What is the use of console.log?
Please explain how to use it in JavaScript, with a code example.
424
votes
23 Answers
459
votes
It's not a jQuery feature but a feature for debugging purposes. You can for instance log something to the console when something happens. For instance:
$('#someButton').click(function() {
console.log('#someButton was clicked');
// do something
});
You'd then see #someButton was clicked in Firebug’s “Console” tab (or another tool’s console — e.g. Chrome’s Web Inspector) when you would click the button.
For some reasons, the console object could be unavailable. Then you could check if it is - this is useful as you don't have to remove your debugging code when you deploy to production:
if (window.console && window.console.log) {
// console is available
}
226
votes
Places you can view the console! Just to have them all in one answer.
Firefox
(you can also now use Firefox's built in developer tools Ctrl+Shift+J (Tools > Web Developer > Error Console), but Firebug is much better; use Firebug)
Safari and Chrome
Basically the same.
https://developers.google.com/chrome-developer-tools/docs/overview
https://developer.apple.com/technologies/safari/developer-tools.html
Internet Explorer
Don't forget you can use compatibility modes to debug IE7 and IE8 in IE9 or IE10
http://msdn.microsoft.com/en-us/library/ie/gg589507(v=vs.85).aspx
http://msdn.microsoft.com/en-us/library/dd565628(v=vs.85).aspx
If you must access the console in IE6 for IE7 use the Firebug Lite bookmarklet
http://getfirebug.com/firebuglite/ look for stable bookmarklet
http://en.wikipedia.org/wiki/Bookmarklet
Opera
http://www.opera.com/dragonfly/
iOS
Works for all iPhones, iPod touch and iPads.
Now with iOS 6 you can view the console through Safari in OS X if you plug in your device. Or you can do so with the emulator, simply open a Safari browser window and go to the "Develop" tab. There you will find options to get the Safari inspector to communicate with your device.
Windows Phone, Android
Both of these have no console built in and no bookmarklet ability. So we use http://jsconsole.com/ type :listen and it will give you a script tag to place in your HTML. From then on you can view your console inside the jsconsole website.
iOS and Android
You can also use http://html.adobe.com/edge/inspect/ to access web inspector tools and the console on any device using their convenient browser plugin.
Older browser problems
Lastly older versions of IE will crash if you use console.log in your code and not have the developer tools open at the same time. Luckily it's an easy fix. Use the below code snippet at the top of your code:
if(!window.console){ window.console = {log: function(){} }; }
This checks to see if the console is present, and if not it sets it to an object with a blank function called log. This way window.console and window.console.log is never truly undefined.
101
votes
You can view any messages logged to the console if you use a tool such as Firebug to inspect your code. Let's say you do this:
console.log('Testing console');
When you access the console in Firebug (or whichever tool you decide to use to inspect your code), you will see whatever message you told the function to log. This is particularly useful when you want to see if a function is executing, or if a variable is being passed/assigned properly. It's actually rather valuable for figuring out just what went wrong with your code.
84
votes
It will post a log message to the browser's javascript console, e.g. Firebug or Developer Tools (Chrome / Safari) and will show the line and file where it was executed from.
Moreover, when you output a jQuery Object it will include a reference to that element in the DOM, and clicking it will go to that in the Elements/HTML tab.
You can use various methods, but beware that for it to work in Firefox, you must have Firebug open, otherwise the whole page will crash. Whether what you're logging is a variable, array, object or DOM element, it will give you a full breakdown including the prototype for the object as well (always interesting to have a poke around). You can also include as many arguments as you want, and they will be replaced by spaces.
console.log( myvar, "Logged!");
console.info( myvar, "Logged!");
console.warn( myvar, "Logged!");
console.debug(myvar, "Logged!");
console.error(myvar, "Logged!");
These show up with different logos for each command.
You can also use console.profile(profileName); to start profiling a function, script etc. And then end it with console.profileEnd(profileName); and it will show up in you Profiles tab in Chrome (don't know with FF).
For a complete reference go to http://getfirebug.com/logging and I suggest you read it. (Traces, groups, profiling, object inspection).
Hope this helps!
34
votes
There is nothing to do with jQuery and if you want to use it I advice you to do
if (window.console) {
console.log("your message")
}
So you don't break your code when it is not available.
As suggested in the comment, you can also execute that in one place and then use console.log as normal
if (!window.console) { window.console = { log: function(){} }; }
23
votes
19
votes
console.log logs debug information to the console on some browsers (Firefox with Firebug installed, Chrome, IE8, anything with Firebug Lite installed). On Firefox it is a very powerful tool, allowing you to inspect objects or examine the layout or other properties of HTML elements. It isn't related to jQuery, but there are two things that are commonly done when using it with jQuery:
install the FireQuery extension for Firebug. This, amongst other advantages, makes the logging of jQuery objects look nicer.
create a wrapper that is more in line with jQuery's chaining code conventions.
This means usually something like this:
$.fn.log = function() {
if (window.console && console.log) {
console.log(this);
}
return this;
}
which you can then invoke like
$('foo.bar').find(':baz').log().hide();
to easily check inside jQuery chains.
16
votes
16
votes
A point of confusion sometimes is that to log a text message along with the contents of one of your objects using console.log, you have to pass each one of the two as a different argument. This means that you have to separate them by commas because if you were to use the + operator to concatenate the outputs, this would implicitly call the .toString() method of your object. This in most cases is not explicitly overriden and the default implementation inherited by Object doesn't provide any useful information.
Example to try in console:
>>> var myObj = {foo: 'bar'}
undefined
>>> console.log('myObj is: ', myObj);
myObj is: Object { foo= "bar"}
whereas if you tried to concatenate the informative text message along with the object's contents you'd get:
>>> console.log('myObj is: ' + myObj);
myObj is: [object Object]
So keep in mind that console.log in fact takes as many arguments as you like.
13
votes
Use console.log to add debugging information to your page.
Many people use alert(hasNinjas) for this purpose but console.log(hasNinjas) is easier to work with. Using an alert pop-ups up a modal dialog box that blocks the user interface.
Edit: I agree with Baptiste Pernet and Jan Hančič that it is a very good idea to check if window.console is defined first so that your code doesn't break if there is no console available.
12
votes
11
votes
10
votes
Beware: leaving calls to console in your production code will cause your site to break in Internet Explorer. Never keep it unwrapped. See: https://web.archive.org/web/20150908041020/blog.patspam.com/2009/the-curse-of-consolelog
10
votes
In early days JS debugging was performed through alert() function - now it is an obsolete practice.
The console.log() is a function that writes a message to log on the debugging console, such as Webkit or Firebug. In a browser you will not see anything on the screen. It logs a message to a debugging console. It is only available in Firefox with Firebug and in Webkit based browsers (Chrome and Safari). It does not work well in all IE releases.
The console object is an extension to the DOM.
The console.log() should be used in code only during development and debugging.
It’s considered bad practice that someone leaves console.log() in the javascript file on the production server.
5
votes
If your browser supports debugging, you can use the console.log() method to display JavaScript values.
Activate debugging in your browser with F12, and select "Console" in the debugger menu.
Console in JavaScript. Try to fix, or "debug," a non-functioning JavaScript program, and practice using the console.log() command. There are shortcuts that is going to help you to access to the JavaScript console, based on the browser that you are using:
Chrome Console Keyboard Shortcuts
Windows: Ctrl + Shift + J
Mac: Cmd + Option + J
Firefox Console Keyboard Shortcuts
Windows: Ctrl + Shift + K
Mac: Cmd + Option + K
Internet Explorer Console Keyboard Shortcuts
F12 key
Safari Console Keyboard Shortcuts
Cmd + Option + C
4
votes
console.log specifically is a method for developers to write code to inconspicuously inform the developers what the code is doing. It can be used to alert you that there's an issue, but shouldn't take the place of an interactive debugger when it comes time to debug the code. Its asynchronous nature means that the logged values don't necessarily represent the value when the method was called.
In short: log errors with console.log (if available), then fix the errors using your debugger of choice: Firebug, WebKit Developer Tools (built-in to Safari and Chrome), IE Developer Tools or Visual Studio.
4
votes
4
votes
It is used to log (anything you pass it) to the Firebug console. The main usage would be to debug your JavaScript code.
4
votes
3
votes
This is nothing to deal with the jQuery. The console.log() is referencing to the console object's log function, which provides methods for logging information to the browser's console. These methods are intended for debugging purposes only and should not be relied on for presenting information to end users.
2
votes
1
votes
console— contains many methods that you can call to perform rudimentary debugging tasks, generally focused around logging various values to the browser's Web Console.By far the most commonly-used method is
console.log, which is used to log the current value contained inside a specific variable.
How to use in Javascript?
let myString = 'Hello World';
console.log(myString);
Also remember console is a part of global window object in the web browser. Thus the following is also technically correct but isn't used in practical scenario.
let myString = 'Hello World';
window.console.log(myString);
0
votes
I try to explain in easy way:
Why Useable:
The console.log() method writes a message to the console and display it.
~very useful for testing purposes.
~Use them under/inside script tags.
How To See:
=>Press F12
Syntax:
console.log(message you want to display);
~Accepts a parameter and display it.
Working With Array:
var myArray = ["Ali", "John", "Shahrukh"];
console.log(myArray);Working With Objects:
var myObject = { firstname : "Ali", lastname : "Rana" };
console.log(myObject);Working With Text:[method 1]:
console.log("Hello StackOverflow");Working With Text:[method 2]
var str = "Hello StackOverflow";
console.log(str);Working With Numbers:
var myvar = '2';
console.log(myvar);Working With Function:
function myfunction() { return (5 * 19); }
console.log(function());Display Message With Argument:
var a = 2;
console.log("The value of a is " + a);Hope, It May Helps.
