javascript权威指南(03~06)

3.12.自动类型转换

	String	Number	Boolean	Object
Undefined value	"undefined"	NaN	false	Error
null	"null"	0	false	Error
Nonempty string	As is	Numeric value of string or NaN	TRue	String object
Empty string	As is	0	false	String object
0	"0"	As is	false	Number object
NaN	"NaN"	As is	false	Number object
Infinity	"Infinity"	As is	true	Number object
Negative infinity	"-Infinity"	As is	TRue	Number object
Any other number	String value of number	As is	true	Number object
true	"true"	1	As is	Boolean object
false	"false"	0	As is	Boolean object
Object	toString( )	valueOf( ), toString( ), or NaN	true	As is

3.13. 简单数据进行运算的时候，会短暂的转换成相应的对象，因为相关方法是定义在对象上面的，运算完了，这个对象也马上销毁。

var s = "hello world";       // A primitive string value

var S = new String("Hello World");  // A String object

typeof运算会返回不同的结果

3.14.对象进行数值运算的时候，通过valueOf() ,toString()进行运算。但是在当对象是Date的+时候有个例外，优先toString()

3.15.4. 传值与传址

Type	Copied by	Passed by	Compared by
number	Value	Value	Value
boolean	Value	Value	Value
string	Immutable	Immutable	Value
object	Reference	Reference	Reference

4.2.1.变量声明

在函数中未通过var声明的变量，且全局也没这变量，程序会自行声明成一个全局变量，所以变量声明，需要var;

 
function a()
{
　　b=10;
}
a(); //window.b=10;

4.3.1.没有块级作用域

主要是要了解js的执行顺序，js是先解释，再执行的，解释的时候会先看见相关的var定义操作。

var scope = "global";
function f() {
　　alert(scope); // Displays "undefined", not "global"
　　var scope = "local"; // Variable initialized here, but defined everywhere
　　alert(scope); // Displays "local"
}
f();
//这是另一个例子
var x; // Declare an unassigned variable. Its value is undefined.
alert(u); // Using an undeclared variable causes an error.会出错
u = 3; // Assigning a value to an undeclared variable creates the variable.

5.10.4.delete操作

涉及到属性的时候，操作只是删除相关的引用，不会删除所指向的对象

var o = {x:1, y:2}; // Define a variable; initialize it to an object
delete o.x; // Delete one of the object properties; returns true
typeof o.x; // Property does not exist; returns "undefined"
delete o.x; // Delete a nonexistent property; returns true
delete o; // Can't delete a declared variable; returns false修正好像IE8和FF都显示为true，可删除
delete 1; // Can't delete an integer; returns true IE8提示错误，FF为returns true
x = 1; // Implicitly declare a variable without var keyword
delete x; // Can delete this kind of variable; returns true
x; // Runtime error: x is not defined

6.9. for/in

for/in循环体中删除还未列举的属性，则该属性不会再被列举

for/in循环体中添加属性，则根据情况可能列举添加的属性，可能是由于列举是无顺序的导致的

for/in不会列举所有属性。

6.15. return

函数通过return;或执行完返回的是 undefined (void 0; 同样)