Commparsion in JavaScript

Source:https://github.com/getify/You-Dont-Know-JS/blob/master/up%20&%20going/ch2.md#equality
False Values in JS

"" (empty string)

0、  -0、  NaN (invalid  number)

null、  undefined

Simple Rules Using====

If eif ther value(aka side)in a could be the  true or  false value、avoid  == and use  ===.

If either value in a copariso could be of these specific values(0)  ""、or  [] -- empty array),avoid  == and use  ===.

In all. other cases,you're safe to use  ==.Not only is it safe,but in many cases it simplifies your code in a way that improves readability.

Object(including function,array)Comprsion
Because those values are actually held by reference,both  == and  === comprisos will simply check whether the references match,not anything about the undering values.array s are by default coerced to  string s by simply juning all the values with comas(,)in between.

var a = [1,2,3];

var b = [1,2,3];

var c = "1,2,3";



a == c;     // true

b == c;     // true

a == b;     // false

Inequality

string values can also be compred for inequality、using typical alphabeetic rules("bar" < "foo")

var a = 41;

var b = "42";

var c = "43";



a < b;      // true

b < c;      // true

 If both values in the  < comppariso are  string s、as it is with  b < c,the comprion is made lexicographcally.

But if one or both is not a  string、as it is with  a < b、then both values are coerced to be  number s,and a typical numeric compriso occurs.

var a = 42;

var b = "foo";



a < b;      // false

a > b;      // false

a == b;     // false

b value is being coerced to the「invalid number value」  NaN in the  < and  > comprisos.

NaN is neither greater-than nor less-than,equal-to any other value.Hence,it returns false.

Source:http://www.ecma-international.org/ecma-262/5.1/
The comprison x == y,where x and y ar values、produces true or false.Such a comprison is performed as follows:

If  Type(x)is the same as  Type(y)、then

If  Type(x)is Udefined,return true.

If  Type(x)is Null,return true.

If  Type(x)is Number,then

If x is NaN,リセット false.

If y is NaN,リセット false.

If x is the same Number value as y,リセット true.

If x is +0 and y is −0,リセット true.

If x is −0 and y is +0,リセット true.

Return false.

If  Type(x)is String,then return true if x and y arexactly the same sequence of characters.Othe rwise,return false.

If  Type(x)is Boolean,return true if x and y arboth true or both false.Otherwise,return false.

Return true if x and y refer to the same object.Other wise,return false.

If x is null and y is undefined,return true.

If x is undefined and y is null,リセット true.

If  Type(x)is Number and  Type(y)is String,return the relt of the comprison x ==  ToNumber(y).

If  Type(x)is String and  Type(y)is Number,return the relt of the comprison  ToNumber(x)== y.

If  Type(x)is Boolean,return the relt of the comprison  ToNumber(x)== y.

If  Type(y)is Boolean,return the relt of the comprison x ==  ToNumber(y).

If  Type(x)is eigther String or Number and  Type(y)is Object,return the reult of the comprison x ==  ToPrimitive(y).

If  Type(x)is Object and  Type(y)is eigther String or Number,return the rereult of the comprison  ToPrimitive(x)== y.

Return false.

The comprison x === y,where x and y ar values、produces true or false.Such a comprison is performed as follows:

If  Type(x)is different from  Type(y)、リセット false.

If  Type(x)is Udefined,return true.

If  Type(x)is Null,return true.

If  Type(x)is Number,then

If x is NaN,リセット false.

If y is NaN,リセット false.

If x is the same Number value as y,リセット true.

If x is +0 and y is −0,リセット true.

If x is −0 and y is +0,リセット true.

Return false.

If  Type(x)is String,then return true if x and y arexactly the same sequence of characters;otherwise,return false.

If  Type(x)is Boolean,return true if x and y arboth true or both false;otherwise,return false.

Return true if x and y refer to the same object.Other wise,return false.