ECMA-262 ===算法说明

Question

I was trying to understand the exact algorithm for the === operator in JavaScript. It is defined as something like

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

1. If Type(x) is different from Type(y), return false.
2. If Type(x) is Undefined, return true.
3. If Type(x) is Null, return true.
4. If Type(x) is Number, then
   • If x is NaN, return false.
   • If y is NaN, return false.
   • If x is the same Number value as y, return true.
   • If x is +0 and y is −0, return true.
   • If x is −0 and y is +0, return true.
   • Return false.
5. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.
6. If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
7. Return true if x and y refer to the same object. Otherwise, return false.

Now if I write something like

var t1 = undefined,t2 = 2;
typeof(t1); //"undefined"
typeof(t2); //"number"

t1 === t2; //returns false ?????

Consider point 2 and 3: It should return true instead. I am testing it in Chrome 15.0.874.106 m. Can somebody explain what exactly is going on in this case?

Answer 1

You have to go in order, If Type(x) is different from Type(y), return false. . Since false is already returned, it never gets to point 2 or 3.

Answer 2

Considering 1: If Type(x) is different from Type(y), return false. ,
t1 === t2 should indeed return false.

t1 is undefined , while t2 is a number.