Should I be removing console.log from production code?

I currently have this JS statement everywhere in my code:

window.console && console.log("Foo");

I am wondering if this is costly at all, or has any negative side-effects in production.

Am I free to leave client-side logging in, or should it go?

EDIT: In the end, I suppose the best argument I (and anyone else?) can come up with is that there is a possibly non-negligible amount of extra data transferred between the server and the client by leaving logging messages left in. If production code is to be fully optimized, logging will have to be removed to reduce the size of javascript being sent to the client.

Another way of dealing with this is to 'stub' out the console object when it isn't defined so no errors are thrown in contexts that do not have the console i.e.

if (!window.console) {
  var noOp = function(){}; // no-op function
  console = {
    log: noOp,
    warn: noOp,
    error: noOp
  }
}

you get the idea... there are a lot of functions defined on the various implementations of the console, so you could stub them all or just the ones you use (e.g. if you only ever use console.log and never used console.profile, console.time etc...)

This for me is a better alternative in development than adding conditionals in front of every call, or not using them.

see also: https://stackoverflow.com/questions/1114187/is-it-a-bad-idea-to-leave-firebug-console-log-calls-in-your-producton-javascri

You should not add development tools to a production page.

To answer the other question: The code cannot have a negative side-effect:

• window.console will evaluate to false if console is not defined
• console.log("Foo") will print the message to the console when it's defined (provided that the page does not overwrite console.log by a non-function).

UglifyJS2

If you are using this minifier, you can set drop_console option:

> Pass true to discard calls to console.* functions

So I would suggest to leave console.log calls as they are for a most trickiest part of the codebase.

If minification is part of your build process, you may use it to strip out debug code, as explained here with Google closure compiler: https://stackoverflow.com/questions/2934509/exclude-debug-javascript-code-during-minification

if (DEBUG) {
  console.log("Won't be logged if compiled with --define='DEBUG=false'")
}

If you compile with advanced optimizations, this code will even be identified as dead and removed entirely

Yes. console.log will throw an exception in browsers that don't have support for it (console object will not be found).

Generally yes, its not a great idea to expose log messages in your production code.

Ideally, you should remove such log messages with a build script before deployment; but many (most) people do not use a build process (including me).

Here's a short snippet of some code I've been using lately to solve this dilemma. It fixes errors caused by an undefined console in old IE, as well as disabling logging if in "development_mode".

// fn to add blank (noOp) function for all console methods
var addConsoleNoOp =  function (window) {
    var names = ["log", "debug", "info", "warn", "error",
        "assert", "dir", "dirxml", "group", "groupEnd", "time",
        "timeEnd", "count", "trace", "profile", "profileEnd"],
        i, l = names.length,
        noOp = function () {};
    window.console = {};
    for (i = 0; i < l; i = i + 1) {
        window.console[names[i]] = noOp;
    }
};

// call addConsoleNoOp() if console is undefined or if in production
if (!window.console || !window.development_mode) {
    this.addConsoleNoOp(window);
}

I'm pretty sure I took much of the above addConsoleNoOp f'n from another answer on SO, but cannot find right now. I'll add a reference later if I find it.

edit: Not the post I was thinking of, but here's a similar approach: https://github.com/paulmillr/console-polyfill/blob/master/index.js

var AppLogger = (function () {
  var debug = false;
  var AppLogger = function (isDebug) {
    debug = isDebug;
  }
  AppLogger.conlog = function (data) {
    if (window.console && debug) {
        console.log(data);
    }
  }
  AppLogger.prototype = {
    conlog: function (data) {
        if (window.console && debug) {
            console.log(data);
        }
    }
  };
return AppLogger;
})();

Usage:

var debugMode=true;
var appLogger = new AppLogger(debugMode);
appLogger.conlog('test');

Yes, its good practice to use console.log for javascript debugging purpose, but it needs to be removed from the production server or if needed can be added on production server with some key points to be taken into consideration:

**var isDebugEnabled="Get boolean value from Configuration file to check whether debug is enabled or not".**
if (window.console && isDebugEnabled) {
    console.log("Debug Message");
}

Above code block has to be used everywhere for logging in order to first verify whether the console is supported for the current browser and whether debug is enabled or not.

> isDebugEnabled has to be set as true or false based on our > environment.

TL;DR

Idea: Logging objects precludes them from being Garbage Collected.

Details

1. If you pass objects to console.log then these objects are accessible by reference from console of DevTools. You may check it by logging object, mutating it and finding that old messages reflect later changes of the object.
2. If logs are too long old messages do get deleted in Chrome.
3. If logs are short then old messages are not removed, if these messages reference objects then these objects are not Garbage Collected.

It's just an idea: I checked points 1 and 2 but not 3.

Solution

If you want to keep logs for sake of client-side troubleshooting or other needs then:

['log', 'warn', 'error'].forEach( (meth) => {
  const _meth = window.console[meth].bind(console);
  window.console[meth] = function(...args) { _meth(...args.map((arg) => '' + arg)) }
});

I basically overwrite the console.log function with the one what has knowledge of where the code is being run. Thus i can keep using console.log as I do always. It automatically knows that I am in dev/qa mode or in production. There is also a way to force it. Here is a working fiddle. http://jsfiddle.net/bsurela/Zneek/

Here is the snippet as stack overflow is intimated by people posting jsfiddle

  log:function(obj)
{
    if(window.location.hostname === domainName)
    {
        if(window.myLogger.force === true)
        {
            window.myLogger.original.apply(this,arguments);
        }
    }else {
        window.myLogger.original.apply(this,arguments);
    }
},

I know this is quite an old question and hasn't had much activity in a while. I just wanted to add my solution that I came up with which seems to work quite well for me.

	/**
	 * Logger For Console Logging 
	 */
	Global.loggingEnabled = true;
	Global.logMode = 'all';
	Global.log = (mode, string) => {	
		if(Global.loggingEnabled){
			switch(mode){
			  case 'debug':
				  if(Global.logMode == 'debug' || Global.logMode == 'all'){
				    console.log('Debug: '+JSON.stringify(string));
				  }
				  break;
			  case 'error':
				  if(Global.logMode == 'error' || Global.logMode == 'all'){
				    console.log('Error: '+JSON.stringify(string));
				  }		  
				  break;
			  case 'info':
				  if(Global.logMode == 'info' || Global.logMode == 'all'){
				    console.log('Info: '+JSON.stringify(string));
				  }
				  break;
			}
		}
	}

Then I typically create a function in my scripts like this or you could make it available in a global script:

Something.fail = (message_string, data, error_type, function_name, line_number) => {
	try{
		
		if(error_type == undefined){
			error_type = 'error';
		}
		
		Global.showErrorMessage(message_string, true);
		Global.spinner(100, false);
		
		Global.log(error_type, function_name);
		Global.log(error_type, 'Line: '+line_number);
		Global.log(error_type, 'Error: '+data);
		
	}catch(error){
		if(is_global){
			Global.spinner(100, false);
			Global.log('error', 'Error: '+error);
			Global.log('error', 'Undefined Error...');
		}else{
			console.log('Error:'+error);
			console.log('Global Not Loaded!');
		}			
	}	
}

And then I just use that instead of console.log like this:

try{
 // To Do Somehting
 Something.fail('Debug Something', data, 'debug', 'myFunc()', new Error().lineNumber);
}catch(error){
 Something.fail('Something Failed', error, 'error', 'myFunc()', new Error().lineNumber);
}

If the workflow is done using the right tools such as parcel/webpack then it's no longer a headache, because with the production build console.log is being dropped. Even few years earlier with Gulp/Grunt it could've been automated as well.

Many of the modern frameworks such as Angular, React, Svelte, Vue.js come with that setup out-of-the-box. Basically, you don't have to do anything, as long as you deploy the correct build, i.e. production one, not development which will still have console.log.

Don't overcomplicate things! I personally use console.log all the time during development, it's just such a timesaver. For production i just add a single line of code (in the "production profile" in my case) that disable all logs:

window.console.log = () => {};

done ;) This monkey patches window.console and replace the log function with an empty function, disabling the output.

This is good enough for me in most cases. If you want to go "all the way" and remove console.logs from your code to decrease bundle size, you have to change the way your js is bundled (e.g. drop console.logs with minifier or something)

Also I think you CAN actually make a strong point for leaving them in - even in production. It doesn't change anything for a normal user but can really speed up understanding weird "exotic-browser" problems. It's not like it's backend-logs that may contain critical information. It's all on the frontend anyway, not showing a log message because you are scared to reveal something a user shouldn't know really is only "security by obscurity" and should make you think why this information is even available on the frontend in the first place. Just my opinion.