Gotchas

Many variable scope nuances can be found throughout JavaScript. The following is by no means a comprehensive list, but it covers the most common cases:

The following will output 4, not 'undefined' as one would expect:
```
for (var n = 4; false; ) { } console.log(n);
```
This is due to the fact that, in JavaScript, variable definition happens at the beginning of the corresponding scope, not just when it is declared.
If you define a variable in the outer scope, and then have an if statement define a variable inside the function with the same name, even if that if branch isn't reached, it is redefined. An example:
```
var x = 1; function foo() {   if (false) {     var x = 2;   }   return x; } foo(); // Return value: 'undefined', expected return value: 2
```
Again, this is caused by moving the variable definition at the beginning of the scope with the undefined value.
In the browser, global variables are really stored in the window object.
```
window.a = 19; console.log(a); // Output: 19
```
a in the global scope means a as an attribute of the current context, so a===this.a and window object in a browser act as an equivalent of the this keyword in the global scope.

The first two examples are a result of a feature of JavaScript known as hoisting, which will be a critical concept in the next section about writing functions.