SOURCE

function writeDocument(variable){
            document.write(variable+"<br\>");
        }
        var welcomeString="Welcome";

        writeDocument(welcomeString.charAt(3));//提取
        writeDocument(welcomeString.charAt(6));

        var sentence="we have fun in our class.Do you have fun in your class?";
        writeDocument(sentence);//替换
        writeDocument(sentence.replace("class","programming class"));// 替换第一个出现的
        writeDocument(sentence.replace(/class/g,"programming class"));//替换所有出现的


        writeDocument(sentence.split(" ")); //分割  用空格来分割字符串
        writeDocument(sentence.split(""));
        writeDocument(sentence.split("."));//没有空格的，所以可以用来分割字符串每一个字符


        writeDocument(welcomeString.substring(3));    //截取
        writeDocument(welcomeString.substring(0,2));

        writeDocument(sentence.indexOf("class"));  //检索字符串的位置
        writeDocument(sentence.lastIndexOf("class"));
        writeDocument(sentence.match("class"));
        writeDocument(sentence.match(/class/g));
        writeDocument(sentence.search(/class/g));

        function countChar(stringToCount,char){
            var charCount=0;
            for(var i=0;i<stringToCount.length;i++){
                var char=stringToCount.charAt(i);
                if (char.toLowerCase() == "c") {
               charCount++
            }
            }
        return charCount;
        }
        
        var stringToCount="Can you can a can as a Canner can can a can";
        writeDocument("There are"+ countChar(stringToCount,"c")+"c in"+ stringToCount);

        
         
function countNumber(stringToCount) {
            var numCount = 0;
            for (var i = 0; i < stringToCount.length; i++) {
                var char = stringToCount.charAt(i);
                if (char != " " && !isNaN(char)) {
                     numCount++;
                }
            }
            return  numCount;
        }
        document.write(countNumber("1fdf4555"));