Variable

Variables are declared using the keyword “var”. For instance, the following statement creates a variable named “message”:

var message;

message holds the value “undefined” because no initial value is assigned to it. To notice that, if a variable is undeclared, it’s also undefined.

var message;

typeof message;           // "undefined"
typeof messageUndeclared; // "undefined"

However, an undeclared variable is different from an variable holding “undefined” value. Trying to use un undeclared variable may cause exceptions.

var message;

alert(message);           // "undefined"
alert(messageUndeclared); // "Uncaught ReferenceError: messageUndeclared is not defined"

Variable is defined at where “var” statement is executed. Do not access a variable declared afterwards because at that point, the variable isn’t available yet.

var a = 5;
(function() {
    console.log("a is " + (typeof a));    // "a is number"
    console.log("b is " + (typeof b));    // "b is undefined"
}());
var b = 6;

It’s noteworthy that assigning value to an undeclared variable accidentally creates a variable in the global scope.

(function() {
    b = 6;
}());
console.log(window.b);   // "6"