JavaScript Debugging
Errors can (will) happen, every time you write some new
computer code.
Code Debugging
Programming code might contain syntax errors, or logical
errors.
Many of these errors are difficult to diagnose.
Often, when programming code contains errors, nothing will
happen. There are no error messages, and you will get no indications where to
search for errors.
Searching for (and
fixing) errors in programming code is called code debugging.
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>
»
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.
Example
var x = 15 * 5;
debugger;
document.getElementbyId("demo").innerHTML
= x;
»
Major Browsers' Debugging Tools
Normally, you activate debugging in your browser with F12,
and select "Console" in the debugger menu.
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