JavaScript provides no good way to check if a variable exists.  If you try to use one that doesn't, you get a big fat error.  I've seen some people use:

if (myVar == undefined)

But this only works if the variable has been declared, but wasn't set to a value.  Sometimes, you don't know if the variable was even declared.

Here's some code that solves that: 

try {
      if (myVar) {}
} catch (err) {
      var myVar = "";
}

You can do whatever you want in the catch block. Initializing the variable, as I show, is a good way to avoid errors you would've gotten if the variable didn't exist.