如果语句起作用，为什么这个简单的JavaScript不起作用？

Question

This if statement doesn't work. I would expect it to write "if" as the variable should be empty. Or failing that I would expect "else" but there are neither

You can also see it on JSFiddle in full not working glory

I think JSFiddle maybe having problems at the moment

checkforthread();

function checkforthread() { // Check to see if it is a new or existing chat

// Set the variable
        var existingthread = "";

      document.write("test");

      if (typeof(existingthread) == 'undefined' || variable == null) {
            document.write("if");
            gotostage3();   
            }
          else {
            document.write("else");
            gotostage3();   
          }
}

Answer 1

In JavaScript, if you try to take the value of an undefined symbol, you get a ReferenceError . That's what's happening here. variable is completely undefined, and so trying to take its value (so you can compare it with null ) is failing with a ReferenceError . You can see this in the Dev Tools of your browser:

This aborts the script code being run, since it's not caught.

If for some reason you need to check if a symbol is defined, there is a way to do that: The typeof operator, which you're already using:

if (typeof existingthread == 'undefined' || typeof variable == 'undefined') {

If you apply typeof to a symbol you haven't defined, it doesn't throw an error; instead, you get back the string "undefined" .

---

Note that typeof is an operator, not a function; no need to wrap its operand in parentheses.

Answer 2

variable是undefined ，这与JSFiddle中显示的错误相同。

Answer 3

If I assume that you mistyped and that variable is supposed to be existingthread , then what you are doing wrong is that existingthread is neither undefined nor null , it is an empty string!

you could simplify your if clause by just saying if (existingthread) { ... } , if I am guessing correctly what you are trying to achieve.