cursor.js

"use strict";

/**
 * An example implementation of cursor-based pagination.
 * The "database" is a singly-linked list of foods.
 * The server uses the cursor to find the last returned record, so
 * it can begin the next page after the cursor.
 *
 * The cursor is base64 encoded JSON with "type" and "id" fields.
 * The "type" allows servers to tag pagination strategies, and change
 * implementations later without users knowing about it.
 * A "version" key would also make sense.
 */

let records = `{"1":{"id":"1","name":"eggs","tastiness":7,"next":"2"},"2":{"id":"2","name":"toast","tastiness":5,"next":"3"},"3":{"id":"3","name":"kale","tastiness":3,"next":"4"},"4":{"id":"4","name":"rice","tastiness":4,"next":"5"},"5":{"id":"5","name":"burrito","tastiness":9,"next":"6"},"6":{"id":"6","name":"hot pot","tastiness":8,"next":"7"},"7":{"id":"7","name":"reuben","tastiness":10,"next":"8"},"8":{"id":"8","name":"potato salad","tastiness":8,"next":null}}`;

Database.prototype = {
  fetch(id) {
    return this.records[id];
  },
  ids() {
    return Object.keys(this.records).sort();
  },
  fetchFrom(id, limit, filters = []) {
    let first = id === "undefined" ? this.ids()[0] : this.fetch(id).next;
    let current = this.fetch(first);
    let results = [];
    while (current && results.length < limit) {
      if (filters.every((f) => f(current))) results.push(current);
      current = this.fetch(current.next);
    }
    return results;
  },
};

function Database(records) {
  this.records = records;
}

Server.prototype = {
  encodeCursor(id) {
    let str = `{"type":"direct", "id":"${id}"}`;
    return Buffer.from(str).toString("base64");
  },

  fetchFrom(id, limit, filters = []) {
    let data = this.db.fetchFrom(id, limit, filters);
    if (data.length === 0) return { data, afterCursor: null };

    let afterCursor = this.encodeCursor(data[data.length - 1].id);
    return { data, afterCursor };
  },

  fetch(limit, filters = [], cursor = this.encodeCursor()) {
    let data = JSON.parse(Buffer.from(cursor, "base64"));
    switch (data.type) {
      case "offset":
        return this.fetchOffset(data.offset, limit, filters);
      case "direct":
        return this.fetchFrom(data.id, limit, filters);
      default:
        console.error(`unexpected cursor type: ${data.type}`);
    }
  },
};

function Server(db, defaultCursor = "direct") {
  this.db = db;
  this.defaultCursor = defaultCursor;
}

const PAGE_SIZE = 3;
const FILTERS = [(x) => x.tastiness > 3];

let server = new Server(new Database(JSON.parse(records)));

let currentPage = server.fetch(PAGE_SIZE, FILTERS);
let i = 1;

while (currentPage.afterCursor) {
  console.log(`PAGE ${i++}`);
  console.log(`--------`);
  console.log(currentPage);
  currentPage = server.fetch(PAGE_SIZE, FILTERS, currentPage.afterCursor);
}

console.log(currentPage); // the last page whose cursor is `null`