JavaScript OSA Handler Invocation

When Apple added support to macOS to support JavaScript for Automation, they did so in a way that more or less allows folks who invoke AppleScripts to invoke JavaScript for Automation scripts as if they were exactly the same. An abstraction in Apple’s Open Script Architecture (OSA) makes it easy for script-running tools to theoretically handle any number of scripting languages without concern for the implementation details of those languages.

This mostly works, but I recently received a bug report that shed light on a problem with Apple’s implementation of JavaScript with respect to invoking a specific named handler. The OSA provides a mechanism for loading and running a specific handler, or function, within a script. My app FastScripts takes advantage of this to query a script about whether it would prefer to be invoked in another process or not. Unfortunately, when it comes to JavaScript, Apple’s implementation runs the whole script in addition to running just the specific, named handler.

If you’ve got Xcode handy, you can use this simple playground content to observe the problem:

import OSAKit

if let javaScriptLanguage = OSALanguage(forName: "JavaScript") {
   let scriptSource = "Application('Safari').activate();" +
         "function boo() { ObjC.import('Cocoa'); $.NSBeep(); }"
   let myScript = OSAScript(source: scriptSource, language: javaScriptLanguage)

  // Only the behavior of boo should be observed
  myScript.executeHandler(withName: "boo", arguments: [], error: nil)

// Give time for the beep to sound Date(timeIntervalSinceNow:5))

The named function “boo()” only invokes NSBeep, so when this playground is run, all that should happen is a beep should be emitted from the Mac. Instead, when it runs Safari becomes the active application. This is because in addition to running the “boo()” handler, it also runs the whole script at the top level.

A workaround to the bug is to wrap the top level functionality of a script in a “run()” handler, so where the scriptSource is declared above, instead use:

   let scriptSource = "function run() { Application('Safari').activate(); }" +
         "function boo() { ObjC.import('Cocoa'); $.NSBeep(); }"

I hope this helps the one other person on earth who cares about invoking JavaScript for Automation methods indvidually! (Radar #33962901, though I’m not holding my breath on this one!)