Unclear Javascript snippet

Question

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 accomplish here?

+1 for mentioning my favourite JS crypto site :)

Polynomial
– Polynomial

2011-11-30 16:12:21 +00:00
Commented Nov 30, 2011 at 16:12 — Polynomial
– Polynomial, Commented Nov 30, 2011 at 16:12

hvgotcodes · Accepted Answer · 2011-11-30 16:15:15Z

8

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.

enter image description here

edited Nov 30, 2011 at 16:15

answered Nov 30, 2011 at 16:09

hvgotcodes

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3 Comments

Steve Over a year ago

Thanks! Is: if (hexcase === undefined) { hexcase = 0; } not a better way of doing this? Google Closure complains about this statement (which seems to lack side-effects).

Jakob Over a year ago

Global variable alert! Defining variables without "var" is like having sex without a condom. Comfortable, but you never know what you leave behind..

Jakob Over a year ago

Not that I support this global variable idea in general, but an even better way to test for undefined is to write if (typeof hexcase == "undefined"). The variable undefined can be changed, but typeof cannot.

pimvdb · Accepted Answer · 2011-11-30 16:16:47Z

6

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

edited Nov 30, 2011 at 16:16

answered Nov 30, 2011 at 16:10

pimvdb

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5 Comments

Steve Over a year ago

Curious; why hexcase = hexcase || 0 and not hexcase === undefined?

hvgotcodes Over a year ago

i get a reference error if I do this without at least var hexcase;

pimvdb Over a year ago

@hvgotcodes: You're entirely correct, good catch. @ Steve: It's a bit cleaner, it reads as "existing value or otherwise 0".

Steve Over a year ago

@pimvdb This is when I wish I could mark more than one answer as accepted. Thanks for the help. Much appreciated.

Matt Over a year ago

@Steve that comparison will work only if hexcase has already been declared. A declared variable without assignment is undefined. But you can't compare an undefined variabled to anything, even if it is 'undefined'

jabclab · Accepted Answer · 2011-11-30 16:10:50Z

2

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;

answered Nov 30, 2011 at 16:10

jabclab

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Comments

Leon · Accepted Answer · 2011-11-30 16:10:18Z

1

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...

answered Nov 30, 2011 at 16:10

Leon

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Comments

Viruzzo · Accepted Answer · 2011-11-30 16:14:31Z

0

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

answered Nov 30, 2011 at 16:14

Viruzzo

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