backend.js

const debug = require('debug')('backend'),
      { Pool } = require('pg');

module.exports = function createTodoBackend(connectionString) {
  const pool = new Pool({
    connectionString: connectionString,
  });

  function query(query, params, callback) {
    debug(`Query ${query} with params ${params}`);
    pool.query(query, params, function(err, result) {
      if (err) {
        debug(`Database error ${err}`);
        callback(err);
        return;
      }

      callback(null, result.rows);
    });
  }

  return {
    all: function(callback) {
      query('SELECT * FROM todos', [], callback);
    },

    get: function(id, callback) {
      query('SELECT * FROM todos WHERE id = $1', [id], function(err, rows) {
        callback(err, rows && rows[0]);
      });
    },

    create: function(title, order, callback) {
      query('INSERT INTO todos ("title", "order", "completed") VALUES ($1, $2, false) RETURNING *', [title, order], function(err, rows) {
        callback(err, rows && rows[0]);
      });
    },

    update: function(id, properties, callback) {
      var assigns = [], values = [];
      if ('title' in properties) {
        assigns.push('"title"=$' + (assigns.length + 1));
        values.push(properties.title);
      }
      if ('order' in properties) {
        assigns.push('"order"=$' + (assigns.length + 1));
        values.push(properties.order);
      }
      if ('completed' in properties) {
        assigns.push('"completed"=$' + (assigns.length + 1));
        values.push(properties.completed);
      }

      var updateQuery = [
        'UPDATE todos',
        'SET ' + assigns.join(', '),
        'WHERE id = $' + (assigns.length + 1),
        'RETURNING *'
      ];

      query(updateQuery.join(' '), values.concat([id]), function(err, rows) {
        callback(err, rows && rows[0]);
      });
    },

    delete: function(id, callback) {
      query('DELETE FROM todos WHERE id = $1 RETURNING *', [id], function(err, rows) {
        callback(err, rows && rows[0]);
      });
    },

    clear: function(callback) {
      query('DELETE FROM todos RETURNING *', [], callback);
    }
  };
};