Output to the command line using Node.js

Basic output using the console module

Node.js provides a console module which provides tons of very useful ways to interact with the command line.

It is basically the same as the console object you find in the browser.

The most basic and most used method is console.log(), which prints the string you pass to it to the console.

If you pass an object, it will render it as a string.

You can pass multiple variables to console.log, for example:

const x = 'x';
const y = 'y';
console.log(x, y);

and Node.js will print both.

We can also format pretty phrases by passing variables and a format specifier.

For example:

console.log('My %s has %d ears', 'cat', 2);

• %s format a variable as a string
• %d format a variable as a number
• %i format a variable as its integer part only
• %o format a variable as an object

Example:

console.log('%o', Number);

Clear the console

console.clear() clears the console (the behavior might depend on the console used)

Counting elements

console.count() is a handy method.

Take this code:

const x = 1;
const y = 2;
const z = 3;

console.count(
  'The value of x is ' + x + ' and has been checked .. how many times?'
);

console.count(
  'The value of x is ' + x + ' and has been checked .. how many times?'
);

console.count(
  'The value of y is ' + y + ' and has been checked .. how many times?'
);

What happens is that console.count() will count the number of times a string is printed, and print the count next to it:

You can just count apples and oranges:

const oranges = ['orange', 'orange'];
const apples = ['just one apple'];

oranges.forEach(fruit => {
  console.count(fruit);
});
apples.forEach(fruit => {
  console.count(fruit);
});

Reset counting

The console.countReset() method resets counter used with console.count().

We will use the apples and orange example to demonstrate this.

const oranges = ['orange', 'orange'];
const apples = ['just one apple'];

oranges.forEach(fruit => {
  console.count(fruit);
});
apples.forEach(fruit => {
  console.count(fruit);
});

console.countReset('orange');

oranges.forEach(fruit => {
  console.count(fruit);
});

Notice how the call to console.countReset('orange') resets the value counter to zero.

Print the stack trace

There might be cases where it's useful to print the call stack trace of a function, maybe to answer the question how did you reach that part of the code?

You can do so using console.trace():

const function2 = () => console.trace();
const function1 = () => function2();
function1();

This will print the stack trace. This is what's printed if we try this in the Node.js REPL:

Trace
    at function2 (repl:1:33)
    at function1 (repl:1:25)
    at repl:1:1
    at ContextifyScript.Script.runInThisContext (vm.js:44:33)
    at REPLServer.defaultEval (repl.js:239:29)
    at bound (domain.js:301:14)
    at REPLServer.runBound [as eval] (domain.js:314:12)
    at REPLServer.onLine (repl.js:440:10)
    at emitOne (events.js:120:20)
    at REPLServer.emit (events.js:210:7)

Calculate the time spent

You can easily calculate how much time a function takes to run, using time() and timeEnd()

const doSomething = () => console.log('test');
const measureDoingSomething = () => {
  console.time('doSomething()');
  // do something, and measure the time it takes
  doSomething();
  console.timeEnd('doSomething()');
};
measureDoingSomething();

stdout and stderr

As we saw console.log is great for printing messages in the Console. This is what's called the standard output, or stdout.

console.error prints to the stderr stream.

It will not appear in the console, but it will appear in the error log.

Color the output

You can color the output of your text in the console by using escape sequences. An escape sequence is a set of characters that identifies a color.

Example:

console.log('\x1b[33m%s\x1b[0m', 'hi!');

You can try that in the Node.js REPL, and it will print hi! in yellow.

However, this is the low-level way to do this. The simplest way to go about coloring the console output is by using a library. Chalk is such a library, and in addition to coloring it also helps with other styling facilities, like making text bold, italic or underlined.

You install it with npm install chalk, then you can use it:

const chalk = require('chalk');

console.log(chalk.yellow('hi!'));

Using chalk.yellow is much more convenient than trying to remember the escape codes, and the code is much more readable.

Check the project link posted above for more usage examples.

Create a progress bar

Progress is an awesome package to create a progress bar in the console. Install it using npm install progress

This snippet creates a 10-step progress bar, and every 100ms one step is completed. When the bar completes we clear the interval:

const ProgressBar = require('progress');

const bar = new ProgressBar(':bar', { total: 10 });
const timer = setInterval(() => {
  bar.tick();
  if (bar.complete) {
    clearInterval(timer);
  }
}, 100);