Javascript string casting problem

Something wierd is happening in my javascript and i dont understand it. Can anyone explain?

var adsl2pSpeed = '9500 - 12500';

        alert(adsl2pSpeed);

        开发者_运维问答if (!adsl2pSpeed) {
            alert(adsl2pSpeed);
            var adsl2pSpeed = 'Unknown';
        }


        var speed = document.getElementById("PredictedSpeed");
        speed.innerHTML = adsl2pSpeed + " b/s";

This alerts "Undefined" twice and sets the innerhtml to be "Unknown". If I comment out the if statment it alerts '9500 - 12500' and sets the innerHTML to be '9500 - 12500'. Whats happening? Is the string is being cast as an object so it becomes null?

EDIT : I am actually registing the adsl2pSpeed as a startup script not in the function. I moved it up for clarity but possibly that is the problem?

---

Speculation:

The indented code is in a function.

   if (!adsl2pSpeed) {
        alert(adsl2pSpeed);
        var adsl2pSpeed = 'Unknown';
    }

You are in that function declaring a local variable, which masks the global variable, so it looks "undefined".

Try to remove the var to avoid making a new variable.

---

I think Thilo is right. If the code is in a function like:

var adsl2pSpeed = '9500 - 12500';

function test() {
 alert(adsl2pSpeed);
 if (!adsl2pSpeed) {
   alert(adsl2pSpeed);
   var adsl2pSpeed = 'Unknown';
 }
 var speed = document.getElementById("PredictedSpeed");
 speed.innerHTML = adsl2pSpeed + " b/s";
}

// some code runs ...  

test();

then the declaration of adsl2pSpeed inside the if-statement is "hoisted" to the top of the function, so that the function actually is interpreted like this:

function test() {
 var adsl2pSpeed;  // declaration hoisted to the top, shadows the global var
 alert(adsl2pSpeed);
 if (!adsl2pSpeed) {
   alert(adsl2pSpeed);
   adsl2pSpeed = 'Unknown';   // assignment to local var
 }
 var speed = document.getElementById("PredictedSpeed");
 speed.innerHTML = adsl2pSpeed + " b/s";
}

This article explains it: http://bustingseams.blogspot.com/2009/08/another-javascript-pitfall-hoisting.html