SOURCE

  function writeDocument(variable) {
        document.write(variable + "<br/>");
    }


    var welcomeString= "welcome";

    writeDocument(welcomeString);
    writeDocument(welcomeString.length);

    writeDocument("//---case-----------------------------------------------");
    writeDocument(welcomeString.toLowerCase());
    writeDocument(welcomeString.toUpperCase());

    writeDocument("//---charAt-----------------------------------------------");
    writeDocument(welcomeString.charAt(2));
    writeDocument(welcomeString.charAt(6));

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

    writeDocument("//---replace-----------------------------------------------");
    var sentence = "We have fun in our class. Do you have fun in your class?"
    writeDocument(sentence);
    writeDocument(sentence.replace("class", "programming class"));
    writeDocument(sentence.replace(/class/g, "programming class"));

    writeDocument("//split-----------------------------------------------");
    var splitSentence = "We have fun in our class";
    writeDocument(splitSentence);
    writeDocument(splitSentence.split(" "));
    writeDocument(welcomeString);
    writeDocument(welcomeString.split(""));
    writeDocument(sentence.split(""));

    writeDocument("//IndexOf-----------------------------------------------");
    writeDocument(sentence.indexOf("class"));
    writeDocument(sentence.lastIndexOf("class"));
    writeDocument(sentence.match("class"));
    writeDocument(sentence.match(/class/g));
    writeDocument(sentence.search(/class/g));

    function countChar(stringValue, char) {
        var charCount = 0;

        for (var i = 0; i < stringValue.length; ++i) {
            if (stringValue.charAt(i).toLowerCase() == char) {
                ++charCount;
            }
        }

        return charCount;
    }
    var stringValue = "Can you can a can as a Canner can can a can";
    writeDocument("There are " + countChar(stringValue, "c") + " 'c' in sentence.");

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(countNumber("45621fsasc"));