每天一篇javascript学习小结（基础知识）

 var s1 = "01";  var s2 = "1.1";  var s3 = "z";//字母'z'无法转换为数字，所以或返回NaN  var b = false;  var f = 1.1;  var o = {    valueOf: function() {    return -1;   }  };    s1 = -s1; //value becomes numeric -1  s2 = -s2; //value becomes numeric -1.1  s3 = -s3; //value becomes NaN  b = -b;  //value becomes numeric 0  f = -f;  //change to -1.1  o = -o;  // 1 先执行对象的valueOf()方法返回-1,--1为1　　　　o = +o;//-1 o的valueOf()值为-1，+-1还是-1

 var result1 = 5 - true; //4 because true is converted to 1  var result2 = NaN - 1;  //NaN NaN不是一个数字，和任何数字做任何运算都是NaN  var result3 = 5 - 3;  //2  var result4 = 5 - "";  //5 because "" is converted to 0  var result5 = 5 - "2";  //3 because "2" is converted to 2  var result6 = 5 - null; //5 because null is converted to 0

 var value1 = 10;  var value2 = true;  var value3 = null;  var value4;//value4 没有赋值就是underfined 转换为字符串就是'underfined'    alert(String(value1));  //"10"  alert(String(value2));  //"true"  alert(String(value3));  //"null"  alert(String(value4));  //"undefined"

var num = 10;  alert(num.toString());  //"10"默认十进制  alert(num.toString(2));  //"1010"二进制  alert(num.toString(8));  //"12"八进制  alert(num.toString(10));  //"10"十进制  alert(num.toString(16));  //"a"十六进制

var result1 = 5 > 3; //true  var result2 = 5 < 3; //false  var result3 = "Brick" < "alphabet"; //true 字符串比较按照字母表顺序进行比较 小写字母在大写字母后面  var result4 = "Brick".toLowerCase() < "alphabet".toLowerCase(); //false字母表顺序比较  var result5 = "23" < "3"; //true 字符2小于字符3  var result6 = "23" < 3; //false 此时'23'转换成23了  var result7 = "a" < 3; //false because "a" becomes NaN 字符'a'无法转换成数字  var result8 = NaN < 3; //false NaN 和任何数字比较，都无法转换成数字，所以一直是false  var result9 = NaN >= 3; //false

  var num1 = parseInt("AF", 16);  //175 按照16进制输出十进制数据 10*16+15*1  var num2 = parseInt("AF");   //NaN 没有指定进制，默认按照10进制转换，由于AF不在十进制范围，返回NaN  alert(num1);  alert(num2);

  var num1 = parseInt("1234blue"); //1234  var num2 = parseInt("");   //NaN 字符''无法转换成数字   var num3 = parseInt("0xA");   //10 - hexadecimal 16进制的A  var num4 = parseInt(22.5);   //22  var num5 = parseInt("70");   //70 - decimal  var num6 = parseInt("0xf");   //15 16进制为15

  var num1 = Number("Hello world!"); //NaN  var num2 = Number("");    //0 空字符串可以转换成0 这个parseInt()不一样   var num3 = Number("000011");  //11  var num4 = Number(true);   //1

  alert(NaN == NaN);  //false  alert(isNaN(NaN));  //true  alert(isNaN(10));  //false ?10 is a number  alert(isNaN("10"));  //false ?can be converted to number 10  alert(isNaN("blue")); //true ?cannot be converted to a number  alert(isNaN(true));  //false ?can be converted to number 1

var result = Number.MAX_VALUE + 1;alert(isFinite(result)); // false

  alert(5 * 6);   //30  alert(5 * NaN);   //NaN  alert(Infinity * 0); //NaN  alert(Infinity * 2); //Infinity  alert("5" * 5);   //25  alert(true * 10);  //10  alert(false * 10);  //0　　　　　　alert(26 % 5); //1　　　　　　alert(Infinity % 3); //NaN　　　　　　alert(3 % 0); //NaN　　　　　　alert(5 % Infinity); //5　　　　　　alert(0 % 10); //0　　　　　　alert(true % 25); //1　　　　　　alert(3 % false); //NaN

for (var propName in window) {    document.write(propName);    document.write("<br />");  }

 alert(null == undefined); //true  alert(null === undefined); //false    alert("NaN" == NaN);  //false  alert("NaN" === NaN);  //false  alert(NaN == NaN);   //false  alert(NaN === NaN);   //false  alert(NaN != NaN);   //true  alert(NaN !== NaN);   //true    alert(false == 0);   //true  alert(false === 0);   //false  alert(true == 1);   //true  alert(true === 1);   //false    alert(null == 0);   //false  alert(undefined == 0);  //false    alert(5 == "5");   //true  alert(5 === "5");   //false  

 var message = "Hello world!";  var messageAsBoolean = Boolean(message);    alert(messageAsBoolean); //true

 var num = 0;  //返回到特定的位置  outermost:  for (var i=0; i < 10; i++) {    for (var j=0; j < 10; j++) {    if (i == 5 && j == 5) {     break outermost;    }    num++;   }  }    alert(num); //55

var iNum = 0;outermost:for (var i=0; i<10; i++) { for (var j=0; j<10; j++) { if (i == 5 && j == 5) { continue outermost; } iNum++; }}alert(iNum); //输出 "95"