so if you have a function like this
var i = 5
function testvar () {
alert(i);
var i=3;
}
testvar();
the alert window will contain undefined. because internally, it's been changed into this:
var i = 5
function testvar () {
var i;
alert(i);
i=3;
}
testvar();
this is called "hoisting". The reason Crockford so strongly advocates var declarations go at the top, is that it makes the code visibly match what it's going to do, instead of allowing invisible and unexpected behavior to occur. function definitions are also hoisted to the top of the scope.
Putting a var inside an if statement is not against "the rules" of the language, but it means that, because of var hoisting, that var will be defined regardless of whether the if statement's condition is satisfied. Javascript also does not have block scope, so declaring a variable inside a block is doubly confusing to those who come from languages that do have block scope, because of hoisting.
Keep in mind also that hoisting does not include the assignment, so the var declarations will be moved to the top, and left undefined until they're assigned later, as in the example above.
This is a feature that must have seemed like a good idea at the time, but it has turned out to be more confusing than helpful.
