Analyzing JavaScript Samples

1. Introduction to JavaScript Analysis:

• Tools Used:

  • Popular Browsers: Microsoft Edge, Google Chrome, Mozilla Firefox.

  • Developer Tools: Integrated within browsers to debug or inspect webpages.

  • Visual Studio Code: Another tool to analyze JavaScript code.

• JavaScript File Locations:

  • Embedded in HTML source code.

  • As standalone .js files.

• HTML Requirement:

  • JavaScript debugging often requires embedding the script in an HTML file since browsers standardly read HTML.

2. Practical Analysis - JavaScript Files:

• Samples to Analyze: (say)

  • re_basicjs.js

  • re_texrjs.html

• Preparation:

  • Restored VM image, copied and extracted course files, and ran fakeness.

3. Analyzing re_basicjs.js:

• Initial Inspection:

  • Use Notepad++ to inspect the JavaScript sample.

  • Rotten Tomatoes Function:

    • Appears to perform string character replacements.

    • Used with variables str1 and str2, which are joined into a variable named flag.

• Loading the JavaScript File in a Browser:

  • Step 1: Write an HTML loader file in Notepad++ to load re_basicjs.js.

  • Step 2: Save the file as loader.html.

  • Step 3: Open loader.html in Firefox by dragging and dropping it into the browser.

4. Debugging with Firefox Developer Tools:

• Opening Developer Tools:

  • Press F12 to open Developer Tools in Firefox.

• Locating the JavaScript File:

  • Go to the Debugger tab, expand the file tree under the Main Thread section.

  • Locate re_basicjs.js.

• Setting a Breakpoint:

  • Place a breakpoint on line 14, where str1 is assigned.

• Reloading the Page:

  • Press F5 to reload the page and trigger the breakpoint.

• Using Debugging Tools:

  • Step Over (F10): Skip over function calls.

  • Step In (F11): Step into function calls for detailed analysis.

• Watching Variables:

  • Add the flag variable to the Watch Expressions window to track its value as the script executes.

  • Alternatively, hover the mouse pointer over the flag variable to reveal its content.

5. Key Takeaways:

• Combining Tools:

  • Use text editors like Notepad++ for initial inspection and browser Developer Tools for dynamic debugging.

• HTML Integration:

  • Essential to house JavaScript within HTML for effective debugging in browsers.

• Debugging Strategies:

  • Breakpoints and watch expressions are crucial for understanding the flow and outcomes of JavaScript variables and functions.

• Practical Approach:

  • Skipping complex functions like Rotten Tomatoes can save time when the main objective is to reveal key variable contents, like the flag in this case.