Prototope, Xcode, and Protocaster all crash when you forget “new” in your script

[jbrennan]

This one was fun. I had the following line in my script,

var layer = TextLayer()

And I couldn’t figure out what was wrong with it. Turns out I forgot new. Anyway, when Prototope hit that line, it crashed, and sometimes this caused Xcode to crash (betas!). Obviously the script is wrong but for some reason Prototope didn’t catch the exception...not quite sure why yet.

[saniul]

Investigated a bit – looks like the constructor functions that get created for the JSExported classes are generated in such a weird way that they crash JavaScriptCore when called straight, without the new keyword. If you look at the JSExport.h documentation, it actually states that those constructors aren’t
callable:

This is an actual crash (not a thrown exception) in JavaScriptCore, couldn’t figure out a workaround :( We can either live with it or stop using the constructor and expose a create method on all bridged types. I think just making sure we always write new is a better option, despite the crashiness.

[andymatuschak]

Yikes. I've got no bright ideas here, other than e.g. preprocessing the JS to look for this kind of mistake, which is probably overboard. @jbrennan?

[kevinbarabash]

It's probably not a bad idea to do some sort of linting before running. If we're looking to support JavaScript as a language it shouldn't look too different from regular JavaScript that you see in the browser/node. For instance, I was surprised that there was no setTimeout, but instead there's a afterDuration function. Hopefully we can support new as well as more of the JavaScript standard library in the future.

[NachoSoto]

How do we define the constructors? I don't know if this is possible because I haven't really looked at the bridging code, but you can detect in a constructor if it's been invoked correctly by checking if (this instanceof Constructor)

[saniul]

Constructor objects are defined by JavaScriptCore automatically based on the exposed init in the XXXJSBridge protocol (the one that extends JSExport).

Sent from my iPhone

On 6 Mar 2015, at 09:23, NachoSoto [email protected] wrote:

How do we define the constructors? I don't know if this is possible because I haven't really looked at the bridging code, but you can detect in a constructor if it's been invoked correctly by checking if (this instanceof Constructor)

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[sophiebits]

Just seeing this in passing via email so I haven't looked at how the code is set up, but if you can run something along these lines before the user code, perhaps it will help:

function wrapNativeClass(OriginalConstructor) {
  function Constructor() {
    if (!(this instanceof Constructor)) {
      throw new Error("Classes should be instantiated with `new`");
    }
    OriginalConstructor.apply(this, arguments);
  }
  Constructor.prototype = OriginalConstructor.prototype;
  return Constructor;
}

TextLayer = wrapNativeClass(TextLayer);

Constructor.prototype.constructor will still be OriginalConstructor which may be a problem? If so, you can probably copy things over from the prototype manually. At the very least, you probably want to make sure that new TextLayer() instanceof TextLayer is still true.