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Prefix increment/decrement

If you place this line of code into an HTML page, and click the link, you see

“[Object]”

printed on the

screen (Figure 2-3). This occurs because

window.open()

returns a reference to the newly opened win-

dow (this and other methods of the window are discussed further in Chapter 5, “JavaScript in the

Browser”). That object is then converted to a string for display.

Figure 2-3

To avoid this, use the

window.open()

call with the void operator:

<a href=”javascript:void(window.open(‘about:blank’))”>Click Me</a>

This makes the

window.open()

call return

undefined

, which is not a valid value and is not displayed

in the browser window. Remember, functions that have no return value actually return

undefined

Prefix increment/decrement

Two operators taken directly from C (and Java) are prefix increment and prefix decrement. Prefix

increment, which adds one to a number value, is indicated by placing two plus signs (++) in front of a

variable:

var iNum = 10;

++iNum

The second line increments

iNum

. This is effectively equal to:

var iNum = 10;

iNum = iNum + 1;

Likewise, the prefix decrement subtracts one from a value. The prefix decrement is indicated by two

minus signs (– –) placed before the variable:

var iNum = 10;

--iNum;

In this example, the second line decreases the value of

iNum

When you use prefix operators, note that the increment/decrement takes place

before

the expression is

evaluated. Consider the following example:

Chapter 2

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