function fn(str) {

    var obj = {};

    for (var i = 0; i < str.length; i++) {

        if (!obj[str.charAt(i)]) {

            obj[str.charAt(i)] = 1;

        } else {

            obj[str.charAt(i)]++;

        }

    }

    var value = '';

    var count = 0;

    for (var i in obj) {

        if (obj[i] > count) {

            count = obj[i];

            value = i;

        }

    }

    console.log('出现次数最多的是' + value + ',出现次数为' + count);

}