typeof '11' //string 
typeof(11) 　//number
'11' < 4 //false

//隐式转换 + - * == / 
// + 
10 + '20' //'2010'
// -
10 - '20' //-10
10 - 'one' //NaN
10 - '100a' //NaN
// *
10*'20' //200
'10'*'20' //200
// /
20/'10' //2
'20'/'10' //2
'20'/'one'　 //NaN

// ==
undefined == null; //true
'0' == 0; 　　 //true,字符串转数字
0 == false; //true,布尔转数字
'0' == false; 　　　//true,两者转数字
null == false; 　 //false
undefined == false; 　//false

Math.sqrt(-2)
Math.log(-1)
0/0
parseFloat('foo')

console.log(typeof NaN); // 'Number'

var x = Math.sqrt(-2);
var y = Math.log(-1);
console.log(x == y); // false

var x = Math.sqrt(-2);
var y = Math.log(-1);
console.log(x === y); // false

console.log(NaN === NaN); // false

console.log(isNaN(NaN)); // true

console.log(isNaN('hello')); // true
console.log(isNaN(['x'])); // true
console.log(isNaN({})); // true

var My = {
 isNaN: function (x) { return x !== x; }
}

console.log(Number.isNaN(NaN)); // true
console.log(Number.isNaN(Math.sqrt(-2))); // true
console.log(Number.isNaN('hello')); // false
console.log(Number.isNaN(['x'])); // false
console.log(Number.isNaN({})); // false

0 == []; // true, 0 == [].valueOf(); ---> 0 == 0;
'0' == []; // false, '0' == [].toString(); ---> '0' == '';
2 == ['2']; // true, 2 == ['2'].valueOf(); ---> 2 == '2' ---> 2 == 2;
'2' == [2]; // true, '2' == [2].toString(); ---> '2' =='2';

[] == ![]; //true, [].valueOf() == !Boolean([]) -> 0 == false ---> 0 == 0;

var valueOf = function (){
 var str = this.toString(); //先调用toString(),转成字符串
 //...
}
0 == []; // true, 0 == [].valueOf(); -> 0 == '0' -> 0 == 0;

var a = [1];

a.valueOf = function (){ return 1;}
a.toString = function (){ return '1';}

a + 1; // 2, valueOf()先调用

var a = [1];

a.valueOf = function (){ return 1;}
a.toString = function (){ return '1';}

a + 1; // 2, 先调用valueOf()
//去掉valueOf
delete a.valueOf;
a + 1; // '11', 调用toString()

var a = [1];

a.valueOf = function (){return ;}
a.toString = function (){return 1 ;};

1 - a; //NaN

var a = {};
a.valueOf(); //Object {}
var a = [];
a.valueOf(); //[] 自己本身
var a = new Date();
a.valueOf(); //1423812036234 数字
var a = new RegExp();
a.valueOf(); // /(?:)/ 正则对象