en,if not defined.

ok,try this:

for (i=2; i>=0; i--) {
if (mc == undefined) {
trace(void(mc));
_root.createEmptyMovieClip("mc", 1);
}
}

(mc == undefined) returns true and then create "mc".
it returns false in the next loop. So nothing in the output window.
if i use the this code:

for (i=2; i>=0; i--) {
if (mc == undefined) {
_root.createEmptyMovieClip("mc", 1);
}
trace(void(mc));
}

three "undefined" in output window.

看这里：
这是FlashMX的参考,它指出undefined只有一个值那就是undefined。
undefined类似与null


undefined

Availability

Flash Player 5.

Usage

undefined

Parameters

None.

Returns

Nothing.

Description

A special value, usually used to indicate that a variable has not yet been assigned a value. A reference to an undefined value returns the special value undefined. The ActionScript code typeof(undefined) returns the string "undefined". ..The only value of type undefined is undefined

When undefined is converted to a string, it converts to the empty string.

The value undefined is similar to the special value null. In fact, when null and undefined are compared with the equality operator, they compare as equal.

Example

In this example, the variable x has not been declared and therefore has the value undefined. In the first section of code, the equality operator (==) compares the value of x to the value undefined and the appropriate result is sent to the Output window. In the second section of code, the equality operator compares the values null and undefined.

// x has not been declared
trace ("The value of x is " + x);
if (x == undefined) {
trace ("x is undefined");
} else {
trace ("x is not undefined");
}

trace ("typeof (x) is " + typeof (x));
if (null == undefined) {
trace ("null and undefined are equal");
} else {
trace ("null and undefined are not equal");
}

The following result is displayed in the Output window:

The value of x is x is undefined
typeof (x) is undefined
null and undefined are equal
Note: In the ECMA-262 specification, undefined converts to the string "undefined", not the empty string. This is a difference between ActionScript and the ECMA-262 specification

拜托各位老大想想： 我对 demons 的疑问在于 无论 名为 "mc " 这个 instance 存不存在， void( mc ) 始终等于 undefined. 所以：

引用:

---

if (void (mc) == undefined) {

_root.createEmptyMovieClip("mc",1);

}

---

这个条件语句，有用么？

后来 demons 举了这个例子：

引用:

---

for (i=2; i>=0; i--) {
if (mc == undefined) {
_root.createEmptyMovieClip("mc", 1);
}
trace(void(mc));
}

---

我觉得这个例子并不能证明 void(mc) 在 mc 存在与否的时候有不同。

Please be PROFESSIONAL !

说的不对的请 各位老大指正 。

updated.

是啊，他这个马大哈！不过赶活的速度倒挺快的！

直接点这个进入吧：
http://www.dnvs.net/tutorial/flash_mx/FlashMX5.htm

师父啊！你的米筐精彩极了！爆笑之余，还学到了不少东西。继续加油啊！
另个问个问题：
txtUser.tabChildren=txtPass;
这句，我不知道有啥作用？去掉后运行依然正常。而且面板中找不到有tabChildren命令。给讲解一下行吗？