Javascript的真值和相等

真值

Argument Type	Result
Undefined	false
Null	false
Boolean	The result equals the input argument (no conversion).
Number	The result is false if the argument is +0, −0, or NaN; otherwise the result is true.
String	The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
Object	true.

在Javascript中只有null, undefined, false, 0, -0, NaN, "" 为假值，其余数值均为真值。

相等

==算法

Type(x)	Type(y)	Result
x and y are the same type		See Strict Equality (===) Algorithm
null	Undefined	true
Undefined	null	true
Number	String	x == toNumber(y)
String	Number	toNumber(x) == y
Boolean	(any)	toNumber(x) == y
(any)	Boolean	x == toNumber(y)
String or Number	Object	x == toPrimitive(y)
Object	String or Number	toPrimitive(x) == y
otherwise…		false

toNumber算法

Argument Type	Result
Undefined	NaN
Null	+0
Boolean	The result is 1 if the argument is true. The result is +0 if the argument is false.
Number	The result equals the input argument (no conversion).
String	In effect evaluates Number(string) “abc” -> NaN “123″ -> 123
Object	Apply the following steps: 1. Let primValue be ToPrimitive(input argument, hint Number). 2. Return ToNumber(primValue).

toPrimitive算法

Argument Type	Result
Object	(in the case of equality operator coercion) if valueOf returns a primitive, return it. Otherwise if toString returns a primitive return it. Otherwise throw an error
otherwise…	The result equals the input argument (no conversion).

===算法

Type(x)	Values	Result
Type(x) different from Type(y)		false
Undefined or Null		true
Number	x same value as y (but not NaN)	true
String	x and y are identical characters	true
Boolean	x and y are both true or both false	true
Object	x and y reference same object	true
otherwise…		false