注：部分摘自《
JavaScript: The Definitive Guide, 5th Edition》
字符型转换为数值型可以使用下列方法
parseInt(stringVar); //parseInt还可以指定二进制、八进制、或16进制
parseFloat(stringVar);
Number(stringVar);
for example:
parseInt("ff") //will throw a error
parseInt("ff", 16) == parseInt("0xff") // return 255
parseInt("77") //return 77
parseInt("077") == parseInt("77", 8) // return 63
注：加"0"或"0x"前缀会自动检测并转换为相应的数制所表示的值
If parseInt( ) or parseFloat( ) cannot convert the specified
string to a number, it returns NaN
数值型转换为字符型
numberVar + "";
numberVar.toString(); //可以指定二进制、八进制、或16进制
String(numberVar);
other useful method
var n = 123456.789;
n.toFixed(0); // "123457"
n.toFixed(2); // "123456.79"
n.toExponential(1); // "1.2e+5"
n.toExponential(3); // "1.235e+5"
n.toPrecision(4); // "1.235e+5"
n.toPrecision(7); // "123456.8"
其实还有一种方法就是使用new操作符，如
new String(numberVar);
new Numer(stringVar);
但是这种方法返回的结果是object类型，而不是原始数据类型，大家可以酌情使用。
另外，在相等判断时使用' == '会自动转型（具体转换情况请参考其他资料），如果想精确判断请使用' === '。
如 1 == '1' //return true
1 === '1' //return false
1 == new Number(1) //return true
1 === new Number(1) //return false
数值连接等相关操作
"21" + "2" == "21" + 2 //return 212
2 + "21" //return 221
"21" * "2" == "21" * 2 == 2 * "21" //return 42
"21" / "3" == "21" / 3 == 21 / "3" //return 7
"21" - "2" == "21" - 2 == 21 - "2" == 21 - " 2 " //return 19
正如和Java中一样，new Number(3) == new Number(3)返回false，同理推广到其他类型，new操作符总是建立一个新对象，
而==只是比较其引用，并不比较对象本身，所以两个new的对象的引用总是不同的。所以在通常的逻辑判断中(如if或while等)，
最好避免使用Primitive Datatype Wrapper，而直接使用Primitive Datatype。
From Wrapper to Primitive, for example:
new Number(3).valueOf()
new String("str").valueOf()
new Date().valueOf() //convert a Date to millisecond representation
[any other object].valueOf() //The primitive value associated with the
object,
if any. If there is no value associated with
object, returns the
object itself.
Finally, note that any number, string, or boolean value can be
converted to its corresponding wrapper object with the Object( )
function:
var number_wrapper = Object(3);
Automatic datatype conversions
|
Value
|
Context in which value is used
|
|
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
|
利用上面的表，我们可以知道if ("") 、if (0)、if (undefined)、if (null)、if (NaN)，这些if语句的条件都为false.