Develop Reference

Stop future object properties from being edited

I'm a JS game dev who's been trying to combat tampermonkey scripts for a while now.
I came up with a solution for people hooking into WebSockets where I'd cause the WebSocket to throw an error new WebSocket(0); (0 throws an error due to it being a number)
let output;
try {
output = new target(...args);
} catch(e) {
let source = e.stack.substring(e.stack.indexOf("("), 1 + e.stack.indexOf(")"));
e.stack = e.stack.replace(source, "nothing to see here");
throw e;
}
this code made the error's stack have all the information I was looking for replaced!
I've been looking at Object.defineProperty, and I was wondering how I could stop an error's stack from being modified before I have access to that specific error. And if anyone has any other ways I could stop a script from being loaded or run, I'd love to hear them!

One thing you could do is Object.freeze the error before throwing it. This would prevent people from altering the object's contents.
So for example:
try {
new WebSocket(0);
} catch (wsErr) {
throw Object.freeze(wsErr);
}
The code catching your error and trying to alter it would fail to be able to alter it. This should work as it will cause the code that was altering the error to throw with the following:
Cannot assign to read only property 'stack' of object ''
The other thing you'll have to consider is that in your code where you're catching the error, you will not be able to alter its contents either. Typically with errors, that's not a huge deal though. Tampering with errors is one of the only reasons I can think of for modifying the error.

How to pass native ReferenceError object stack to console in a JavaScript try catch?

In JavaScript, when using a try catch, how can I get the native Chrome ReferenceError object to the console as it would normally be logged? I can get close by using the Error object's stack property, but it logs the errors differently:
try {
bet you can't run this you dumb computer;
}
catch(error) {
console.log(error.stack);
}
When running the above, I get the following in the console:
While this is slightly helpful, clicking on the line number takes me to the console.log(error.stack) line in the code, not the error.
Without the try catch, the error looks like this:
and clicking on the line number will take me exactly to where the original error occurred. So how can I get this native Chrome error object to pass to the catch and then log to the console so that I can navigate to the original error and line that was thrown as it would normally be without the try catch in place?

I believe it'll work with the error method from the console object:
try {
bet you can not run this you dumb computer;
}
catch(error) {
console.error(error);
}
Hope this helps!

JS - how to prevent script from stopping after thrown error

How can I prevent the script from stopping after an error has been thrown? Is there something like exception handling in JS?
Console text
Not allowed to load local resource: file:///C:/Users/Temp/image.png

Javascript does have exception handling. There are two possible types of error you can encounter:
1) Places in your application where you proactively guard against errors being thrown, for example, AJAX request. You can handle them like this:
try {
AJAX-code or other code susceptible to errors
} catch(error){
// Log error
}
2) Script errors or compile-time error, for example, undefined variables. In browsers, window.onerror is a global event handler which is called on script or compile errors. However, it's implementation is inconsistent across browsers. You can use it like this:
window.onerror = function(message, url, lineNo) {
// Code to handle the error
}
The main problem with onerror is that no stack trace is passed through which is not very helpful. However, Chromium has added column number and errorObj, so hopefully other browsers will implement the same in near future.

There surely is: try {} catch (exception) {}

Sure, wrap your code inside try/catch:
try
{
//Run some code here
}
catch(err)
{
//Handle errors here
}
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/try...catch

You can use a try/catch/finally block to handle errors:
try {
// Whatever you need
} catch(error) {
// Handle error if one occurs
} finally {
// Last resort
}
Note that you can have multiple catch blocks in-between your try and finally as needed.
Here's some more information.

try and catch javascript unexpected identifier

i'm tryin to identify the error i get in a javascrip function in my webpage, so i added
function guardarMisDatos() throws Exception {
try{
...
} catch (Exception e){
alert("error: ", e);
}
but when i open the page, the chrome web console gives me error at
function guardarMisDatos() throws Exception {
and the error type is "Uncaught syntaxerror: unexpected identifier"
where is the error? is it a correct way to check way the function is not fired on the first click?

It is JavaScript not Java. Lose the throws Exception!

Your code looks a lot like Java, not javaScript. The syntax for try/catch in javaScript goes like this:
try {
// do stuff
} catch (e) {
// something bad happened
}
Notice there is no throws and no type on e (since javascript is loosely typed)

Remove "throws Exception" and the catch reference to "Exception". To know what kind of exception it is, look at the e.name property, it'll be one of six things:
EvalError - An error in the eval() function has occurred.
RangeError - ut of range number value has occurred.
ReferenceError - An illegal reference has occurred.
SyntaxError - A syntax error within code inside the eval() function has occurred. All other syntax errors are not caught by try/catch/finally, and will trigger the default browser error message associated with the error. To catch actual syntax errors, you may use the onerror event.
TypeError - An error in the expected variable type has occurred.
URIError - An error when encoding or decoding the URI has occurred (ie: when calling encodeURI()).
These aren't constants, they're the actual string, as in if (e.name.toString()=="TypeError") There are a lot of other good things on the error object too, read more at http://www.javascriptkit.com/javatutors/trycatch2.shtml

Remove the throws Exception from your function definition. You do not need this in JavaScript. Besides that, why would your function ever throw an exception - you already catch it!

How to create, design and throw built-in Error objects

UPDATE
[Rewriting question to focus on the problem I am trying to understand.]
Is there a means in JavaScript to throw Exceptions that notify the line number where the problem occurs? Similar to C#'s debugger, if an error is thrown on line 50 then I will be taken to line 50.
For example, according to MDN EvalError represents an error with eval(). So, let's say I have a function that uses eval(). I want to use a specific error that is representative of the problem at hand, EvalError:
//As written here the error implies there is a problem on this line. See Firebug console window
var evalErra = new EvalError('required element missing from evaluation');
var stringFunc = "a=2;y=3;document.write(x*y);";
EvalString(stringFunc);
function EvalString(stringObject) {
//Some arbitrary check, for arguments sake let's say checking for 'x' makes this eval() valid.
if(stringObject.indexOf('x') !== -1) {
throw evalErra;
//throw 'required element missing from evaluation';//This way offers no line number
}
eval(stringFunc);//The problem really lies in the context of this function.
}
If I'm going about this all wrong, then please tell me how I should approach these kinds of issues.

When you throw an error, execution of the current code will stop and JS will work its way back up the execution tree until it finds a catch () which handles the particular type of error being thrown, or gets all the way up to the top of the tree, causing an "unhandled exception" error: You threw an error, and nothing caught it, and now someone's window got broken.
try {
if (true) {
throw 'yup'
}
} catch (e) { // catches all errors
... handle the error
}

When doing error handling you want to do the following
throw new Error("message");
Then if you ever handle this error look at err.stack (firefox/opera/chrome) or err.line (Safari) or err.IE_Y_U_NO_SHOW_ME_ERROR_LINE_NUMBER (IE) to find the line number.
If you want you can subclass Error.