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Javascript: undefined Script error without any external scripts

We have a landing page for mobile customers. And we use window.onerror to catch and log JS errors in this way:
window.onerror = function(message, url, line, col, error) {
log(message + ', ' + error + ', ' + url + ', ' + line + ', ' + col)
}
As a result in our logs we see multiple undefined errors: 'Script error., null, , 0, 0'. I know that such errors may appear in case of CORS problems. When some external script doesn't have "crossorigin=anonymous" attribute. But the problem is that we don't have any external scripts at our landing page.
To double check this we even log the whole page:
log(document.documentElement.outerHTML)
Sometimes browsers insert something into our page. But mostly they don't 😵‍💫
So what else can cause such undefined error?
Maybe something from the following list?
SVG images such as logo
Google Fonts
<link href="https://fonts.googleapis.com/css2?family=Roboto:wght#300;400;500&family=Cairo&display=swap" rel="stylesheet">
Newest unsupported yet JS features?
UPD 02.07.2022

Detect a console.warn message

I have a need to find out when a warning (not error) message is sent to the browser's console.log . From research, I know that I can detect error messages with JavaScript code similar to this:
window.onerror = function(errorMsg, url, lineNumber){
// any action you want goes here
// errorMsg is the error message itself.
// url should be the file presenting the error, though i have
// found that it only presents to me the address of the site.
// lineNumber is the line number the error occoured on.
// here is an example of what you could do with it:
alert("Error in " + url + " at " + lineNumber + ":\n" + errorMsg);
}
But that only shows error messages. It will not show any warning messages.
Is there a similar way to detect a specific warning message in the console.log? Or, alternately, is there some JS code that will allow me to search the console.log for a specific string?
Note that I can see the warning message in the browser's debugging (console logging) window. But I want to 'intercept' the warning message and perform some action if that warning is found.
Added
The intent is to find out when a specific warning message occurs; the message being similar to "Loading failed for the <script> with source "https://www.example.com/js/somescript.js" .
But the question of how to intercept a warning message (not error; the code I included does that) that is output by the browser (not my code).
Added
So, here's what I need. Assume this code
console.warn("Here is a warning!");
What code would you write to see if that message was written to the console as a warning?

The warnings you are talking about are generated by the browser, not console.warn. This is why you cannot override it. Most likely, you need to manually add listeners for each event you need. For example, if you want to handle a script loading error use the onerror event:
<script src="https://www.example.com/js/somescript1.js" onerror="console.log('Cannot load script from ' + this.src)"></script>
<script src="https://www.example.com/js/somescript2.js" onerror="console.log('Cannot load script from ' + this.src)"></script>

How about overriding the console.log()?
let tmpConsoleLog = console.log;
console.log = function(msg){
// Intercept code goes here using msg variable
alert(msg);
// then perform the normal logging
tmpConsoleLog(msg);
}
console.log("Something");

It's very late, but here's an answer that actually directly answers your question.
var originalConsoleWarn = console.warn;
console.warn = function(message) {
doSomethingWithWarn(message);
originalConsoleWarn(message);
};
function doSomethingWithWarn(message){
console.log("DOING SOMETHING WITH: " + message);
}
console.warn("hi");

Generic Ajax Error Handling - interceptor in Angular JS

I want to implement the global error handling for Ajax calls in my angular Js Application . I think $httpprovider works for $http requests only. Kindly guide me in achieving this. How can i write the interceptor for capturing the failure of Ajax calls. My main goal is to capture the failure of Ajax call into google Analytics.

Write ajax call inside javascripts try-catch
http://www.w3schools.com/js/js_errors.asp

You can try to listen to window.onerror event but it will catch all js errors, so you need to filter them.
window.onerror = function(msg, url, line, col, error) {
// Note that col & error are new to the HTML 5 spec and may not be
// supported in every browser. It worked for me in Chrome.
var extra = !col ? '' : '\ncolumn: ' + col;
extra += !error ? '' : '\nerror: ' + error;
// You can view the information in an alert to see things working like this:
alert("Error: " + msg + "\nurl: " + url + "\nline: " + line + extra);
// TODO: Report this error via ajax so you can keep track
// of what pages have JS issues
var suppressErrorAlert = true;
// If you return true, then error alerts (like in older versions of
// Internet Explorer) will be suppressed.
return suppressErrorAlert;
};
You can read more about it in this question.

Phantomjs check if javascript exists and is working

I am quite new to Phantomjs and am starting to getting to know how to use it. However all of the semi advanced tutorials does not cover the thing i want to use Phantomjs for.
Now my question is how would i check if a Javascript is active on the site and if it is working correcly (i.e not throwing erros in the console).
I hope someone is able to point me in the right direction or know how to do this.

you can interact with the open page using the webpage.evaluate method:
var page = require('webpage').create();
page.open('http://m.bing.com', function(status) {
var title = page.evaluate(function(s) {
//what you do here is done in the context of the page
//this console.log will appear in the virtual page console
console.log("test")
//here they are just returning the page title (tag passed as argument)
return document.querySelector(s).innerText;
//you are not required to return anything
}, 'title');
console.log(title);
phantom.exit(); //closes phantom, free memory!
});
in order to see the virtual page console, you have to add a onConsoleMessage callback:
page.onConsoleMessage = function(msg, lineNum, sourceId) {
console.log('CONSOLE: ' + msg + ' (from line #' + lineNum + ' in "' + sourceId + '")');
};
EDIT: by default javascript is executed.

Logging value of a variable in MonkeyTalk IDE Javascript file

I'm using MonkeyTalk IDE Beta2 for testing iPad application. I exported the javascript from the MonkeyTalk IDE and got a new .js file. I am storing the Boolean value of a Verify command in a var and want to see what is its value, and accordingly do custom logic. I tried document.write, console.log and alert used in javascript but got an error that they are not defined. Please help me with this.
Also, is it possible to output the result of a test as XML (as in FoneMonkey) or as an Excel spreadsheet or something like that?
Thank you in advance.

Believe it or not*, but to date there is no way direct way to cause MonkeyTalk to log messages to the console. What you can do, however, is abuse a command like verifyNot which will result in a log message. In a MonkeyTalk .mt this would be done like:
View * VerifyNot Message
I created the following helper script called log.js for this purpose. Timestamps are automatically added by Eclipse, but not elsewhere so I have prepended the time.
load("libs/Executor.js");
function getTimeStamp() {
var now = new Date();
return now.getHours() + ":" + now.getMinutes() + ":" + now.getSeconds();
}
EXECUTOR.defineScript("Log", function(msg) {
this.app.view().verifyNot(getTimeStamp() + ": " + msg);
});
Finally, you don't need the executor boilerplate (only the verifyNot line), but we use that with scripts by Doba in order to be able to organize files in different directories (Doba.js renamed to Executor.js) -- another feature not available out of the box.
* It's almost like GorillaLogic doesn't want you to be able to resolve your own problems. ;)