SOURCE

//字符串
var a="welcome";
 document.write(a.charAt(2) + "<br/>");
 document.write(a.charAt(6) + "<br/>");
 document.write(a.substring(3) + "<br/>");
 document.write(a.substring(0,2) + "<br/>");
 //换词
 var b="you are my best miss.would you miss me?"
 document.write(b+"</br>");
 document.write(b.replace("miss","love")+"</br>");
 document.write(b.replace(/miss/g,"love")+"</br>");
 //分割
 document.write(b+"</br>");
 document.write(b.split("")+"</br>");
 document.write(b.split(" ")+"</br>");
 document.write(b.split(".")+"</br>");
 //找字
 document.write(b+"</br>");
 document.write(b.indexOf("you")+"</br>");//首次出现
 document.write(b.lastIndexOf("you")+"</br>");//最后一次出现
 document.write(b.match("you")+"</br>");//寻找有没有
 document.write(b.match(/you/g)+"</br>");//找所有
 document.write(b.search(/you/g)+"</br>");//找首次出现
 //如何找出字符串“Can you can a can as a Canner can can a can”中找出所有c的个数，不区分大小写。 
 function countChar(stringToCount, char) {
    var charCount = 0;
        for(var i=0;i<stringToCount.length;i++)
        {
            var char = stringToCount.charAt(i);
            //将每一个字符转换为小写，然后判断是否与“c”相等
            if (char.toLowerCase() == "c") {
                charCount += 1;
            }
        }
    return charCount;
}
var stringToCount = "Can you can a can as a Canner can can a can";
document.write("There are " + countChar(stringToCount, "c") + " c in " + stringToCount); 
function writeDocument(variable) {
    document.write(variable + "<br/>");
} 
var a=["HTML","CSS","JavaScript","jQuery","Vue.js" ];
 document.write(a + "<br/>");
 document.write(a[2] + "<br/>");
 a[3]="hello";
 a[5]="you";
 a[8]="me";
  document.write(a + "<br/>");
  //作业
  /*给大家一个任意字符串，然后统计一下里面有多少个数字。
  实现方法很简单，使用for循环结合charAt()方法来获取字符串中的每一个字符，然后判断该字符是否是数字就行。
使用isNaN（　）函数来判断一个值是否为NaN值。
NaN，意为Not a Numer（非数字）。如果该值不是数字，会返回true；如果该值是数字，会返回false。

isNaN会空格会转化成0，返回false*/


var stringtoCount="1a2b3c4d5eaabbcc889900";
function countNumber(stringtoCount) {

    var numCount = 0;
    for(var i=0;i<stringtoCount.length;i++){
        var char=stringtoCount.charAt(i);
        //isNaN()对空格字符会转化为0，需要加个判断charAt(i)不能为空格 
         if (char != " " && !isNaN(char)) { numCount++; }
    }
     return numCount;

}
document.write(stringtoCount+"有"+countNumber(stringtoCount)+"个数字");