Eval and vm module with custom errors

[Samuel Miller]

• Version: v6.2.2
• Platform: Ubuntu 16.04
• Subsystem: vm

I was playing around with dynamically running code and found a difference between standard JS eval and the vm module with custom error code. Here is a code snippet that can be run

var vm = require("vm");

var customError = `
function MyError(message) {
  this.name = this.constructor.name;
  this.message = message;
};
MyError.prototype = Error.prototype;
`;
var errorThrower = "throw new MyError('MyError');";


console.log("Eval - MyError");
try {
  var result = eval(customError + "\n" + errorThrower);
} catch (e) {
  console.log(e.message + "\n");
}


console.log("Good VM - Error");
try {
  var result = vm.runInThisContext("throw new Error('Not MyError');", "repl");
} catch (e) {
  console.log(e.message + "\n");
}

console.log("Bad VM - MyError");
try {
  var result = vm.runInThisContext(customError + "\n" + errorThrower, "repl");
} catch (e) {
  console.log(e.message + "\n");
}

console.log("Bad VM - MyError Again");
try {
  var result = vm.runInThisContext(customError + "\n" + errorThrower, "repl");
} catch (e) {
  console.log(e.message + "\n");
}

and the output

kamn@kamn:~/Dev/VmEvalTest$ node index.js 
Eval - MyError
MyError

Good VM - Error
Not MyError

Bad VM - MyError

repl:7
throw new MyError('MyError');
^
MyError

Bad VM - MyError Again
MyError

It seems like custom errors in vm output to stdin(?) the first time they happen. They are even caught afterwards. This is not true for a standard Error in vm and both Errors work in JS Eval. I am not sure this is a expected behavior but it does not seem like it. Any feedback would be appreciated.

Thanks!

[mscdex]

The reason is that the vm.runInThisContext()'s displayErrors option is true by default. Explicitly setting it to false will get you:

Eval - MyError
MyError

Good VM - Error
Not MyError

Bad VM - MyError
MyError

Bad VM - MyError Again
MyError

The name displayErrors is a bit misleading, but what it actually does is decorate the exception message if true, which is what you're seeing (the filename is added and it points to the source line). When it's false, the error is not decorated, so you just get the exception message itself. Originally when this option was added it did print the exception to stderr IIRC (hence the 'display' part of the name), but that behavior changed some time ago.

[Anna Henningsen]

@mscdex Well, there’s also a real bug here; none of the vm methods should print anything to stderr under normal circumstances.

The reason it does here is this line, and it shouldn’t be too hard to fix this.

[Anna Henningsen]

PR: #7398

[Samuel Miller]

@mscdex: Thank you for the explanation and work around.
@addaleax: Thank you for the quick fix.

[ADoyle]

There is another question.

if (env->printed_error())
return;
env->set_printed_error(true);

uv_tty_reset_mode();
PrintErrorString("\n%s", arrow);

Why print the error only once? The exception may occur repeatedly. If the output to stderr is allowed, I think it should be printed always rather than once.

[Anna Henningsen]

Hm, maybe that made sense back in nodejs/node-v0.x-archive#7049, but I don’t see any real point right now either… @indutny you don’t happen to remember, do you?