var s = new MyString('hello');

    s.length; //

    s[0]; // "h"

    s.toString(); // "hello"

    s.valueOf();  // "hello"

    s.charAt(1); // "e"

    s.charAt('2'); // "l"

    s.charAt('e'); // "h"

    s.concat(' world!'); // "hello world!"

    s.slice(1, 3); // "el"

    s.slice(0, -1); // "hell"

    s.split('e'); // ["h", "llo"]

    s.split('l'); // ["h", "", "o"]

    s.split(); // ["hello"]

    s.reverse(); // "olleh"

function MyString(str) {

    var str = str.toString();

    var arr = [];

    for (var i in str) {

        this[i] = arr[i] = str[i];

    }

    this.length = arr.length;

    this['[[PrimitiveValue]]'] = str;

}

MyString.prototype.toString = function () {

    return this['[[PrimitiveValue]]']+'';

}

MyString.prototype.valueOf = function () {

    return this['[[PrimitiveValue]]'];

}

MyString.prototype.charAt = function (pos) {

    var pos = pos*1 || 0;

    return this[pos] || '';

}

MyString.prototype.concat = function (strings) {

    return this.toString()+strings;

}

MyString.prototype.slice = function (start,end) {

    var start = start>=0?start : this.length+start || 0; //-1

    var end   = end  >=0?end   : this.length+end || 0; //-1

        start = start <= this.length ? start:this.length;

        end   = end   <= this.length ? end:this.length;

    var str = '';

    for (var i = start; i<end; i++) {

        str+=this[i];

    }

    return str;

}

MyString.prototype.split = function (separator) {

    var arr = [];

    var index=0;

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

        if(this[i]==separator) {

            arr[arr.length] = this.slice(index,i);

            index = i+1;

        }

    }

    arr[arr.length] = this.slice(index,i);

    return arr;

}

// for循环版

MyString.prototype.reverse = function () {

    var str = '';

    for (var i = this.length-1; i>-1; i--) {

        str += this[i];

    }

    return str;

}

//借助数组的reverse()方法

MyString.prototype.reverse = function () {

    var arr = [];

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

        arr[arr.length] = this[i];

    }

    return arr.reverse().join('');

}