在javascript /可能的javascript错误中测试未定义的变量？

Question

So I have been working with javascript for a website I am designing, shocker I know, I was trying to find a way to test if a variable did not exist or wasn't defined. After getting through this I think being undefined and not existing are two different things. Also I think its highly unlikely I found a bug but maybe someone with a better understanding of Javascript can explain to me why the following code works the way it does.

<script type="text/javascript">
var t1="";
var t2;
if (t1==undefined) {document.write("t1 is undefined");}
if (t2==undefined) {document.write("t2 is undefined");}
</script> 

The above code returns "t2 is undefined".

<script type="text/javascript">
var t1="";
if (t1==undefined) {document.write("t1 is undefined");}
if (t2==undefined) {document.write("t2 is undefined");}
</script> 

This second code crashes I believe. So in the first code t2 exists but is not defined? and in the second code it needs to exist before it can be undefined? I just figured that if I did not write "var t2;" then tested for it, it would be undefined. Hopefully I have explained this question enough.

Answer 1

It's not a bug. In fact, the typeof operator is the only place where you can use an undeclared variable without getting an error.

See my answer Internet Explorer: "console is not defined" Error for a detailed explanation

edit:

This is how it's defined in the specs :

The production UnaryExpression : typeof UnaryExpression is evaluated as follows: 1. Let val be the result of evaluating UnaryExpression.
2. If Type(val) is Reference, then
a. If IsUnresolvableReference(val) is true, return "undefined".
...

Everywhere else, IsUnresolvableReference==true results in an error.

Answer 2

Well in your case if you have:

var t1 = "";

You will declare an empty string so it is normal to be "defined";

Whilst by doing:

var t2;

You are not defining it as any type of javascript object

Answer 3

It seems that there's a confusing naming convention here; We can say in the first case, you define variable t2 itself but it does not have a value defined. You can think the "undefined" property as the "value-undefined" to be more precise.

In the second case, you have t2 variable itself is NOT defined, thus you will get an error when you try to use it in code: "variable undefined", this error is because of a different reason but it is named similarly, making the situation a little confusing. Think this error as "variable-undefined" to be more precise.

I think you can just use the property "null" instead of the "undefined" in the first case to be clear.

var t1="";
var t2;
if (t1==null) {document.write("t1 is null");} 
if (t2==null) {document.write("t2 is null");} 

and;

var t1="";
if (t1==null) {document.write("t1 is null");} 
if (t2==null) {document.write("t2 is null");} // this line gives error because t2 variable is undefined