Building Expressions

As we mentioned earlier in this lesson, an expression is a phraseor a collection of variables, numbers, text, and operatorsthat evaluates to a value. To understand this concept, take a look at this example:

var oneDozen:Number = 6 * 2;

To the right of the equals sign, you see 6 * 2this is an expression. When this script is executed, the expression 6 * 2 is replaced by the result of 6 multiplied by 2, or 12. Thus, oneDozen equals 12.

An expression can contain variables, arrays, even function callsanything that, when evaluated, returns a value. For example:

var total:Number = subTotal + tax;

The value of total is based on the result of adding the variable subTotal to the variable tax. The expression is subTotal + tax.

var discount:Number = totalPrice * employeeDiscount[2];

The value of discount is based on the result of multiplying the variable totalPrice by the value of the third element in the employeeDiscount array; totalPrice * employeeDiscount[2] is the expression.


var usDollars:Number = 10;

var japaneseYen:Number = convertToYen(usDollars);

the value of japaneseYen is based on the value returned by a call to the convertToYen() functionmaking the function call itself the expression.


For more information about functions, see Lesson 5, "Using Functions."

Expressions are used to do the following:

  • Set variable values

  • Set array element values

  • Determine whether conditions are being met (using the comparison operators)

  • Dynamically name movie clips, variables, and objects

  • Call functions dynamically

  • And more

Expressions enable you to avoid hard-coding values that will remain the same no matter what. By assigning and manipulating values via expressions, you can make the data used by your scripts dynamicresulting in more interactive projects. Many of the scripts in this book rely on expressionsthat's because without them, your project plays back in exactly the same way each time it's viewed.