Safari: Error#column is read only

[ariya]

This is related to #1284. See also: https://bugs.webkit.org/show_bug.cgi?id=146047

Some unit tests (that check for invalid syntax) fail on Safari. This is because column property of an Error object is read only.

Run this on Safari (or check this snippet: http://jsbin.com/pexocaruce/edit?js,console):

function createError(msg) {
  var error = new Error(msg);
  error.column = 42;
  console.log('error.column', error.column);
 return error;
}

function throwError(msg) {
  throw createError(msg);
}

try {
  throwError('Hello Universe');
} catch (e) {
  console.log('e.column', e.column);
}

It should say that column is 25, despite the fact that its value is set to 42. Chrome and Firefox behave as expected, showing 42.

[ariya]

With this situation, any Error object that Esprima is throwing is going to have a wrong value for column on Safari >= 8. While we need a long term solution for this, for now we need to workaround it by having the test suite excludes column when comparing expected vs actual, if this situation is detected.

[gibson042]

Why not use the Object.create + Object.defineProperty solution from #1284 (comment) in strict-mode Safari so the property is correct there too?

[ariya]

@gibson042 Because that doesn't change the value of column property. In other word, the result for the above example is still 25, not the expected 42.

[michaelficarra]

@ariya It should because you're adding an own-property to the freshly created object that has the Error object as its prototype. The problem with leaving it this way for now is that, in strict mode, the write throws an error instead of just silently failing. I think the best thing to do for now would be either use Object.defineProperty on an object that has the error as its prototype as @gibson042 suggests or testing whether it is writable using Object.getOwnPropertyDescriptor. If we can't depend on ES5+ stdlib functions, we'll just have to wrap the assignment with a try-catch.

[gibson042]

@ariya http://jsbin.com/zihasukaku/edit?js,console

function createError(msg) {
  var error = Object.create(new Error(msg));
  Object.defineProperty(error, 'column', { value: 42 });
  return error;
}

[ariya]

@gibson042 With Safari 8, your JSBin snippet produces:

"e.column"
33

With Chrome and Firefox, the output is

"e.column"
"Corinthian"

[gibson042]

Ah, I didn't see the problem introduced by throwing the error in Safari 8 (Safari 9 was fine). But it too can be resolved: http://jsbin.com/tecusujowu/edit?js,console

And note that even though the above works on other browsers, it is only needed for Safari.

[ariya]

The problem with leaving it this way for now is that, in strict mode, the write throws an error instead of just silently failing.

That's the difference between Safari 8 and 9:

• Safari 8: no TypeError is thrown, but column has the wrong value
• Safari 9: "TypeError: Attempted to assign to readonly property."