Node Core Modules Exploration - Console

In the previous post we took a look at some of the command line options available when starting node.

Now we are going to take a look at probably the most used core module, especially for those just getting started in debugging. Thats right, console

This is another "Global" module, so require is not, ummm, require'd here

I'm sure this is very common in everyones code:

console.log('It Worked');

And if you've ever done client side(html, js) programming, then console and most of its methods should be familiar.

While console is on both browsers and Node.js, don't expect them to be completely implemented the same way.

For example, the console.assert in Node will throw an AssertionError on a falsey value instead of printing the message out, which is what the browser will do

Console Class

The console module has a Console class that can be used to create loggers whos output can be configurable.

new Console(output, errorOutput)

In fact, the global console is a special console. It is acutally the same as calling this:

new Console(process.stdout, process.stderr)

We could create a simple logger that outputs to a file. This example is taken from the official docs

const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');

const logger = new Console(output, errorOutput);

logger.log('Say Something Cool');
// in stdout.log:  Say Something Cool

Timings

console has 2 methods that can be used to compute the duration of an operation. They are console.time and console.timeEnd. Both take a label as an argument. Timers are identified by this label, so you would need to use the same labels for time and timeEnd.

console.time('myTimer');

someFunctionThatTakesTwentyThreeMS();

console.timeEnd('myTimer');
// prints myTimer: 23.232ms

Just a quick note on the timeEnd function in versions of node prior to 6.0.0. You could actually call console.timeEnd multiple times with the same label, this wasn't intended. Newer versions will delete the timer once console.timeEnd is called.

Other Neat things

There are 2 other sort of neat things, those are console.dir and console.trace.

Lets take a look at trace first. It is pretty simple, but can be powerful. It prints to stderr or where ever you have your err outputting, the string Trace: <Stack Trace of current position in code>

Here is an example from my REPL:

> console.trace('Trace Me');
Trace: Trace Me
at repl:1:9
at sigintHandlersWrap (vm.js:22:35)
at sigintHandlersWrap (vm.js:96:12)
at ContextifyScript.Script.runInThisContext (vm.js:21:12)
at REPLServer.defaultEval (repl.js:313:29)
at bound (domain.js:280:14)
at REPLServer.runBound [as eval] (domain.js:293:12)
at REPLServer.<anonymous> (repl.js:504:10)
at emitOne (events.js:101:20)
at REPLServer.emit (events.js:188:7)

This next one is something i always forget is there. console.dir

If you pass an object into console.dir, it prints out the String representation of that Object.(It uses the util.inspect function behind the scenes).

There is also some options you can pass. For example, if an object is large and complicated, use the depth option, to make inpsect recurse through the object.

The depth defaults to 2, so with this example:

const a = {
    a: 1, 
    b: {
        c: 2, 
        d:{
            e: 3, 
            f: { 
                d: 4
            }
        }
    }
};

console.dir(a);

//outputs: { a: 1, b: { c: 2, d: { e: 3, f: [Object] } } }

If i wanted to see what that [Object] is, then i could do:

console.dir(a, {depth: 3});

// outputs: { a: 1, b: { c: 2, d: { e: 3, f: { d: 4 } } } }

But what if i had more depth, then passing null will recurse idefnitely.

One more tidbit

console functions are actually asynchronous.

Next up is a very easy topic, Crypto

comments powered by Disqus