After we've created a variable, we may assign and reassign its value as often as we like, as shown in Example 2-1.
var firstName; // Declare the variable firstName firstName = "Graham"; // Set the value of firstName firstName = "Gillian"; // Change the value of firstName firstName = "Jessica"; // Change firstName again firstName = "James"; // Change firstName again var x = 10; // Declare x and assign a numeric value x = "loading...please wait..."; // Assign x a text value
Notice that we changed the variable x's datatype from numeric to text data by simply assigning it a value of the desired type. Some programming languages don't allow the datatype of a variable to change but ActionScript does.
Of course, creating variables and assigning values to them is useless if you can't retrieve the values later. To retrieve a variable's value, simply use the variable's name wherever you want its value to be used. Anytime a variable's name appears (except in a declaration or on the left side of an assignment statement), the name is converted to the variable's value. Here are some examples:
newX = oldX + 5; // Set newX to the value of oldX plus 5 ball._x = newX; // Set the horizontal position of the // ball movie clip to the value of newX trace(firstName); // Display the value of firstName in the Output window
Note that in the expression ball._x, ball is a movie clip's name, and the ._x indicates its x-coordinate property (i.e., horizontal position on stage). We'll learn more about properties later. The last line, trace(firstName), displays a variable's value while a script is running, which is handy for debugging your code.
Occasionally we may wish to verify that a variable has been assigned a value before we make reference to it. As we learned earlier, a variable that has been declared but never assigned a value contains the special "non-value," undefined. To determine whether a variable has been assigned a value, we compare that variable's value to the undefined keyword. For example:
if (someVariable != undefined) { // Any code placed here is executed only if someVariable has a value }
Note the use of the inequality operator, !=, which determines whether two values are not equal.
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