js参数转换，各种类型参数在另一种类型里代表什么

3.12. Type Conversion Summary

As each datatype has been described in the previous sections, I've discussed how values of each type convert into values of other types. The basic rule is that when a value of one type is used in a context that requires a value of some other type, JavaScript automatically attempts to convert the value as needed. So, for example, if a number is used in a Boolean context, it is converted to a boolean. If an object is used in a string context, it is converted to a string. If a string is used in a numeric context, JavaScript attempts to convert it to a number. Table 3-3 summarizes each of these conversions and shows the conversion that is performed when a particular type of value is used in a particular context.

Table 3-3. 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