> For the complete documentation index, see [llms.txt](https://nuclei-av.gitbook.io/malware-handbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nuclei-av.gitbook.io/malware-handbook/malware-analysis/malware-analysis-methodologies/analyzing-javascript-samples.md).

# 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.