Javascript Window.onerror Script Error
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each other. Join them; it only takes a minute: Sign up Cryptic “Script Error.” reported in Javascript in Chrome and Firefox up vote 144 down vote favorite 52 I have a script that detects Javascript errors on my website and sends them to my backend for reporting. It reports the first error encountered, the supposed line number, script onerror and the time. EDIT to include doctype: ... Because of this script, I'm acutely aware of any javascript errors that are happening on my site. One of by biggest offenders is "Script Error." on line 0. in Chrome 10+, and Firefox 3+. This error doesn't exist (or may be called something else?) in
almost a year ago. If you look at any javascript error report, you will see a cryptic error polluting the reports under the name "Script error." without any information about the script crossorigin error. This happens in Firefox, Safari, and Chrome when an exception violates the
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browser's same-origin policy - i.e. when the error occurs in a script that's hosted on a domain other than the domain
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of the current page. This tech post details how you can fix this error and decrypt the error message. Table of Contents Background Why does the browser say "Script error." instead of something meaningful http://stackoverflow.com/questions/5913978/cryptic-script-error-reported-in-javascript-in-chrome-and-firefox ? How do I fix the "Script error." ? Server Client Example Setup HTML CDN JS "Script error." demo without CORS Actual JS Error with CORS enabled Related articles on web Background The "Script error." happens in Firefox, Safari, and Chrome when an exception violates the browser's same-origin policy - i.e., when the error occurs in a script that's hosted on a domain other than the domain of the http://ravikiranj.net/posts/2014/code/how-fix-cryptic-script-error-javascript/ current page. This is a very common scenario when the javascript served on a webpage is hosted on a CDN (Content Delivery Network) such as Akamai where the domain of the javascript file is different from the webpage that is including the JS and running it. This behavior is intentional. It prevent scripts from leaking information to external domains. For an example, imagine that you accidentally visited a site called evilsite.com that serves a script pointing to your bank's home page such as "mybank.com/index.html". 1 Please note that the script tag is pointing to an html file rather than JavaScript. This will result in a script error, but the error might be interesting because it might tell us if you are logged in or not. Imagine that if you're logged in, the error spitted out might be something like "Welcome Ravi.. is undefined", whereas if you're not logged in, it might be "Please Login ... is undefined" or something similar. If evilsite.com does this for the top 20 or so bank institutions, they'd have a pretty good idea of which banking sites you visit, and enables it to target you with a superior phishing page. This is one such example as t
to log client-side errors and report them to your servers. It’s also one of the major mechanisms by which https://blog.sentry.io/2016/01/04/client-javascript-reporting-window-onerror.html Sentry’s client JavaScript integration (raven-js) works. You listen to the onerror event by assigning a function to window.onerror: window.onerror = function (msghttps://raygun.com/blog/2015/05/fixing-script-errors/ class="p">, url, lineNo, columnNo, error) { // ... handle error ... return false; } script error When an error is thrown, the following arguments are passed to the function: msg – The message associated with the error, e.g. “Uncaught ReferenceError: foo is not defined” url – The URL of the script or document associated with the error, e.g. “/dist/app.js” lineNo – The line number (if available) columnNo – The column number (if javascript window.onerror script available) error – The Error object associated with this error (if available) The first four arguments tell you in which script, line, and column the error occurred. The final argument, Error object, is perhaps the most valuable. Let’s learn why. The Error object and error.stack At first glance the Error object isn’t very special. It contains 3 standardized properties: message, fileName, and lineNumber. Redundant values that already provided to you via window.onerror. The valuable part is a non-standard property: Error.prototype.stack. This stack property tells you at what source location each frame of the program was when the error occurred. The stack trace can be a critical part of debugging an error. And despite being non-standard, this property is available in every modern browser. Here’s an example of the Error object’s stack property in Chrome 46: "Error: foobar\n at new bar (
from cross-domain JS errors Callum GavinMay 28, 2015Javascript, Raygun Labs, Tech Stuff, Web DevelopmentLeave a Comment If you're working on a website and have Raygun hooked into its client-side JavaScript, one of the first things you probably noticed was the rather unhelpful "Script Error" appearing in your dashboard. You may have also seen this appear in the browser console while developing, and noticed that Script Errors are thrown when errors from scripts loaded from a different domain than the origin are caught by the global window.onerror handler. Browsers may behave in this way as a security feature to protect against potentially malicious scripts injected or hosted by other sites from reading user data such as cookies (quite rightly, I'm sure you'll agree). This is termed the Same-Origin Policy, and means that scripts only have full access rights if they are loaded from the same origin domain as the original document (when the script passes the CORS validation). The Same-Origin Policy CORS and the Same-Origin Policy do however present a problem with regards to the architecture of modern web sites and applications. Due to the nature of HTTP 1.1, frequently key resources including JavaScripts are hosted on non-origin (also called ‘third-party') domains - in particular CDNs, as using the massive resources of public clouds keeps both costs and response times low. The main problem is that if your web application code is defined and loaded in a script hosted on a different domain to the one in the address bar, errors that hit window.onerror won't have any stack trace or message context for you to debug. This is not a problem when developing locally, but becomes a critical issue when trying to figure out why a site is breaking on a user's machine. This is most obvious when Raygun4JS reports these errors, and the error groups lack any indication as to what happened. The spec and implementation for controlling the Same-Origin Policy is documented nicely here. In particular the Browser Compatibility matrix is a valuable read; you'll notice proper implementations are only available in recent versions of the evergreen browsers, come with caveats in the bug tickets, or are plain not supported. There are two key pieces of metadata needed to allow a cross-domain script to report errors correctly in a modern browser. As the documentation lists above, the first is the presence of the ‘crossorigin' attribute on the appropriate script tag. By example:
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