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Errors can (will) happen, every time you write some new computer code.
JavaScript Debuggers
Debugging is not easy. But fortunately, all modern browsers have a built-in JavaScript 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:
Example
<!DOCTYPE html>
<html>
<body>
<h1>My First Web Page</h1>
<script>
a = 5;
b = 6;
c = a + b;
console.log(c);</script>
</body>
</html>
Tip: Read more about the console.log() method in our JavaScript Console Reference.
<!DOCTYPE html>
<html>
<body>
<h1>My First Web Page</h1>
<script>
a = 5;
b = 6;
c = a + b;
console.log(c);</script>
</body>
</html>
Tip: Read more about the console.log() method in our JavaScript Console Reference.
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).
Major Browsers' Debugging Tools
Normally, you activate debugging in your browser with F12, and select "Console" in the debugger menu.
Otherwise follow these steps:
Chrome
- Open the browser.
- From the menu, select tools.
- From tools, choose developer tools.
- Finally, select Console.
Firefox Firebug
- Open the browser.
- Go to the web page:
http://www.getfirebug.com
- Follow the instructions how to:
install Firebug
http://www.getfirebug.com
install Firebug
Internet Explorer
- Open the browser.
- From the menu, select tools.
- From tools, choose developer tools.
- Finally, select Console.
Opera
- Open the browser.
- Go to the webpage:
http://dev.opera.com
- Follow the instructions how to:
add a Developer Console button to your toolbar.
http://dev.opera.com
add a Developer Console button to your toolbar.
Safari Firebug
- Open the browser.
- Go to the webpage:
http://safari-extensions.apple.com
- Follow the instructions how to:
install Firebug Lite.
http://safari-extensions.apple.com
install Firebug Lite.
Safari Develop Menu
- Go to Safari, Preferences, Advanced in the main menu.
- Check "Enable Show Develop menu in menu bar".
- When the new option "Develop" appears in the menu:
Choose "Show Error Console".
Choose "Show Error Console".
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.
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.
The first known computer bug was a real bug (an insect) stuck in the electronics.
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