JavaScript Debugging

It is easy to get lost writing JavaScript code without a debugger.

JavaScript Debugging


It is difficult to write JavaScript code without a debugger.

Your code might contain syntax errors, or logical errors, that are difficult to diagnose.

Often, when JavaScript code contains errors, nothing will happen. There are no error messages, and you will get no indications where to search for errors.

Normally, errors will happen, every time you try to write some new JavaScript code.

JavaScript Debuggers


Searching for errors in programming code is called code debugging.

Debugging is not easy. But fortunately, all modern browsers have a built-in debugger.

Built-in debuggers can be turned on and off, forcing errors to be reported to the user.

With a debugger, you can also set breakpoints (places where code execution can be stopped), and examine variables while the code is executing.

Normally, otherwise follow the steps at the bottom of this page, you activate debugging in your browser with the F12 key, and select "Console" in the debugger menu.

The console.log() Method


If your browser supports debugging, you can use console.log() to display JavaScript values in the debugger window:


<!DOCTYPE html>

<h1>My First Web Page</h1>

a = 5;
b = 6;
c = a + b;

Try it Yourself

Setting Breakpoints


In the debugger window, you can set breakpoints in the JavaScript code.

At each breakpoint, JavaScript will stop executing, and let you examine JavaScript values.

After examining values, you can resume the execution of code (typically with a play button).

The debugger Keyword


The debugger keyword stops the execution of JavaScript, and calls (if available) the debugging function.

This has the same function as setting a breakpoint in the debugger.

If no debugging is available, the debugger statement has no effect.

With the debugger turned on, this code will stop executing before it executes the third line.


var x = 15 * 5;
document.getElementbyId("demo").innerHTML = x;
Try it Yourself

Major Browsers' Debugging Tools


Normally, you activate debugging in your browser with F12, and select "Console" in the debugger menu.

Otherwise follow these steps:


Firefox Firebug

Internet Explorer


Safari Firebug

Safari Develop Menu

Did You Know?

Debugging is the process of testing, finding, and reducing bugs (errors) in computer programs. The first known computer bug was a real bug (an insect) stuck in the electronics.