• Distribution:
  • Firebug (which is both a debugger and a profiler) is distributed as a Firefox plug-in
• Setting Up:
  • Downloading the plug-in will start the installation process (which you will need to "Allow")
  • You may need to re-start Firefox to complete the installation

1. Start Firefox
2. Start Firebug by clicking on in the toolbar
3. Select the Script tab
4. Load the HTML page containing the code to debug
5. Select the file containing the JavaScript of interest using the drop-down
6. Set breakpoints (by clicking on the line number) as needed
7. Use the code (e.g., by generating mouse or keyboard events, by re-loading the page) which will cause the execution to pause at the breakpoint
8. Step through the code as needed

• Normal Breakpoints:
  • Set/clear by clicking on the line number
• Conditional Breakpoints:
  • Set by right-clicking on the line number
  • Enter the boolean expression (which can involve any variable/function that is in scope)
  • Clear by clicking on the line number

• Continue :
  • Run continuously until the next breakpoint is reached
• Step Into :
  • Execute a single instruction (if it is a function, invokes the function)
• Step Over :
  • Execute a single instruction (if it is a function, execute the entire function)
• Step Out :
  • Execute the rest of the current function

• The Watch Window:
  • See the contents of all variables that are in-scope
  • Expand/contract objects
  • See the current value of this
  • Create watch expressions involving variables that are in-scope
• The Call Stack:
  • All of the function calls that were required to reach the currently executing statement

Debugging the BMI Calculator

ecmascriptexamples/clientsidebasics/bmi.js

/**
 * A function to calculate and display the body mass index (BMI)
 *
 * @param {number} weight - The weight in pounds
 * @param {number} height - The height in pounds
 */
function calculateBMI(weight, height)
{
   "use strict"; // ECMAScript 5
   var bmi;
   
   bmi = weight / (height * height) * 703;

   if (bmi < 15){
      bmi = 15;
   }else if (bmi > 60){
      bmi = 60;
   }

   return bmi;
}