CODEWITHSUNDEEP
javascriptexceptionphantomjs
Jonathan.Brink
Jonathan.Brink

Reputation: 25443

PhantomJS error handling

I'm having a hard time understanding how PhantomJS handles errors.

I have a locally installed Apache server running (xampp), and when I manually visit "http://localhost/" I get the "It Works!" page.

As a test, I wrote a small file (called forceError.js) that purposely causes an unchecked exception:

var page = require('webpage').create(),
    url = 'http://localhost/';

page.onError = function(msg, trace) {
  console.log("page.onError");
  var msgStack = ['ERROR: ' + msg];
  if (trace && trace.length) {
    msgStack.push('TRACE:');
    trace.forEach(function(t) {
      msgStack.push(' -> ' + t.file + ': ' + t.line + (t.function ? ' (in function "' + t.function +'")' : ''));
    });
  }
  console.error(msgStack.join('\n'));
};

phantom.onError = function(msg, trace) {
  console.log("phantom.onError");
  var msgStack = ['PHANTOM ERROR: ' + msg];
  if (trace && trace.length) {
    msgStack.push('TRACE:');
    trace.forEach(function(t) {
      msgStack.push(' -> ' + (t.file || t.sourceURL) + ': ' + t.line + (t.function ? ' (in function ' + t.function +')' : ''));
    });
  }
  console.error(msgStack.join('\n'));
  phantom.exit(1);
};

page.open(url, function (status) {
    console.log("status: " + status);

    // an undefined function
    thisShouldForceAnError();
});

When I run this using:

phantomjs.exe forceError.js

First I get "status: success" and then the process just hangs. I don't see either page.onError or phantom.onError being invoked.

Is there some property or something I need to turn on to get general error handling?

I'm on Windows 7, PhantomJS version 2.0.0, and running this in my "git bash" shell.

Upvotes: 12

Views: 13753

Answers (3)

maxi-code
maxi-code

Reputation: 321

Your app hangs because it looks there is a loop when you call console.error within phantom.onError. Check this: Phantomjs v2, consume huge memory+cpu, after throwing exception.

Upvotes: 1

Justin Pavatte
Justin Pavatte

Reputation: 1208

The accepted answer was very helpful, but I'll supplement it with a code sample.

page.open("https://www.google.com/", function (status) {
    try {
        if (status !== "success") {
            console.log("Unable to access network");
        } else {
            //do some stuff with the DOM
        }        
    } catch (ex) {
        var fullMessage = "\nJAVASCRIPT EXCEPTION";
        fullMessage += "\nMESSAGE: " + ex.toString();
        for (var p in ex) {
            fullMessage += "\n" + p.toUpperCase() + ": " + ex[p];
        }
        console.log(fullMessage);
    }
});

Output will look similar to this screen snip: Output

UPDATE: This appears to be a bug specifically with page.open. I noticed that phantom.onError was catching things from callbacks, just not directly inside page.open. Here is another possible workaround. This at least allows you to have all the error handling code in one spot instead of having a bunch of try/catches. NOTE: You still need page.onError for things inside page.evaluate.

page.open(genericSignInPageUrl, function (status) {
    setTimeout(function () { //hack for page.open not hooking into phantom.onError
        if (status !== "success") {
            throw new Error("Unable to access network");
        }
        //do some stuff
    }, 0);
});

When actually doing things with the page, I've started using this to make sure the element I am looking for is there. Since my code is in a callback, the onError methods work fine. Code for the waitFor method is here: https://github.com/ariya/phantomjs/blob/master/examples/waitfor.js

page.open(genericSignInPageUrl, function () {
    waitFor(function () {
        return page.evaluate(function () {
            return document.getElementById("idOfElementToIndicatePageLoaded");
        });
    }, function () {
        //do some stuff with the page
    });
});

Upvotes: 6

artur grzesiak
artur grzesiak

Reputation: 20348

Tested on MacOS and experienced exactly the same behaviour, which indeed is a bit unintuitive and most likely just a bug. The weird thing is that, if you call an undefined function from the top most scope phantom.onError is invoked correctly1.

As a workaround you can just wrap body of the open callback with a try/catch. Hopefully it will do the job.

Just to clarify: page.onError is invoked if an error occurred while executing code of the requested page - not the phantom script itself. I have been relying on page.onError for a while now and it seems to work pretty stable. (Although some errors only occur in phantomjs engine, but not in regular browsers.)

1 Actually: "phantom.onError" gets printed on the console infinitely as console.error is not supported by phantomjs.

Upvotes: 7

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