javascript学习基础知识介绍

 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"