Override console.log in coffeescript - javascript

Sometimes we leave console.log debug statements in our javascript. Just so they don't accidentally make it into production (where some old browsers will die on these), we tried doing this in our first coffeescript file:
if !console?
console = {log: ->}
But this seems to kill all ability to use console.log in development, even in browsers that support it (webkit).
console? returns false, so the if block doesn't run, but it still seems to overwrite the functionality of console.log.
Any ideas? Thanks!

The problem is that this:
if !console?
console = {log: ->}
becomes this JavaScript:
var console;
if (typeof console === "undefined" || console === null) {
console = {
log: function() {}
};
}
The result is that you have a local console variable that shadows the window.console that you're looking for.
The solution is to say exactly what you mean:
if 'console' !of window
window.console = { log: -> }
Note that console is always localized to window here so you don't accidentally create any shadows.
There are various ways to say if 'console' !of window that will have the same effect, use whichever one is clearest for you.

My approach has been to define a DEBUG (or print) function to use instead of console.log. The default, production definition is
DEBUG = () ->
and prior to any test code (e.g. at the end like Python if __name__... block):
if not module.parent? and (!process.argv[2]? or process.argv[2]!='nodebug')
DEBUG = (arg...) ->
arg.unshift('==> ')
console.log arg...
or simply
DEBUG = console.log

Related

Getting the stack trace of a console message

Sometimes I see stray log statements when I load up my application, and I don't remember how the log statement was called. Is there a way to find the stack trace of a console log message from the Chrome developer tools?
Right now, I wrap console.log with a function that prints the stack trace:
console.log = (function () {
var oldLog = console.log.bind(console);
return function () {
try { throw new Error(); } catch (e) {
oldLog(
e.stack.split('\n')
.slice(2)
.map((line) => line.trim().replace('at', 'from'))
.join('\n')
);
}
oldLog.apply(null, Array.prototype.slice.call(arguments));
};
}());
This will print the stack trace in addition to the log message. For this to be useful, I have to insert this code just before any of my code runs. Is there a way to find out this information from just the console, without having to add this wrap?
I primarily use Chrome developer tools, but I'd be interested if this was possible in any other browser.
Have you tried console.trace()? It is available in Chrome and Firefox. For your use-case, you could alias log to trace.
That said, I usually wrap logger statements in a custom logger function that itself wraps whatever I need to use, but which can be programmatically disabled in production to prevent leaking logs.
EDIT:
There aren't currently any chrome://flags that would let you set that behaviour, but you could do this before your console gets invoked (such as at the top of your minified js): console.log=console.trace;
Which will have this effect, (example in Chrome Canary's console):
> console.log("Hello, Robz");
VM929:2 Hello, Robz
undefined
> console.log = console.trace;
function trace() { [native code] }
> console.log("Hello, Robz");
VM935:2 Hello, Robz
VM935:2 (anonymous function)
VM468:777 InjectedScript._evaluateOn
VM468:710 InjectedScript._evaluateAndWrap
VM468:626 InjectedScript.evaluate
Basically. To show the source of a logging-message was a very bad idea. It leads you to write logging messages that neither belongs to the code nor can be localized without a stack-trace.
How do you work in a Team with it?
hey, i got a `ohoh!`-logging in file abc.js in line 3487!
To understand the problem by inspecting the call-stack means: Hey i do not need a text-message, i do not need a message what exactly is going on, just where.

Is it possible to override console logs

If I receive an error from a framework or an error from the browser. Basically a runtime error of any kind. Without modifying the framework, is it possible for me to override the console logs that these frameworks make and the browser's errors. I want to use my own framework with own error handling system when informing the user of errors of practically anything runtime (not syntax errors). I don't know if you would class it all as runtime errors because of the way javascript is executed in the browser but hopefully you will get me?
Is this possible if all the frameworks are written in Javascript?
How is this achieved?
What considerations do I have to make between different browsers?
Thanks
You are probably looking for a try-catch block:
try {
alert(foo);
} catch(e) {
alert('The code got the following error: '+e.message);
}
Whenever the code between the try {} receives an error, the catch(e) {} block will execute, with the argument e being the error object for the error that occurred. In this case, the variable foo is not defined, so executing this code will result in an alert message saying "The code got the following error: foo is not defined"
While not over-riding console.log, you may be achieve the same effect by overriding window.onerror.
From the MDN documentation
window.onerror = function myErrorHandler(errorMsg, url, lineNumber) {
// Log the error here -- perhaps using an AJAX call
}
You could try overriding the console.log() function.
//Save original reference
var originalConsole = console;
//Override
console = {};
console.log = function()
{
//According to MDN the console.log function can receive a variable number of params
for(var i=0; i<arguments.length; i++)
{
//Make your changes here, then call the original console.log function
originalConsole.log("Change something: "+arguments[i]);
}
//Or maybe do something here after parsing all the arguments
//...
}
console.log("one", "two");
JSFiddle here.
You can override the console logs by creating a "console" object and overriding it's .log() function:
var console = {};
console.log = function(){};
Some browsers require this to be added to the window object directly; so, for browser compatibility, also add:
window.console = console;
Additionally, you can override other console functions (e.g. console.info, console.warn and console.error) if you're using those too.
Also, consider reading this blog post from Udi Talias on overriding the console functions. Good and quick read!
You can custom your console.log here
// copy the original
let originalConsole = Object.assign({}, console);
// do something with your log.
console.log = (value) => {
//some cool condition
if (true) {
value = "new_log : " + value
}
originalConsole.log(value);
};

How can I use console.log on IE by using Modernizr.js [duplicate]

This question already has answers here:
What happened to console.log in IE8?
(17 answers)
Closed 8 years ago.
Can anyone plase let me know that how can I use console.log in my application throgh modernizr.js which needs to be openned in almost every browser.
When I tried to run it in IE 8, it is giving me an error that console is undefined.
You may create a simple console polyfill for IE:
window.console = window.console || {
log: function () {}
};
This won't have any effect on modern browsers. It'll just prevent undefined console error in IE.
You can include console polyfill in your page https://github.com/paulmillr/console-polyfill which add empty function if console.log is not available(ex: IE6)
You can also try console.log wrapper https://github.com/patik/console.log-wrapper
Console.log object is a features of some browser (Firefox, Chrome,...)
In IE console.log is only available after you have opened the Developer Tools F12
to avoid those errors you have to check for existence:
if ( window.console && window.console.log ) {
// console is available
}
or you can create a console fallback to avoid checking every time writing the code below on top of you .js file or in your case before referencing modernzer.js:
window.console = window.console || {
log: function (msg) {
alert(msg); //if you don't want alerts instead of logs comment this line
}
};

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.

Preventing console errors

Whats the best approach for preventing errors when console.log calls have been left in JavaScript and it is executed on Browsers without a console or with console deactivated. Is there a way it can be automatically overridden to become a javascript alert for example?
if(!window.console) console = {log: function(s) {alert(s);}};
You can of course add more of the functions that console normally has.
You have to check if the console identifier is available, you can do it either by using the typeof operator, or by checking window.console, because if you access directly an identifier and it's not defined, you will get a ReferenceError.
For example:
if (typeof console == "undefined") {
window.console = {
log: function () {
// do nothing
}
};
console.warn = console.debug = console.log;
}
Here's what I use :-
if(typeof(console) != "undefined")

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