This Javascript MD5 implementation has me confused.

In the global space, the author declares a var:

var hexcase = 0; 

Later on, the following method appears:

function rstr2hex(input)
{
  try { hexcase } catch(e) { hexcase=0; }
  var hex_tab = hexcase ? "0123456789ABCDEF" : "0123456789abcdef";
  var output = "";
  var x;
  for(var i = 0; i < input.length; i++)
  {
    x = input.charCodeAt(i);
    output += hex_tab.charAt((x >>> 4) & 0x0F)
           +  hex_tab.charAt( x        & 0x0F);
  }
  return output;
}

The line that I don't understand is:

try { hexcase } catch(e) { hexcase=0; }

What is the author trying to acplish here?

This Javascript MD5 implementation has me confused.

In the global space, the author declares a var:

var hexcase = 0; 

Later on, the following method appears:

function rstr2hex(input)
{
  try { hexcase } catch(e) { hexcase=0; }
  var hex_tab = hexcase ? "0123456789ABCDEF" : "0123456789abcdef";
  var output = "";
  var x;
  for(var i = 0; i < input.length; i++)
  {
    x = input.charCodeAt(i);
    output += hex_tab.charAt((x >>> 4) & 0x0F)
           +  hex_tab.charAt( x        & 0x0F);
  }
  return output;
}

The line that I don't understand is:

try { hexcase } catch(e) { hexcase=0; }

What is the author trying to acplish here?

• javascript

• +1 for mentioning my favourite JS crypto site :) – Polynomial Commented Nov 30, 2011 at 16:12

5 Answers 5

He is just making sure hexcase is defined, and if it isn't, he is defining it.

Try putting

try {amIdefined} catch(e) {console.log('was not defined');}

in your console and you'll see...

Note that this is the safest way of making sure the variable is defined. In order to do

hexcase = hexcase || 0;

you need to do var hexcase first, or else you will get an error.

If hexcase does not exist, a ReferenceError is thrown, and the catch block is executed. If it does exist, the catch block is not executed.

So it sets hexcase to 0 if it does not exist.

It is a creative way of doing this, though. The more usual way is:

hexcase = window.hexcase || 0; // you have to add window because
                               // otherwise you would still get the error

Looks like hexcase is a global variable that the author is trying to check the existence of. Not sure that's the best way to do it though :-)

I'd go for:

if (typeof hexcase === "undefined") {
    hexcase = 0;
}

Just to make it explicit. You could use this too:

hexcase = hexcase || 0;

he's just checking to see if hexcase was defined, and if not sets a default.

apparantely that will decide whether the result is all uppercase or not...

That try-catch statement will set hexcase to 0 if it's undefined.