var i = 2;

Number.prototype.valueOf = function() {

return i++;

};

var a = new Number( 42 );

if (a == 2 && a == 3) {

console.log( "Yep, this happened." );

}

============================

"0" == null; // false

"0" == undefined; // false

"0" == false; // true -- UH OH!

"0" == NaN; // false

"0" == 0; // true

"0" == ""; // false

false == null; // false

false == undefined; // false

false == NaN; // false

false == 0; // true -- UH OH!

false == ""; // true -- UH OH!

false == []; // true -- UH OH!

false == {}; // false

"" == null; // false

"" == undefined; // false

"" == NaN; // false

"" == 0; // true -- UH OH!

"" == []; // true -- UH OH!

"" == {}; // false

0 == null; // false

0 == undefined; // false

0 == NaN; // false

0 == []; // true -- UH OH!

0 == {}; // false

[] == ![]; // true

2 == [2]; // true

"" == [null]; // true

0 == "\n"; // true

var a = null;

var b = Object( a ); // same as `Object()`

a == b; // false

var c = undefined;

var d = Object( c ); // same as `Object()`

c == d; // false

var e = NaN;

var f = Object( e ); // same as `new Number( e )`

e == f; // false

======================================================

ES5规范中的解释：

Comparing: objects to nonobjects

If an object/function/array is compared to a simple scalar primitive

(string, number, or boolean), the ES5 spec says in clauses

11.9.3.8-9:

1. If Type(x) is either String or Number and Type(y) is Object,

return the result of the comparison x == ToPrimitive(y).

2. If Type(x) is Object and Type(y) is either String or Number,

return the result of the comparison ToPrimitive(x) == y.

Comparing: nulls to undefineds

Another example of implicit coercion can be seen with == loose

equality between null and undefined values. Yet again quoting the

ES5 spec, clauses 11.9.3.2-3:

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

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

Comparing: anything to boolean

1. If Type(x) is Boolean, return the result of the comparison

ToNumber(x) == y.

2. If Type(y) is Boolean, return the result of the comparison x ==

ToNumber(y).

Comparing: strings to numbers

1. If Type(x) is Number and Type(y) is String, return the result of

the comparison x == ToNumber(y).

2. If Type(x) is String and Type(y) is Number, return the result of

the comparison ToNumber(x) == y.

==============================直观的比較图

http://dorey.github.io/JavaScript-Equality-Table/