When we first define a variable, it is assigned the value undefined by default:

var velocity;

To the interpreter, the preceding statement reads:

var velocity = undefined;

To check whether a variable has a value, we can compare the variable to undefined, as in:

if (myVariable != undefined) {
  // myVariable has a value, so proceed as desired...
}

Note that an undefined value is converted to the empty string when used as a string. For example, if firstName is undefined, the following trace( ) statement will display "" (the empty string):

var firstName;
trace(firstName);  // Displays nothing (the empty string)

This same code in JavaScript would display the string "undefined" instead of the empty string. ActionScript converts undefined to "" for the sake of backward compatibility.

if (myVar eq "") { 
  // Don't do anything yet: myVar is undefined 
}

If Flash 5 converted undefined to anything other than "" in a string context, old code like that would break in the Flash 5 player.

Note that ActionScript returns undefined both for variables that do not exist and variables that have been declared but have no value. This is also a departure from JavaScript, where references to variables that do not exist cause an error.

4.7. The Boolean Type		4.9. Null