Tuesday, 23 October 2018

Debug JavaScript Application in Chrome Browser

In this post, I am going to explain how to debug an application in Chrome Browser.

You are going to learn
a.   Key Terms in debugging the application
b.   How to add debug points and inspect the code

Key Terms in debugging the application
Breakpoint: It is the place, where you are informing debugger to stop execution. Once debugger stopped at breakpoint, you can inspect the data, stack traces etc.,

Step Over: When you click on this button, debugger move the control to next line.

Step Into: It is used to step into the function body.

Step out: Used to stepping out of the currently executing function.

How to add debug points and inspect the code?
Let me try to explain with an example.

HelloWorld.html
<!DOCTYPE html>
<html>

<body>

    <p>Click on the buttons to perform Arithmetic Operations</p>

    <button onclick="sum()">Add Numbers</button>

    <p id="demo"></p>

    <script>
        function sum() {
            var input1 = parseInt(prompt("Enter first Argument", 0));
            var input2 = parseInt(prompt("Enter Second Argument", 0));
            var result = input1 + input2;

            console.log("Sum of %d and %d is %d", input1, input2, result);
            document.getElementById("demo").textContent = "Result : " + result;
        }
    </script>

</body>

</html>

Open ‘HelloWorld.html’ in chrome browser.


Right click on the body of html page -> Inspect.

It opens developer tools on right side of the window. Click on Source tab, you can able to see the source code of the application.



Click on the file ‘HelloWorld.html’ under Page tab, it opens the source code.

Click on line numbers in the source code to add debug points.


For example, I added debug points on line 13 and 18.

Click on the button ‘Add Numbers’.

You can see that application stops at debugger point.


Click on ‘Step Over’ button to move to the new line.

After entering the user input, you can see the data at Scope section. This section displays all the local and global variable information. Beside scope section, there is a ‘Call Stack’ window, that gives information about application call stack.

At the bottom of Call Stack section, there is Breakpoints section, it shows all the breakpoints. You can disable the breakpoint by unchecking the checkbox.



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