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

NOTE This part of the resource is designed with version 22.11 which notes styleText as ‘Active development’.

In many cases, you will be tempted to paste certain text to get a nice output at the terminal.

There is a styleText function provided by the node:util module. Let's discover how to use it.

First of all, you need to import the styleText function from the node:util module:

import { styleText } from 'node:util';

Then, you can use it to style your text:

console.log(
  styleText(['red'], 'This is red text ') +
    styleText(['green, bold'], 'and this is green bold text ') +
    'this is normal text'
);

The first argument is an array of styles, and the second argument is the text you want to style. We invite you to read the docs