Function Expression

　　One of the key characteristics of function declarations is function declaration hoisting, whereby function declarations are read before the code excutes. That means a function declaration may appear after code that calls it and still work:

sayHi();
function sayHi(){
    alert("Hi!");
}

　　Function expressions act like other expressions and, therefore, must be assigned before usage. The following causes an error:

sayHi();              // error - function doesn't exist yet
var sayHi = function(){
    alert("Hi!");
};

　　It's advisable to always use arguments.callee of the function name whenever you're writing recursive functions.