The built-in console module

Anyone familiar with browser-side development has probably used console.log for debugging purposes - Node.js has implemented a built-in console object to mimic much of this experience. Since we're working server-side, however, it wraps stdout, stdin, and stderr instead of the browser's debugging console.

Because of this browser parallel, the console module has become home to quite a bit of standard output functionality of Node.js. The simplest is console.log().

console.log('Hi, everybody!');
console.log('This script is:', __filename);
console.log(__filename, process.title, process.argv);

The first, simplest example just prints the provided string to stdout. It can also be used to output the contents of variables, as evidenced in #2; furthermore, console.dir() is called on any objects passed in as arguments, enumerating their properties.

NODE.JS PRO TIP: console.log() accepts three format characters, %s, %d, and %j. These format characters can be used to insert string, integer, or JSON data into your output - the order of format characters must match the order of arguments.

var name = 'Harry',
    number = 17,
    myObj = {
      propOne: 'stuff',
      propTwo: 'more stuff'
    };
console.log('My name is %s, my number is %d, my object is %j', name, number, myObj);

A gotcha with console.log, and all functions that depend on it, is that it buffers the output. So if your process ends suddenly, whether it be from an exception or from process.exit(), it is entirely possible that the buffered output will never reach the screen. This can cause a great deal of frustration, so watch out for this unfortunate situation.

console.error() works the same as console.log, except that the output is sent to stderr instead of stdout. This is actually an extremely important difference, as stderr is always written to synchronously. Any use of console.error, or any of the other functions in Node.js core that write to stderr, will block your process until the output has all been written. This is useful for error messages - you get them exactly when they occur - but if used everywhere, can greatly slow down your process.

console.dir(), as mentioned above, is an alias for util.inspect() - it is used to enumerate object properties. Read More

That covers the basic console module functionality, but there are a few other methods worth mentioning as well. First, the console module allows for the marking of time via console.time() and console.timeEnd(). Here is an example:

console.time('myTimer');
var string = '';
for (var i = 0; i < 300; i++) {
  (function (i) {
    string += 'aaaa' + i.toString();
  })(i);
}
console.timeEnd('myTimer');

This would determine the amount of time taken to perform the actions in between the console.time and console.timeEnd calls.

One last function worth mentioning is console.trace(), which prints a stack trace to its location in your code without throwing an error. This can occasionally be useful if you'd like to figure out where a particular failing function was called from.

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