LearnCode.live
Console I/O in Node.js · learncode.live

The console Module

Node.js’s console module is more powerful than browser console.log. It writes to stdout and stderr streams and includes formatting, timing, and grouping.

console.log()    ───► stdout
console.error()  ───► stderr

Basic Output

// stdout
console.log('Hello World');

// stderr
console.error('Something went wrong');

// Format strings (%s = string, %d = number, %j = JSON)
console.log('User: %s, Age: %d', 'Alice', 30);
console.log('Data: %j', { id: 1, name: 'Bob' });

// Output:
// User: Alice, Age: 30
// Data: {"id":1,"name":"Bob"}

Why stderr matters: console.error goes to stderr (fd 2), not stdout (fd 1). This lets you do node app.js 2> errors.log to separate errors from normal output.

Redirecting stdout and stderr

// redirect.js
console.log('This is normal output');
console.error('This is an error message');

node redirect.js 2> err.txt
# Terminal shows: This is normal output
# err.txt contains: This is an error message

node redirect.js > output.txt 2>&1
# Both stdout and stderr go to output.txt

Console Methods

console.table - Tabular Data

const users = [
  { name: 'Alice', age: 30, role: 'admin' },
  { name: 'Bob', age: 25, role: 'user' },
  { name: 'Charlie', age: 35, role: 'moderator' },
];

console.table(users);

┌─────────┬─────────┬─────┬────────────┐
│ (index) │  name   │ age │    role    │
├─────────┼─────────┼─────┼────────────┤
│    0    │ 'Alice' │ 30  │  'admin'   │
│    1    │  'Bob'  │ 25  │  'user'    │
│    2    │ 'Charlie'│ 35  │ 'moderator'│
└─────────┴─────────┴─────┴────────────┘

console.group - Indented Output

console.group('User details');
console.log('Name: Alice');
console.log('Age: 30');
console.group('Permissions');
console.log('Can read:', true);
console.log('Can write:', true);
console.groupEnd();
console.groupEnd();

console.count - Counting

function processUser(name) {
  console.count(`Processed user`);
  console.count(`Name: ${name}`);
}

processUser('Alice');
processUser('Bob');
processUser('Alice');

Processed user: 1
Name: Alice: 1
Processed user: 2
Name: Bob: 1
Processed user: 3
Name: Alice: 2

Timers

console.time / console.timeEnd

console.time('fetch-data');
// Simulate async work
setTimeout(() => {
  console.timeEnd('fetch-data');
  // fetch-data: 1002.345ms
}, 1000);

// Multiple timers
console.time('loop');
let sum = 0;
for (let i = 0; i < 1000000; i++) sum += i;
console.timeEnd('loop');
// loop: 2.456ms

console.timeLog

console.time('request');
// ... part 1 ...
console.timeLog('request', 'after query');
// request: 12ms after query
// ... part 2 ...
console.timeEnd('request');
// request: 34ms

Colouring Output

Use ANSI escape codes or the chalk package:

Raw ANSI Codes

const RESET = '\x1b[0m';
const RED = '\x1b[31m';
const GREEN = '\x1b[32m';
const YELLOW = '\x1b[33m';
const BLUE = '\x1b[34m';
const BOLD = '\x1b[1m';

console.log(RED + 'Error message' + RESET);
console.log(GREEN + 'Success!' + RESET);
console.log(BOLD + 'Important heading' + RESET);

npm install chalk

import chalk from 'chalk';  // ES modules

console.log(chalk.red('Error: Something failed'));
console.log(chalk.green('✓ Build successful'));
console.log(chalk.blue.bold('Info:'));
console.log(chalk.yellow.bgRed(' WARNING '));

Progress Bar (Manual)

function progressBar(current, total, barLength = 30) {
  const filled = Math.round((current / total) * barLength);
  const empty = barLength - filled;
  const percent = ((current / total) * 100).toFixed(1);
  process.stdout.write(
    `\r[${'█'.repeat(filled)}${'░'.repeat(empty)}] ${percent}%`
  );
}

const total = 100;
for (let i = 0; i <= total; i++) {
  progressBar(i, total);
  // Simulate work
  Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, 30);
}
process.stdout.write('\nDone!\n');

Assertions

console.assert(2 + 2 === 5, 'Math is broken');
// Assertion failed: Math is broken

// Only prints when assertion fails
console.assert(2 + 2 === 4, 'This will not print');

Tracing the Call Stack

function foo() {
  bar();
}
function bar() {
  console.trace('Trace at bar');
}

foo();
// Trace: Trace at bar
//     at bar (app.js:5:11)
//     at foo (app.js:2:5)
//     at Object.<anonymous> (app.js:8:1)

Key Takeaways

  • console.log → stdout, console.error → stderr - use them intentionally
  • console.table, console.group, console.count for structured output
  • console.time / console.timeEnd for simple performance measurement
  • ANSI escape codes or chalk for coloured terminal output
  • process.stdout.write('\r...') for progress bars and live-updating output
  • console.trace() prints the call stack - invaluable for debugging
Courses