3

In the following code, I intentionally throw an error, but in Chrome (used simply for testing purposes) it does not roll up to the catch. How can I roll up the error into the parent's scope? 

try {
  setTimeout(function() {
    console.log("Throwing Error...");
    throw({message:"Ouch!"});
  }, 500);
} catch(e) {
  console.log(e.message);
}

Chrome replies with:

Uncaught #<Object>
  (anonymous function)

Here is the full example I'm working with; when I require "bob" it (intentionally) times out. I want to catch the requirejs error so I could use my application's error system, which is more robust, to notify the learner.

(function() {
try {
  var scriptVersion = "1.0.0.1"
  window.onload = function() {
    var script = document.createElement("script");
    script.type = "text/javascript";
    script.src = "//content.com/pkg/" + scriptVersion + "/require-jquery.js";
    script.async = false;
    script.done = false;
    // OnReadyStateChange for older IE browsers
    script.onload = script.onreadystatechange = function() {
      if(!(this.done) && (!this.readyState || this.readyState == "loaded" || this.readyState == "complete")) {
        this.done = true;
        require.config({
          baseUrl: "//content.com/pkg/" + scriptVersion
        });
        require(["bob"]);
      }
    }
    document.getElementsByTagName("head")[0].appendChild(script);
  }
} catch(e) {
  console.log(e);
}
})();
scader
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  • I suggest you use try-catch and setTimeout in your title/description. That way when other people are searching for something similar they may find your question and benefit. – pilavdzice Apr 26 '12 at 16:13

4 Answers4

6

See the edit below for how to solve the actual problem with requireJS.

The problem is that the setTimeout() function runs in the parent's scope and completes without error. It schedules (with the system) a future callback event, but when that callback occurs in the future, the parent's scope of execution has finished and the callback is initiated from the system at the top level much like a new system event (e.g. a click event handler).

While the parent closure still exists because the anonymous function inside the setTimeout() can still reference those variables, the actual execution of the parent scope is done, thus the scope of the try/catch is done.

The execution context of the setTimeout() anonymous function is top level (initiated by the system) so there is no parent context that you can put a try/catch in. You can put a try/catch within the anonymous function, but throwing from there will just go back to the system which is what called the setTimeout() callback.

To have your own code catch any exceptions that occur inside the setTimeout() callback, you will need to put a try/catch inside the callback.

  setTimeout(function() {
    try {
        console.log("Throwing Error...");
        throw({message:"Ouch!"});
    } catch(e) {
        console.log(e.message);
    }
  }, 500);

If you explained what the real problem is that you're trying to solve (rather than this manufactured test case), we may be able to offer some useful options.

Edit now that you've shown what problem you're really trying to solve. The require.js library initiates every error by calling the onError method. The default implementation of the onError method is what throws the exception. You can assign your own onError handler and handle the errors in a callback rather than with exceptions. This sounds like the right way to go.

From the requirejs source:

/**
 * Any errors that require explicitly generates will be passed to this
 * function. Intercept/override it if you want custom error handling.
 * @param {Error} err the error object.
 */
req.onError = function (err) {
    throw err;
};
jfriend00
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  • I wanted a "if anything goes wrong" catch. For example, I'm using requirejs, and if the script request times out it throws an error, but I watched to catch that error to better format the error for my application. – scader Apr 26 '12 at 16:20
  • I've updated the question to include the full script. Thanks for calling me out on the generic test case--I'll keep that in mind going forward. – scader Apr 26 '12 at 16:26
  • I don't think Javascript has the ability to do a "catch everything" try/catch. Events or asynchronous callbacks that come from the system (timer callbacks, ajax callbacks, etc...) are inherently not in a try/catch block unless you put each one into one. You could establish a convention in your own coding that EVERY system callback or event handler went through a shell function that did the try/catch before calling the actual callback, but even that won't protect you from libraries doing their own event handling or callback handling. – jfriend00 Apr 26 '12 at 16:27
  • @scader - I added more detail to my answer, including how to override the requireJS errors. – jfriend00 Apr 26 '12 at 16:56
1

Your throw happens some time after the catch block, when the browser calls the setTimeout callback.

(catch uses logical scoping, not lexical scoping)

SLaks
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1

The previous answerer explained it correctly.

Another way of thinking about it is that it is not working because setTimeout completes fine and does not throw and exception when it is initially run. It then executes later when you are no longer within the try-catch block.

It will work if you put the try catch inside the setTimeout function like this:

  setTimeout(function() {
     try {
         console.log("Throwing Error...");
         throw({message:"Ouch!"});
     } catch(e) {
       console.log(e.message);
     }
      }, 500);

Let me know if you still have questions.

pilavdzice
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1

Use wrapper function like this.

// wrapper function
var tryable = function(closure, catchCallback) {
    closure(function(callback) {
        return function() {
            try {
                callback();
            } catch(e) {
                catchCallback(e);
            }
        };
    });
};


function throwException() {
    throw new Error("Hi");
}
tryable(function(catchable) {
    setTimeout(catchable(throwException), 1000);
}, function(e) {
    console.log("Error:)", e);
});
wan2land
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