The use of undefined variables in if-statements

Question

This snippet results in a JavaScript runtime error: (foo is not defined)

if (foo) {
    // ...
}

I have to define foo

Answer 1

It seems strange question to me. Lets say I wanted to call a function:-

thing();

If thing hasn't be defined anywhere surely I would expect it to fail.

Why would you expect to use a variable called foo without defining it anywhere in your code?