var number;
document.write(number); //undefined

document.write(str); //错误

var num;
document.write(typeof num); //undefined
document.write(typeof str); //undefined

var num = null;
document.write(typeof num); //object

alert(null == undefined); //true

var num1 = null;
var num2;
document.write(typeof num1 + " " + typeof num2); //object undefined

var str = "helloworld";
document.write(Boolean(str)); //true

var str = "helloworld";
if (str){
  document.write("hellothere");
} //hellothere

var num1 = 070; //八进制的56
var num2 = 032; //八进制的32
var num3 = 079; //错误的八进制（>7）
var num4 = 09; //错误的八进制（>7）

var num1 = 0xA; //十六进制的10
var num1 = 0xa; //也是十六进制的10（字母不区分大小写）

var a = 0.1;
var b = 0.2;
if (a + b == 0.3){
  document.write("you are right")
} //因为0.1 加上0.2 实际上等于0.30000000000000004

document.write(isFinite(Number.MAX_VALUE + Number.MAX_VALUE)); //false

document.write(isNaN("a")); //true;
document.write(isNaN("324")); //false;
document.write(isNaN(true)); //false;
document.write(isNaN(false)); //false;
document.write(isNaN(NaN)); //true;

document.write(Number(true)); //1
document.write(Number(false)); //0
document.write(Number("789")); //789
document.write(Number(null)); //0
document.write(Number(undefined)); //NaN
document.write(Number("02.0942")); //2.0942
document.write(Number(0xa)); //10
document.write(Number("")); //0
document.write(Number("fdsa")); //NaN

document.write(parseInt("fds")); //NaN
document.write(parseInt("")); //NaN
document.write(parseInt("1111112abc")); //1111112
document.write(parseInt("-1111112abc")); //-1111112
document.write(parseInt("+1111112abc")); //-1111112
document.write(parseInt("0xa")); //10
document.write(parseInt("0022.00009")); //22
document.write(parseInt("070")); //ECMAScript 3认为是56（八进制）， ECMAScript 5认为是70（十进制）

document.write(parseInt("070",10)); //70
document.write(parseInt("070",8)); //56
document.write(parseInt("070",16)); //112

document.write(parseFloat("421")); //421
document.write(parseFloat("0421.32.1")); //421.32
document.write(parseFloat("0xaafd")); //0
document.write(parseFloat("070")); //70
document.write(parseFloat("070abc")); //70
document.write(parseFloat("")); //NaN
document.write(parseFloat("abc")); //NaN

document.write((533).toString(10)); //"533"
document.write((0xa).toString(10)); //"10"
document.write((0xa).toString(2)); //"1010"
document.write((true).toString(10)); //"true"
document.write((false).toString(10)); //"false"

document.write((null).toString(10)); //
document.write((undefined).toString(10)); //

document.write(String(null)); //"null"
document.write(String(undefined)); //"undefined"