Basic output using the console module
Node.js provides a console module which provides many 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:
const x = 'x';
const y = 'y';
console.log(x, y);
console.log('My %s has %d ears', 'cat', 2);
| Specifier | Description |
|---|
%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 |
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 that counts the number of times a string is printed and displays the count next to it.
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?'
);
const oranges = ['orange', 'orange'];
const apples = ['just one apple'];
oranges.forEach(fruit => {
console.count(fruit);
});
apples.forEach(fruit => {
console.count(fruit);
});
Reset counting
console.countReset() resets a counter used with console.count().
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);
});
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 — for example, 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();
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 console.time() and console.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
console.log prints to the standard output (stdout). console.error prints to the stderr stream. It will appear in the console, but can be handled separately from regular output.
Color the output
This section covers styleText, which was marked as ‘Active development’ in Node.js v22.11.
node:util module provides a styleText function to style terminal output.
First, import styleText:
import { styleText } from 'node:util';
const { styleText } = require('node:util');
console.log(
styleText(['red'], 'This is red text ') +
styleText(['green', 'bold'], 'and this is green bold text ') +
'this is normal text'
);