Relay Inspired Pagination

relay-inspired-pagination.js

const mockData = require("./mock-data.json");

const resolvers = {
  Query: {
    courses: (parent, args, context) => {
      let edges = mockData;

      let first = args.first;
      let after = args.after;
      let before = args.before;
      let last = args.last;
      let hasPreviousPage = false;
      let hasNextPage = false;

      if (typeof after === 'string') {
        let opaqueCursor = Number(Buffer.from(after, 'base64').toString('ascii'));
        let edgeIndex = mockData.findIndex(data => data.id === opaqueCursor);

        if (edgeIndex !== -1) {
          edges = edges.slice(edgeIndex + 1)
        }
      }

      if (typeof before === 'string') {
        let opaqueCursor = Number(Buffer.from(before, 'base64').toString('ascii'));
        let edgeIndex = mockData.findIndex(data => data.id === opaqueCursor);

        if (edgeIndex !== -1) {
          edges = edges.slice(0, edgeIndex);
        }
      }

      if (typeof first !== "undefined" && first < 0) {
        throw new Error('"first" should be gth 0');
      }

      if (typeof last !== "undefined" && last < 0) {
        throw new Error('"last" should be gth 0');
      }

      if (edges.length > first) {
        hasNextPage = edges.length > first;
        edges = edges.slice(0, first);
      }

      if (edges.length > last) {
        edges = edges.slice(edges.length - last, edges.length + 1);
      }

      let start = 0;
      let startCursor = null;
      let endCursor = null;

      edges = edges.map((data, index) => {
        endCursor = Buffer.from(`${data.id}`).toString('base64');

        if (index === 0) {
          startCursor = endCursor
        }

        return {
          cursor: endCursor,
          node: data
        };
      });

      return {
        totalCount: edges.length,
        edges,
        pageInfo: {
          startCursor,
          endCursor,
          hasPreviousPage,
          hasNextPage
        }
      };
    }
  }
};

module.exports = resolvers;