Do not hard code err instanceof Error checks

[brian-mann]

Specifically this line: https://github.com/mochajs/mocha/blob/master/lib/runner.js#L205

Long story short, err may be a valid Error instance but in situations such as iframes each window will have its own Error object, and this will therefore always always fail.

I would suggest taking after what lodash does in its isError function.

It just ensures its "object like", has a string for message property, and is [object Error]

//lodash isError implementation
function isError(value) {
  return (isObjectLike(value) && typeof value.message == 'string' && objToString.call(value) == errorTag) || false;
}

[danielstjules]

Good idea! We wouldn't be able to use lodash's implementation out of the box because of how pending tests are defined and propagated up to the runner/runnables. https://github.com/mochajs/mocha/blob/master/lib/pending.js If I recall correctly, there's cases where the instance of Pending will have an undefined message. That logic will need to be tweaked a bit.

[brian-mann]

Just as another reference, in other parts of Mocha, specifically here: https://github.com/mochajs/mocha/blob/master/lib/runnable.js#L234

Mocha already toStrings() the object as a fallback to the instanceof checks. That's all that's needed IMO.

[outdooricon]

See #1758 for related PR.

[outdooricon]

@brian-mann, does that pr solve what you're looking for? The object toString method that you mentioned doesn't work because it can still return something other than [object Object], as it does for AssertionError.

@danielstjules I don't think checking for error message is a problem here. Pending doesn't appear to be an instance of Error, so it should continue to work as implemented if a Pending object does indeed reach this.

[outdooricon]

@boneskull I think we can remove the pr-please label at this point.

[danielstjules]

so it should continue to work as implemented if a Pending object does indeed reach this.

Good point :) Not sure what I was thinking

[danielstjules]

Closed by #1758