JavaScript - Chapter 15 - Debugging Techniques

Team Emertxe
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Debugging Techniques
JavaScript

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JavaScript Errors
(Capturing errors using JavaScript constructs)

Errors
In JavaScript three types of error are:
 Syntax Error: Occurs at compile / interpreting time
 Run Time Errors: Happens during execution. Exceptions and handling
happens here
 Logical Error: Occurs when we make mistake in logic

The try-catch
Syntax:
try
{
//statements
}
catch (error)
{
//statements
}
The Statement try is used to enclose and test parts of
the program where some problem is expected. If an
exception is encountered the control is shifted to catch
block.
To the catch block the problem is returned in form of
error object, which has two properties:
Name : Name of the error (category)
Description: Details about the error

The try-catch example
<script>
function errorFunc()
{
try {
// Write some junk
somejunk();
}
catch(e){
alert("Error Name : " + e.name);
alert("Error Message : " + e.message);
}
}
</script>
<body>
<button onclick=“errorFunc()”>Test Error Function</button>
</body>

The Error Object
Methods Description
RangeError A number "out of range" has occurred
ReferenceError An illegal reference has occurred
SyntaxError A syntax error has occurred
TypeError A type error has occurred
URIError An error in encodeURI() has occurred

The finally statement
The finally clause is used to execute
statements after the end of try block,
whether or not an exception occurred
within the try block.
Syntax:
try
{
// statements
}
catch (error)
{
// statements
}
finally
{
// statements
}

The throw statement
The throw statement allows to create
user defined conditions for
exceptions.
Syntax:
try
{
// statements
throw “statements”;
}
catch (error)
{
// statements
}

JavaScript Throw Example
<script>
function errFunc()
{
var x = Number(prompt("enter x value"));
var y = Number(prompt("enter y value"));
try{
if ( y == 0 ){
throw( "Divide by zero error." );
}
else
{
var z = x / y;
document.write("z ="+z+"<br>");
}
}
catch ( e ) {
alert("Error: " + e );
}
}
</script>

Exercise
• Write a JavaScript program to enter the age of any person and if age is less
than 18 then throw an exception “not eligible for voting”
• Write a JavaScript program to enter the number between 5 to 20. If the
number is not within range then throw an user defined exception

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Debugging
(Debugging the running program using debugger provided by browser)

Debugging
• All modern browsers have a built-in debugger. In our case Chrome browser is provided as a
case study
• These debuggers provide facility to walk through the program during run-time
• It will give you the live snapshot of the program, even alter the flow of the program by
forcing variable values
• By carefully investigating all the facilities provided by the debugger developers can empower
themselves
• However please note before getting into this run-time debugging, ensure previous steps
(Requirements understanding, Algorithm Design, Pseudo-code, Dry-run) are followed well.
• Best way to solve a problem is to avoid them 

The “debugger” keyword
• The debugger keyword stops the execution of JavaScript and calls the debugging
function
• To view the debugger window press F12
<script>
var x = 4 * 5;
debugger; //stop executing before its executes next line
document.getElementById("ex").innerHTML = x;
</script>

Exercise
• Check the given program and do the following:
• Various facilities provided
• Run-time walk through of the code
• Understand step-in and step-through options and differences between them
• Understand various segments of a running program (Code, Data, Stack, Heap)
• What is the call-stack and how it plays a role in function handling?

Breakpoints
 Break Points can be used to stop the execution of the code. They can
be set directly in the debugger without using ‘debugger’ keyword
 Multiple break-points can be set at required placed to monitor the
code flow. It will help you to investigate the source code at various
locations
 You can resume the execution of code by pressing the ‘play’ button, it
will further run or pause in the next break-point
 Depending on the issue, breakpoints will help you to narrow down to
the code area where the potential problem is there. Upon careful
investigation fixes can be done

Changing variable values in runtime
 There can be some situations during development / testing as follows:
 Developer not able to test various paths of the code (ex: if…else)
 Developer not able to exactly recreate the problem but knows the path
 Developer want to force certain conditions and see how the source code handles
• In such situations it would be helpful if the developer is able to force certain
variable value during runtime
• This is a crude approach, but helps during the development time. Achieving
100% test coverage using external methods may not be possible all the
times
• In Chrome you need to use the ‘console’ tab to set values

Exercise
• Check the given program and do the following:
• Remove debugger statement
• Setting multiple break-points and use play button to move between
• Investigate run-time snapshot between multiple break-points
• Force the code to take a different path in the if…else condition
• Add / Remove multiple break-points

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Avoiding Mistakes
(JavaScript – Strict Mode)

Strict Mode
• Strict mode is declared by adding "use strict"; to the beginning of a script
or a function.
• Declared at the beginning of a script, it has global scope.
"use strict";
// This will cause an error (x is not declared).
x = 3.14;

Strict mode
• Strict mode makes it easier to write "secure" JavaScript.
• Objects are variables too so without declaring object we cannot
initialize.
• In strict mode certain operations are not permitted (ex: Deleting an
object) are not allowed. This also helps to keep code safe by avoiding
some mistakes.
"use strict";
x = {p1:10, p2:20}; // error

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JavaScript - Chapter 15 - Debugging Techniques

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