The following tip allows you to quickly perform calculations via JavaScript in Firefox and Chrome.

Basic idea: If you enter a javascript: URL in Firefox, the result of the expression is shown in the window. For example, try the following URL:

javascript:7*45

• Name: Compute with JavaScript
• Location: javascript:%s
• Keyword: js

    js 7*45

Google Chrome: handles output and keywords differently than Firefox.

• JavaScript code to display a result: As Webkit browsers don’t replace the page content with the result of a JavaScript expression, you need to explicitly display the result, e.g. via alert():

      javascript:alert(7*45)
  

• Creating a keyword: Go to “Preferences → Basics → Search → Manage Search Engines...” and create a new entry with the following URL:

      javascript:alert(%s)
  

• Arithmetic operators: addition (+), subtraction (-), multiplication (*), and division (/), modulo (%).

      > 77 / 11
      7
      > 3.16 * 5
      15.8
      > 10 % 7
      3
  

• Properties and methods of the global Math object [2]:
  • Math.PI
  • Math.pow(base, exponent)
  • Math.sqrt(x)
  • Math.sin(x)
• Converting to and from hexadecimal, binary, etc.

      > (255).toString(16)
      'ff'
      > parseInt("ff", 16)
      255
  

  You need to put the number in parenthesis to invoke a method on it. Otherwise the dot will be mistaken as a decimal point. Alternatively, you can type two dots or append .0 to the number.

From http://www.2ality.com/2011/06/javascript-calculator.html