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override Nodejs console to add the line the function were called

is there a way to send the line and file a console function has been called?
i got an app with many console log and console error in it. thats a good thing for me, the problem is i need to know from where they came from.
i tried override console.error and add console.trace inside but that just created a recursive call to console error(i guess they both triggered by process strerr)
here is what i have tried
const tempConsoleError = console.error.bind(console);
console.error = function (err) {
console.log('i am in error')
tempConsoleError(err);
console.trace()
};
console.error('error string ')

ended up writing a packages to do it. is uses the "stack-trace" and node global
console-from

Javascript throw vs. return error object vs. callback

I'm writing an assembler and simulator for a toy assembly language that I have my CS students use in class. I'm writing it in javascript with the idea that I could then build a simple UI in the browser which would show students how each instruction changes the state of the machine and such.
One question that I'm grappling with is the best way to return error information from the assembler when invalid assembly code is passed. The assembler has an extremely simple API at the moment:
var assembler = ... // Get the assembler object
var valid_source = "0 mov r1 r2\n1 halt";
var valid_binary = assembler.assemble(valid_source); // String containing 0's and 1's
var invalid_source = "foo bar baz!";
var invalid_binary = assembler.assemble(invalid_source); // What should happen here?
I have a few thoughts about how this might work:
Construct and throw a new javascript Error object. This seems like overkill (and ultimately maybe not even helpful since the user wouldn't care about the javascript stacktrace, etc).
Return a string or object containing error information. Then the user of the assembler gets to make the choice about what to do with errors (if anything).
Change the assembler API to use a callback instead:
assembler.assemble(source, function(binary, error) {
if (error) {
// Handle the error
}
// Otherwise, do stuff with the binary
});
Something else entirely?
Any ideas, thoughts, or feedback would be much appreciated.

I think your three options would work fine. Now from my perspective:
I would keep away from the third option because it gives the feeling it is an async function when it is not.
I would go for option 1 or 2. The first one is a little overkill but I think it is the most realistic approach to what compilers do. Exit with no zero code. But then you would need to add a try/catch block to handle the error.
So the next option is to return an error object. Seems the best option for me.
I recommend you to return an Error object. It is as simple as:
return new Error('Parsing error');
// Or with an error name
var error = new Error('Parsing error');
error.name = 'PARSING_ERROR';
return error;
One advantage to use the error object is that it gives you the stack trace and other handy stuff. More info here.
Also, to check if there was any error just need to check the variable type:
if (typeof valid_binary === 'string') { /* no error */ }
// Or
if (typeof valid_binary === 'object') { /* error */ }
Good luck!

How can I override/extend ReferenceError in Chrome's JavaScript?

To make debugging easier, I'm capturing all of the console logs in Chrome so that users who submit a feedback entry will also submit all of the logs to our server. When someone encounters a problem in production, I can first and foremost get them back to work so that I can then sit down and more thoroughly go through all of the logs to determine the root cause of whatever issue the user encountered in production.
The technique I use to capture the logs involves overriding console.log so that all text entered in the first argument gets stored in an array while simultaneously invoking the legacy function so that I can still see the logs in the console too.
The problem is when there's the occasional uncaught exception. These aren't included in the uploaded logs, so it's not always clear what caused the problem. So I tried overriding ReferenceError by writing a JavaScript function that takes a function as an argument, then returns a new function that does stuff with it, like storing data in a variable, and then invoking the legacy function as the last step:
function overrideException(legacyFn) {
/** arguments for original fn **/
return function() {
var args = [];
args[0] = arguments[0];
// pass in as arguments to original function and store result to
// prove we overrode the ReferenceError
output = ">> " + legacyFn.apply(this, args).stack;
return legacyFn.apply(this, arguments);
}
}
To test the overrideException function, I ran the following code on the console:
ReferenceError = overrideException(ReferenceError);
Afterwards, I tested the returned function, the new ReferenceError, by manually throwing a ReferenceError:
throw new ReferenceError("YES!! IT WORKS! HAHAHA!");
The resulting output on the console is:
ReferenceError: YES!! IT WORKS! HAHAHA!
And checking the global variable output from the overrideException function shows that it did indeed run:
output
">> ReferenceError: YES!! IT WORKS! HAHAHA!
at ReferenceError (<anonymous>)
at new <anonymous> (<anonymous>:18:35)
at <anonymous>:2:7
at Object.InjectedScript._evaluateOn (<anonymous>:562:39)
at Object.InjectedScript._evaluateAndWrap (<anonymous>:521:52)
at Object.InjectedScript.evaluate (<anonymous>:440:21)"
Now, here's where things start to fall apart. In our code, we're not going to know when an uncaught exception occurs, so I tested it by attempting to run a function that doesn't exist:
ttt();
Which results in:
ReferenceError: ttt is not defined
However, unlike the case where we explicitly throw an error, in this case, the function doesn't fire, and we're left with only the legacy functionality. The contents of the variable output is the same as in the first test.
So the question seems to be this: How do we override the ReferenceError functionality that the JavaScript engine uses to throw errors so that it's the same one we use when we throw a ReferenceError?
Keep in mind that my problem is limited only to Chrome at this time; I'm building a Chrome Packaged app.

I have done quite a bit of research for the same reason: I wanted to log errors and report them.
"Overriding" a native type (whether ReferenceError, String, or Array) is not possible.
Chrome binds these before any Javascript is run, so redefining window.ReferenceError has no effect.
You can extend ReferenceError with something like ReferenceError.prototype.extension = function() { return 0; }, or even override toString (for consistency, try it on the page, not the Dev Tools).
That doesn't help you much.
But not to worry....
(1) Use window.onerror to get file name, 1-indexed line number, and 0-indexed position of uncaught errors, as well as the error itself.
var errorData = [];
onerror = function(message, file, line, position, error) {
errorData.push({message:message, file:file, line:line, position:position, error:error});
};
See the fiddle for an example. Since the OP was Chrome-specific, this has only been tested to work in Chrome.
(2) Because of improvements to (1), this is no longer necessary, but I leave this second technique here for completeness, and since onerror is not guaranteed to work for all errors on all browsers. You will also sometimes see the following:
var errors = [];
function protectedFunction(f) {
return function() {
try {
f.apply(this, arguments);
} catch(e) {
errors.push(e);
throw e;
}
};
}
setTimeout = protectedFunction(setTimeout);
setInterval = protectedFunction(setInterval);
etc...
FYI, all this is very similar to what has been done in the Google Closure Compiler library, in goog.debug, created during Gmail development with the intent of doing exactly this. Of particular interest is goog.debug.ErrorHandler and goog.debug.ErrorReporter.

What is “assert” in JavaScript?

What does assert mean in JavaScript?
I’ve seen something like:
assert(function1() && function2() && function3(), "some text");
And would like to know what the method assert() does.

There is no standard assert in JavaScript itself. Perhaps you're using some library that provides one; for instance, if you're using Node.js, perhaps you're using the assertion module. (Browsers and other environments that offer a console implementing the Console API provide console.assert.)
The usual meaning of an assert function is to throw an error if the expression passed into the function is false; this is part of the general concept of assertion checking. Usually assertions (as they're called) are used only in "testing" or "debug" builds and stripped out of production code.
Suppose you had a function that was supposed to always accept a string. You'd want to know if someone called that function with something that wasn't a string (without having a type checking layer like TypeScript or Flow). So you might do:
assert(typeof argumentName === "string");
...where assert would throw an error if the condition were false.
A very simple version would look like this:
function assert(condition, message) {
if (!condition) {
throw message || "Assertion failed";
}
}
Better yet, make use of the Error object, which has the advantage of collecting a stack trace and such:
function assert(condition, message) {
if (!condition) {
throw new Error(message || "Assertion failed");
}
}

If using a modern browser or nodejs, you can use console.assert(expression, object).
For more information:
Chrome API Reference
Firefox Web Console
Firebug Console API
IE Console API
Opera Dragonfly
Nodejs Console API

The other answers are good: there isn't an assert function built into ECMAScript5 (e.g. JavaScript that works basically everywhere) but some browsers give it to you or have add-ons that provide that functionality. While it's probably best to use a well-established / popular / maintained library for this, for academic purposes a "poor man's assert" function might look something like this:
const assert = function(condition, message) {
if (!condition)
throw Error('Assert failed: ' + (message || ''));
};
assert(1 === 1); // Executes without problem
assert(false, 'Expected true');
// Yields 'Error: Assert failed: Expected true' in console

assert() is not a native javascript function. It is a custom function someone made. You will have to look for it on your page or in your files and post it for anybody to help determine what it's doing.

check this:http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-quick-and-easy-javascript-testing-with-assert/
it is for testing JavaScript. Amazingly, at barely five or six lines, this code provides a great level of power and control over your code, when testing.
The assert function accepts two parameters:
outcome: A boolean, which references whether your test passed or failed
description: A short description of your test.
The assert function then simply creates a list item, applies a class of either “pass” or “fail,” dependent upon whether your test returned true or false, and then appends the description to the list item. Finally, that block of coded is added to the page. It’s crazy simple, but works perfectly.

If the assertion is false, the message is displayed. Specifically, if the first argument is false, the second argument (the string message) will be be logged in the developer tools console. If the first argument is true, basically nothing happens. A simple example – I’m using Google Developer Tools:
var isTrue = true;
var isFalse = false;
console.assert(isTrue, 'Equals true so will NOT log to the console.');
console.assert(isFalse, 'Equals false so WILL log to the console.');

It probably came with a testing library that some of your code is using. Here's an example of one (chances are it's not the same library as your code is using, but it shows the general idea):
http://chaijs.com/guide/styles/#assert

Word or function "assert" is mostly used in testing parts of application.
Assert functions are a short way of instructing the program to check the condition (also called "assertion") and if the condition is not True, it will throw error.
So let's see how it would look like in "normal code"
if (typeof "string" === "array") {
throw Error('Error: "string" !== "array"');
}
With assert you can simply write:
assert(typeof "string" === "array")
In Javascript, there's no native assert function, so you have to use one from some library.
For simple introduction, you can check this article:
http://fredkschott.com/post/2014/05/nodejs-testing-essentials/
I hope it helps.

Assertion throws error message if first attribute is false, and the second attribute is the message to be thrown.
console.assert(condition,message);
There are many comments saying assertion does not exist in JavaScript but console.assert() is the assert function in JavaScript
The idea of assertion is to find why/where the bug occurs.
console.assert(document.getElementById("title"), "You have no element with ID 'title'");
console.assert(document.getElementById("image"), "You have no element with ID 'image'");
Here depending on the message you can find what the bug is.
These error messages will be displayed to console in red color as if we called console.error();
You can use assertions to test your functions eg:
console.assert(myAddFunction(5,8)===(5+8),"Failed on 5 and 8");
Note the condition can be anything like != < > etc
This is commonly used to test if the newly created function works as expected by providing some test cases and is not meant for production.
To see more functions in console execute console.log(console);

In addition to other options like console.assert or rolling your own, you can use invariant. It has a couple of unique features:
It supports formatted error messages (using a %s specifier).
In production environments (as determined by the Node.js or Webpack environment), the error message is optional, allowing for (slightly) smaller .js.

Java has a assert statement, the JVM disables assertion validation by default. They must be explicitly enabled using command line argument -enableassertions (or its shorthand -ea),
while JavaScript supports console.assert(), it's just a logging method and won't interrupt current procedure if assertion failed.
To bring things together and satisfy various needs, here is a tiny js assertion lib.
globalThis.assert = (()=> {
class AssertionError extends Error {
constructor(message) {
super(message);
this.name = 'AssertionError';
}
}
let config = {
async: true,
silent: false
};
function assert(condition, message = undefined) {
if (!condition) {
if (config.silent) {
//NOOP
} else if (config.async) {
console.assert(condition, message || 'assert');
} else {
throw new AssertionError(message || 'assertion failed');
}
}
}
assert.config = config;
return assert;
})();
/* global assert */
Object.assign(assert.config, {
// silent: true, // to disable assertion validation
async: false, // to validate assertion synchronously (will interrupt if assertion failed, like Java's)
});
let items = [
{id: 1},
{id: 2},
{id: 3}
];
function deleteItem(item) {
let index = items.findIndex((e)=> e.id === item.id);
assert(index > -1, `index should be >=0, the item(id=${item.id}) to be deleted doesn't exist, or was already deleted`);
items.splice(index, 1);
}
console.log('begin');
deleteItem({id: 1});
deleteItem({id: 1});
console.log('end');

Node.js has an assert function you can import:
const assert = require('assert')
It works as one would expect, in that assert(false) throws an error, and assert(false, message) throws an error with a message.
The other answers have already pointed out that JS itself has no native assert function, and that remains true as of this writing (April 2021).

Previous answers can be improved in terms of performances and compatibility.
Check once if the Error object exists, if not declare it :
if (typeof Error === "undefined") {
Error = function(message) {
this.message = message;
};
Error.prototype.message = "";
}
Then, each assertion will check the condition, and always throw an Error object
function assert(condition, message) {
if (!condition) throw new Error(message || "Assertion failed");
}
Keep in mind that the console will not display the real error line number, but the line of the assert function, which is not useful for debugging.

If you use webpack, you can just use the node.js assertion library. Although they claim that it's "not intended to be a general purpose assertion library", it seems to be more than OK for ad hoc assertions, and it seems no competitor exists in the Node space anyway (Chai is designed for unit testing).
const assert = require('assert');
...
assert(jqXHR.status == 201, "create response should be 201");
You need to use webpack or browserify to be able to use this, so obviously this is only useful if those are already in your workflow.

As mentioned by T.J., There is no assert in JavaScript.
However, there is a node module named assert, which is used mostly for testing. so, you might see code like:
const assert = require('assert');
assert(5 > 7);

assert() is the assert function in JavaScript. The main idea of assertion is to find why/where the bug occurs.

Chrome devtools support console.assert
You can open devtools and create a snippet in devtools-source-navigator-Snippets. And code some code... and run the snippet...

How to log exceptions in JavaScript

As a C# developer I'm used to the following style of exception handling:
try
{
throw SomeException("hahahaha!");
}
catch (Exception ex)
{
Log(ex.ToString());
}
Output
------
SomeNamespace.SomeException: hahahaha!
at ConsoleApplication1.Main() in ConsoleApplication1\Program.cs:line 27
Its really simple, and yet tells me everything I need to know about what the exception was and where it was.
How do I achieve the equivalent thing in JavaScript where the exception object itself might just be a string. I really want to be able to know the exact line of code where the exception happened, however the following code doesn't log anything useful at all:
try
{
var WshShell = new ActiveXObject("WScript.Shell");
return WshShell.RegRead("HKEY_LOCAL_MACHINE\\Some\\Invalid\\Location");
}
catch (ex)
{
Log("Caught exception: " + ex);
}
Output
------
Caught exception: [object Error]
EDIT (again): Just to clarify, this is for internal application that makes heavy use of JavaScript. I'm after a way of extracting useful information from JavaScript errors that may be caught in the production system - I already have a logging mechanism, just want a way of getting a sensible string to log.

You don't specify if you are working in the browser or the server. If it's the former, there is a new console.error method and e.stack property:
try {
// do some crazy stuff
} catch (e) {
console.error(e, e.stack);
}
Please keep in mind that error will work on Firefox and Chrome, but it's not standard. A quick example that will downgrade to console.log and log e if there is no e.stack:
try {
// do some crazy stuff
} catch (e) {
(console.error || console.log).call(console, e.stack || e);
}

As Eldar points out, you can use e.message to get the message of the exception. However, in Chrome, Firefox and IE10+, you can also get the stack trace using e.stack. The stack trace will include the file and line number of the exception.
So to assemble a string with exception info, you would write something like this:
var exmsg = "";
if (e.message) {
exmsg += e.message;
}
if (e.stack) {
exmsg += ' | stack: ' + e.stack;
}
Note that you will only get a stack trace if
the exception was thrown by the browser (such as in response to a
syntax error);
the exception object is an Error object or has the Error object as its prototype.
So just throwing a string (throw 'Exception!!') won't give you a stack trace.
To take this a bit further, to catch all uncaught exceptions, you would use a window.onerror handler (similar to .Net Application_Error handler in global.asax). The drawback of this used to be (and mostly still is) that this wouldn't give you access to the actual exception object, so you couldn't get the stack trace. You'd only get the message, url and line number.
Recently, the standard has been extended to give you the column (great for minified files) and the exception object as well:
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#errorevent
Right now (April 2014), only Chrome 32 implements all this. IE10+ gives you the column but not the exception object. Firefox 28 still only gives you message, url and line number. Hopefully, this will improve soon. I've written about this for the JSNLog project, at:
http://jsnlog.com/Documentation/GetStartedLogging/ExceptionLogging
(disclaimer: I am the author of JSNLog and jsnlog.com)
Secondly, the .Net Exception object supports inner exceptions. It also has a Data property so you can attach key value pairs with for example variable values. I sort of missed that in JavaScript Error object, so I created my own Exception object, also as part of the JSNLog project. It is in the jsnlog.js file in the jsnlog.js Github project (https://github.com/mperdeck/jsnlog.js).
Description is at:
http://jsnlog.com/Documentation/JSNLogJs/Exception
Finally a shameless plug - the JSNLog project I'm working on lets you insert loggers in your JavaScript, and automatically inserts the log messages in your existing server side log. So to log JavaScript exceptions with their stack traces to your server side log, you only need to write:
try {
...
} catch (e) {
JL().fatalException("something went wrong!", e);
}

You can use almost in the same manner ie.
try
{
throw new Error("hahahaha!");
}
catch (e)
{
alert(e.message)
}
But if you want to get line number and filename where error is thrown i suppose there is no crossbrowser solution. Message and name are the only standart properties of Error object. In mozilla you have also lineNumber and fileName properties.

I'm not sure whether or not it is cross browser or if it's what you are looking for, but I suggest you try:
window.onerror = function (err, file, line) {
logError('The following error occurred: ' +
err + '\nIn file: ' + file + '\nOn line: ' + line);
return true;
}

I had a similar problem.
Using console.table(error); worked well for me.
It displays information in a table, and also lets me expand/collapse to see more details.

It is almost identical, see the manual: https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/Exception_Handling_Statements/try...catch_Statement

You can use logging tools like Yahoo! UI Library - Logger to log the errors/informative messages.

I wrote a handy function for it
const logError = (e: any) => {
if (console.error) console.error(e, e.stack);
else console.log(e)
}

The modern best practice (as I understand it) is to log the error as a separate argument to console.error (or console.log, console.warn, etc...)
try {
maybeThrows()
} catch (e) {
console.error('it threw', e);
}
Trying out this approach in practice:
try {
throw Error('err') // Error object
} catch (e) {
console.error('it threw', e); // it threw Error: err
}
try {
throw 'up' // not an error object
} catch (e) {
console.error('it threw', e); // it threw up
}
I ran the above in Chrome, and Node v16. Note that node did not include a stack trace for throw 'up', but did for the proper error. Chrome included the stack for both.