Cookbooks
JavaScript
Chapter 9. Testing and Debugging
Printing debug output

JavaScript:
Printing debug output

How to:

Javascript makes it super easy to print debug output using console.log(). Here’s how:

console.log('Hello, debug world!');

let number = 42;
console.log('The number is:', number);

function add(a, b) {
  console.log(`Adding ${a} + ${b}`);
  return a + b;
}

let result = add(3, 4);
console.log('Result:', result);

Sample output in your browser’s console or Node.js terminal would look like this:

Hello, debug world!
The number is: 42
Adding 3 + 4
Result: 7

Deep Dive

The console.log() method comes from the Console API, which has been a debugging friend in browsers and Node.js environments for ages. But there’s more than just log; you’ve got console.warn(), console.error(), and console.info(), all spitting out messages with different levels of severity.

Long ago, developers would use alert() for debugging, but that quickly became tedious—it blocks user interaction by popping up a dialog box.

There’s also console.dir() which gives you a JSON-like view of an object, handy for deep inspection. If you want to track how long something takes, console.time() and console.timeEnd() are your pals.

For those loving a good, clean output, console.table() displays data in a neat table format. And when you go beyond simple debugging and step into performance land, the Console API has even more tools like console.trace() for call stack info, console.profile() for performance profiling, among others.

The exact way console methods are implemented can vary between JavaScript environments, but the essence remains the same: they help developers make sense of what’s going under the hood quickly and with minimal fuss.

See Also

Using an interactive shell (REPL)