Buying as many items for as much money I cant find a way to implement a certain solution. Let me flesh out the problem.

Somebody gives you X amount of money and sends you to the shop to buy Y amount of items. You must spend as much money as you can and buy as many items as you can. You cant have more money they X. You cant buy more than Y amount of items. Some money can left afterwards, but as little as it can. You can buy less than Y items, but only if there is no other option. You cant buy same item more than once - items are unique.

Examples:

You have 15£, and need to buy 2 items. There are 4 items for 10£, 5£, 4£ and 3£. You buy 10£ and 5£ items. Perfect scenario - 15£ spend, 2 items aquired.

You have 15£ and have to buy 3 items. There are 4 items for 4£, 3£, 2£, 1£. You buy 4£, 3£ and 2£ items. 9£ and 3 items aquired.

You have 15£ and have to buy 2 items. There are 4 items for 20£, 19£ and 14£. You buy only 14£ item.

I think you understand by now how it works.

Implementation(Javascript):

var arrayOfItems = [];
arrayOfItems[0] = {"name":"Sandwich","price":15};
arrayOfItems[1] = {"name":"Water","price":10};
arrayOfItems[2] = {"name":"Cookie","price":5};

function getItems(moneyAmount,itemsAmount){
    //your code here....
}

var items1 = getItems(21,2); //returns [0,2]
var items2 = getItems(25,2); //returns [0,1]
var items3 = getItems(14,2); //returns [1]


I need to invent the inside of getItems() function. Can you lead me how to do this?

Thank you for help.
This is a classic Knapsack problem  some examples: Knapsack.java, JS example

Here is a pseudo code from Introduction to Algorithms, MIT: