanted!

In practical terms, this means that if you're using MuJS and you're formatting errors with toString(), you might be getting more information than you bargained for. This could clutter your logs, mess up your error reporting, or even expose internal details that you'd rather keep hidden. Imagine you're building a web application, and suddenly your error messages are spitting out file paths and line numbers – that's not ideal for security or user experience.

So, what's the solution? Well, for now, it's about being aware of this behavior in MuJS. If you're relying on strict error formatting, you might need to adjust your code to handle the extra stack trace information. Or, you might consider using a different method to format your errors, one that gives you more control over what's included in the output. Ultimately, the goal is to ensure that your error handling is robust and predictable, no matter which JavaScript engine you're using.

Version

We've pinpointed this behavior in MuJS version 1.3.7.

Command

To reproduce this, you can use the following command in your terminal:

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

Testcase

Here’s the JavaScript code snippet we used to test this:

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

This test case is pretty straightforward. We're creating a new Error object with the message "test", and then we're using toString() (via String.prototype.trim.call) to convert it into a string. The key here is what we expect to get back. According to the ECMAScript 5.1 specification, the result should simply be "Error: test". No extra fluff, no stack trace – just the error name and message.

The first line of the test case is a direct comparison. We're checking if the trimmed output of toString() is exactly equal to "Error: test". This is our baseline. If MuJS is behaving according to the spec, this should return true. The second line is a simple print statement, just to show the expected output. And the third line is where we actually call toString() and print the result. This is where we see the discrepancy.

By running this test case, we can clearly see how MuJS deviates from the standard. The first line will return false, because the actual output includes the stack trace. The third line will print the error message followed by the stack trace, which is not what we want. This simple test case effectively highlights the bug and demonstrates the importance of adhering to standards in JavaScript engines.

Actual Output

When we run the testcase, here’s what we actually see:

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

Expected Output

But here’s what we should be seeing, according to the ECMAScript spec:

true
Error: test
Error: test

Okay, let's break down the expected output and why it's so crucial. As we've discussed, the ECMAScript 5.1 specification dictates that the Error.prototype.toString method should return a string in the format ": ". In our specific test case, we're creating an error with the message "test", so the expected output is simply "Error: test". No more, no less.

The first line of the expected output, true, is the result of our direct comparison. We're checking if the trimmed output of toString() is exactly equal to "Error: test". If MuJS were behaving correctly, this comparison would return true, indicating that the output matches our expectation. The second and third lines are straightforward – they simply print the expected error message, "Error: test". This reinforces what we should be seeing if everything is working as it should.

The significance of this expected output lies in its adherence to the standard. When a JavaScript engine follows the ECMAScript specification, it ensures consistency and predictability. Developers can rely on certain behaviors, knowing that their code will work the same way across different environments. Deviations from the standard, like the one we've observed in MuJS, can lead to unexpected results and compatibility issues. Imagine you're writing a library that formats error messages, and you expect toString() to return a specific string. If an engine adds extra information, your library might break.

So, the expected output isn't just a matter of aesthetics – it's a matter of correctness. It's about ensuring that JavaScript engines behave in a consistent and predictable manner, allowing developers to build reliable and portable code. By highlighting the discrepancy between the actual and expected output, we're underscoring the importance of standards compliance in JavaScript development.

Specification Reference

To be precise, ECMAScript 5.1, Section 15.11.4.4 states that Error.prototype.toString should return a string in the form:

"<error name>: <error message>"

You can find the details here: https://262.ecma-international.org/5.1/#sec-15.11.4.4

There’s no mention of tacking on stack trace info in the toString() output, which makes MuJS’s behavior a bit of a rebel move.

To really drive home the point, let's dive a bit deeper into the specification reference and why it matters so much. ECMAScript 5.1, Section 15.11.4.4 is the holy grail when it comes to understanding how Error.prototype.toString should behave. It clearly states that the method should return a string in the format ": ". This is a concise, standardized way to represent an error, providing just the essential information: what kind of error it is and a brief description.

The beauty of this specification is its simplicity and clarity. By defining a specific format, it allows developers to rely on a consistent output across different JavaScript engines. This is crucial for building robust applications. Imagine you're writing a logging system that parses error messages. If toString() returns different formats in different engines, your system might fail to correctly interpret the errors. This is where the specification becomes essential – it ensures that everyone is speaking the same language.

Now, the fact that the specification makes no mention of including stack trace information in the toString() output is a deliberate choice. Stack traces can be incredibly useful for debugging, but they're often verbose and contain sensitive information, like file paths and line numbers. Including them by default in the toString() output would clutter error messages and potentially expose internal details that you'd rather keep hidden. This is why the specification keeps it simple and focused on the error name and message.

MuJS's deviation from this specification, by automatically appending the stack trace, is therefore a significant issue. It not only violates the standard but also introduces potential problems for developers who are relying on the specified behavior. This is why it's so important to reference the specification when discussing bugs and inconsistencies in JavaScript engines. It provides a clear and authoritative basis for understanding what's right and what's wrong. In this case, the specification clearly shows that MuJS's behavior is non-compliant, highlighting the need for a fix to bring it in line with the standard.

So, there you have it! A deep dive into a MuJS bug that's a bit of a stickler for ECMAScript 5.1 standards. Stay tuned for more tech adventures, guys!