Question

Why does this piece of code return - null - when it should return - - by my understanding – it seems to be treating null like a string.

var testvar = null;
alert(" - "+testvar+" - ");

Thats it. The same goes for undefined. I need this to work as I have an array, and I loop to loop through the array, and add each item to a variable, which is a string.

I have this:

//'resp' variable is a JSON response, decoded with JSON.parse.  This part works fine.
var addOnEnd=null;
for (item in resp) {
    console.log(">"+item);
    addOnEnd += item+"\n";
}

The console.log reads what I’d expect – a list of all the items in the response.

However, if I alert(addOnEnd) after the for loop, it returns ‘undefined’ (literally the string) and then the rest of the array as it should.

What am I doing wrong?

Answer 1

Change it to this:

alert(" - " + (testvar || "") + " - ");

…and this…

addOnEnd += (item || "") + "\n";

You will also need to initialize addOnEnd as an empty string instead of null.

This way, if the value is undefined (which when evaluated as a boolean returns false) it’ll use the ‘default’ value of an empty string.