PaginationHelper.js

// https://www.codewars.com/kata/515bb423de843ea99400000a/train/javascript

// The constructor takes in an array of items and a integer indicating how many
// items fit within a single page
class PaginationHelper {
  
  constructor(collection, itemsPerPage) {
    this.collection = collection;
    this.itemsPerPage = itemsPerPage;
  }
  
  // returns the number of items within the entire collection
  itemCount() {
    return this.collection.length;
  }

  // returns the number of pages
  pageCount() {
    return Math.ceil(this.itemCount() / this.itemsPerPage);
  }

  // returns the number of items on the current page. page_index is zero based.
  pageItemCount(pageIndex) {
    // return -1 for pageIndex values that are out of range
    if(pageIndex < 0 || pageIndex >= this.pageCount()) {
      return -1;
    }
    
    if(pageIndex < this.pageCount() - 1) {
      return this.itemsPerPage;
    }
    
    const leftOver = this.itemCount() % this.itemsPerPage
    if(leftOver === 0) {
      return this.itemsPerPage;
    }
    
    return leftOver;
  }

  // determines what page an item is on. Zero based indexes
  pageIndex(itemIndex) {
    // return -1 for itemIndex values that are out of range
    if(itemIndex < 0 || itemIndex >= this.itemCount()) {
      return -1;
    }
    
    return Math.floor(itemIndex / this.itemsPerPage)
  }
  
  // return a new array of the items on that page
  getItemsOnPage(pageIndex) {
    // TODO: Write this method
  }
}

const emailList = [
    {
        id: 234,
        author: "Natalie",
        to: "Calvin",
        message: "Heyyyy",
        read: false,
    },
    {
        id: 1654,
        author: "Natalie",
        to: "Calvin",
        message: "What's up?",
        read: true,
    },
    {
        id: 474,
        author: "Dylan",
        to: "Calvin",
        message: "Good day",
        read: false,
    },
    {
        id: 234,
        author: "Natalie",
        to: "Calvin",
        message: "Heyyyy",
        read: false,
    },
    {
        id: 1654,
        author: "Natalie",
        to: "Calvin",
        message: "What's up?",
        read: true,
    },
    {
        id: 474,
        author: "Dylan",
        to: "Calvin",
        message: "Good day",
        read: false,
    },
    {
        id: 234,
        author: "Natalie",
        to: "Calvin",
        message: "Heyyyy",
        read: false,
    },
    {
        id: 1654,
        author: "Natalie",
        to: "Calvin",
        message: "What's up?",
        read: true,
    },
    {
        id: 474,
        author: "Dylan",
        to: "Calvin",
        message: "Good day",
        read: false,
    },
    {
        id: 234,
        author: "Natalie",
        to: "Calvin",
        message: "Heyyyy",
        read: false,
    },
    {
        id: 1654,
        author: "Natalie",
        to: "Calvin",
        message: "What's up?",
        read: true,
    },
    {
        id: 474,
        author: "Dylan",
        to: "Calvin",
        message: "Good day",
        read: false,
    },
    {
        id: 234,
        author: "Natalie",
        to: "Calvin",
        message: "Heyyyy",
        read: false,
    },
    {
        id: 1654,
        author: "Natalie",
        to: "Calvin",
        message: "What's up?",
        read: true,
    },
    {
        id: 474,
        author: "Dylan",
        to: "Calvin",
        message: "Good day",
        read: false,
    },
    {
        id: 234,
        author: "Natalie",
        to: "Calvin",
        message: "Heyyyy",
        read: false,
    },
    {
        id: 1654,
        author: "Natalie",
        to: "Calvin",
        message: "What's up?",
        read: true,
    },
    {
        id: 474,
        author: "Dylan",
        to: "Calvin",
        message: "Good day",
        read: false,
    },
]

const emailPagination = new PaginationHelper(emailList, 4);

let pageToShow = prompt("What page do you want to see?");
pageToShow = parseInt(pageToShow);

const emailsOnPage = emailPagination.getItemsOnPage(pageToShow).map(email => email.author + ": " + email.message).join("\n")
alert( "Emails:" + emailsOnPage )