String.prototype.mysubstring=function(beginIndex,endIndex){

    var str=this,

        newArr=[];

    if(!endIndex){

        endIndex=str.length;

    }

    for(var i=beginIndex;i<endIndex;i++){

        newArr.push(str.charAt(i));

    }

    return newArr.join("");

}

//test

"Hello world!".mysubstring(3);//"lo world!"

"Hello world!".mysubstring(3,7);//"lo w"

String.prototype.mysubstring=function(beginIndex,endIndex){

    var str=this,

        strArr=str.split("");

    if(!endIndex){

        endIndex=str.length;

    }

    return strArr.slice(beginIndex,endIndex).join("");

}

//test

console.log("Hello world!".mysubstring(3));//"lo world!"

console.log("Hello world!".mysubstring(3,7));//"lo w"

String.prototype.mysubstring=function(beginIndex,endIndex){

    var str=this,

        beginArr=[],

        endArr=[];

    if(!endIndex){

        endIndex=str.length;

    }

    for(var i=0;i<beginIndex;i++){

        beginArr.push(str.charAt(i));

    }

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

        endArr.push(str.charAt(i));

    }

    return str.replace(beginArr.join(""),"").replace(endArr.join(""),"");

}

//test

console.log("Hello world!".mysubstring(3));//"lo world!"

console.log("Hello world!".mysubstring(3,7));//"lo w"

　模拟一个HashTable类，有add、remove、containes、length方法

var HashTable =function(){

    this.container={

        length:0

    };

}

HashTable.prototype={

    add:function(key,value){

        if(key in this.container){

            return false;

        } else {

            this.container[key] = value;

            this.container.length++;

            return true;

        }

    },

    remove:function(key){

        if(key in this.container){

            delete this.container[key];

            this.container.length--;

            return true;

        }

    },

    containes:function(key){

        return (key in this.container);

    },

    length:function(){

        return this.container.length;

    }

}

var test = new HashTable();

test.add(1,123);

test.add(1,123);

test.add(2,123);

test.add(3,123);

test.add(4,123);

test.add(5,123);

console.log(test.containes(3));//true

console.log(test.length());//5

test.remove(3);

console.log(test.containes(3));//false

console.log(test.length());//4