How to get errors stack trace in Chrome extension content script?

Question

There is a Google Chrome extension with content script that handles JS errors occured on all tabs pages. But the problem is that no one of usual methods of gett

Answer 1

As you mention, the error property of the event object is null when capturing the event in Content Script context, but it has the required info when captured in webpage context. So the solution is to capture the event in webpage context and use messaging to deliver it to the Content Script.

// This code will be injected to run in webpage context
function codeToInject() {
    window.addEventListener('error', function(e) {
        var error = {
            stack: e.error.stack
            // Add here any other properties you need, like e.filename, etc...
        };
        document.dispatchEvent(new CustomEvent('ReportError', {detail:error}));
    });
}

document.addEventListener('ReportError', function(e) {
    console.log('CONTENT SCRIPT', e.detail.stack);
});

//Inject code
var script = document.createElement('script');
script.textContent = '(' + codeToInject + '())';
(document.head||document.documentElement).appendChild(script);
script.parentNode.removeChild(script);

The techniques used are described in:

• https://stackoverflow.com/a/9517879/1507998
• https://stackoverflow.com/a/9636008/1507998

Answer 2

The problem

The main problem is JS context isolation, i.e. the fact that "Content scripts execute in a special environment called an isolated world". This is a good thing, of course, because it avoids conflicts and enhances security, but yet a problem if you want to catch errors.

Each isolated world sees its own version of the (window) object. Assigning to the object affects your independent copy of the object...

...neither one can read the other's event handler. The event handlers are called in the order in which they were assigned.

---

A solution

On posiible solution (a.k.a. hack) consists of the following steps:

1. Inject some code into the web-page itself (and try to make is as non-obtrusive as possible to avoid conflicts).
2. In that code register an event listener for errors and store the error data in a DOM-node's dataset.
3. Finally, from within the content script, use a MutationObserver to watch for changes in this DOM-node's attributes.
4. Once a change in the specified data-<...> attribute is detected, grab its new value and there you have your error info right in your content script :)

---

The source code

Below is the source code of a sample extension that does exactly that.

manifest.json:

{
    "manifest_version": 2,

    "name":    "Test Extension",
    "version": "0.0",

    "content_scripts": [{
        "matches":    ["*://*/*"],
        "js":         ["content.js"],
        "run_at":     "document_start",
        "all_frames": true
    }],
}

content.js:

/* This <script> element will function as an "error-proxy" 
 * for the content-script */
var errorProxy = document.createElement('script');
errorProxy.id = 'myErrorProxyScriptID';
errorProxy.dataset.lastError = '';

/* Make the content as non-obtrusive as possible */
errorProxy.textContent = [
    '',
    '(function() {',
    '    var script = document.querySelector("script#' + errorProxy.id + '");',
    '    window.addEventListener("error", function(evt) {',
    '        script.dataset.lastError = evt.error.stack;',
    '    }, true);',
    '})();',
    ''].join('\n');

/* Add the <script> element to the DOM */
document.documentElement.appendChild(errorProxy);

/* Create an observer for `errorProxy`'s attributes
 * (the `data-last-error` attribute is of interest) */
var errorObserver = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
        if ((mutation.type === 'attributes')
                && (mutation.attributeName === 'data-last-error')) {
            console.log('Content script detected new error:\n',
                        errorProxy.dataset.lastError);
        }
    });
});
errorObserver.observe(errorProxy, { attributes: true });