#javascript: console lesser known features.

We’ve all used the console tools for age (Thank you Firebug) ; but most of us use only use the basic features like console.log() or console.error().

However, the console API is really powerful and offers a lot of very interesting features.

Always keep in mind that the console API is not standard and is not going to be standardized. There is absolutely no guarantee that these features will be available and you should never use console on production.

String substitutions

console.log() and other printing message methods (info, warn and error) supports string substitution (like the C printf function).

You can so use :

console.log('User %s has %d items', 'John', 5);
>> "User John has 5 items"

It’s often useful for string concatenation, to avoid the “ + “ agony and prevent the quote/double-quotes errors.

var example = " This -> ' and this -> '"; console.log('Here is my string "%s"', example);
>> "Here is my string " This -> ' and this -> '""

Currently, here are the supported identifiers :

%s String : IE, Chrome, Firefox

%d or %i Integer : IE, Chrome, Firefox

%f Floating point value : IE, Chrome, Firefox

%o Javascript object : IE, Chrome, Firefox
Object will be pretty-printed or a link to the inspector.
DOM Object are also handled.

%c Apply these CSS rules to the following text. Chrome, Firefox
Exemple :

console.log(‘There are now %c%d%c listeners’, ‘font-weight: bold;’, 2, ‘font-weight: normal;’);
There are now 2 listeners

%b Value as binary: IE

%x Value as hexadecimal: IE

Grouping messages

Messages can be grouped with console.group() and console.groupCollapsed() and console.groupEnd().

console.group('First group');
console.group('Second group');
console.group('Embeded subgroup');
console.groupEnd(); // For the "Embeded subgroup" console.groupEnd(); // For the "Second group"
console.groupCollapsed('Pre-collapsed to save your eyes');
console.log('Never Gonna %s', 'Give You Up');
console.log('Never Gonna %s', 'Get you down !');
console.info('This is a potato');

Measuring & profiling

console.time() and console.timeEnd() allows you to measure the time elapsed between their call.

They both takes a label as argument so you can start simultaneously several timers (docs says up to 10,000) and know which one you want to stop.

var slowInitializer = function() {
var collection = [];
for (var i = 20000000; i > 0 ; i--) {
if (i === 1000) {
console.time('Last iterations');
console.timeEnd('Last iterations');
console.time('Slow initializer');
console.timeEnd('Slow initializer');
>> Last iterations: 0.123ms Slow initializer: 2778.002ms

console.profile() and console.profileEnd() allows you to profile a part of your code.

console.profile() takes a label as argument so you can start simultaneously several timers (no infos on how many) ; console.profileEnd() will end the last started profiler.

The code profiling will be shown in the profiles or profiler (how any other related name) on your brower ; with the memory/cpu/calls etc usages.

var fibonateIt = function(n) {
return ((n < 2) ? n : (fibonateIt(n-1) + fibonateIt(n-2)));
console.profile('Fibonnaci generation');


On Chrome

You may also use console.count() to count the times a label has been executed :

$('#image').on('click', function() {
console.count('Click on my image');
>> Click on my image : 1
>> Click on my image : 2
>> // [...]
>> Click on my image : 12

Don’t use console.count() on fast and numerous loops (like previous previous example on the fibonacci numbers. This will cause your console to print a lot of information and make your browser slow/unstable/angry.

Make a conditional logging

console.assert() allows you to condition your debug by using a condition as its first argument.

If your first argument is false (loose comparison aka == and not ===), it will print out your message (or object), elsewhere it will be ignored.

For example, if you want to log every 100 iteration of a loop :

for (var i = 0; i <= 10000; i++) {
// Do something awesome.
console.assert(i % 1000, 'Iteration #%d', i);
>> "Iteration #0"
>> "Iteration #1000"
>> "Iteration #2000"
>> "Iteration #3000"
>> "Iteration #4000"
>> "Iteration #5000"
>> "Iteration #6000"
>> "Iteration #7000"
>> "Iteration #8000"
>> "Iteration #9000"
>> "Iteration #10000"

assert may sound like unit test to you, and of course, you can use it also to make some kind of unit testing, like :

(fibonateIt(-1) === -1),
'Fibonacci for -1 should be -1'
(fibonateIt(0) === 0),
'Fibonacci for 0 should be 0'
(fibonateIt(10) === 55),
'Fibonacci for 10 should be 55'

Pretty-print tabular datas (Arrays, objects, etc.)

console.table() allows you to debug some tabular datas in a graphical table in your console :

console.table([['a', 'b', 'c'], ['easy as'], [1,2,3]]);

Some browsers “decides” if whether a table is needed or not to display your data. For example, console.table([1,2,3]); will probably not be displayed in a table.

You can filter out the fields you would like to show :

var Crush = function(name, hobby, salary, cute) {
this.name = name;
this.hobby = hobby;
this.salary = salary;
this.cute = cute;
var venal_crushes = [
new Crush('john', 'animals', '70K', true),
new Crush('steeve', 'cars', '0K', false),
new Crush('peter', 'computers', '160K', false),
new Crush('marcel', 'france', '20K', true)
console.table(venal_crushes, ['name', 'salary']);

Log a stack trace

console.trace() allows you to show a stack trace to the line where you called it.

var a = function() {
console.trace('Hello I\'m a stack trace');
var b = function() {
var c = function() {
var d = function() {
try {
throw new Error('Ouch');
} catch(err) {
(function() { d(); })();

Originally published at github.com.