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Refactoring a Blog Commenting System | Applying SOLID Principles to TypeScript

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Aug 28th, 2019 / 4 min read / Share / Edit on GitHub
In this article, we take some hastily written Node.js code for a blog commenting system and improve it by applying the SOLID principles.

This article is part of Solid Book - The Software Design & Architecture Handbook w/ TypeScript + Node.js. Check it out if you like this post.

Any code I don't care about the quality of, I call throwaway code. Throwaway code manifests when:

  • We're exploring new technology and testing something out.
  • We're more focused on speed of development and deployment > quality.
  • The code isn't expected to live a long lifetime.

In these scenarios, I might let my inner barbarian out 🦍.

But the moment we write code intended to solve a real problem or address a real need for a business, we should pay close attention to quality.

In fact, it's a good idea to practice writing quality code for even the throwaway stuff. You never know when something that you thought was throwaway code will turn into production code.


Recently, I deployed my own comment system on this blog using Express.js, TypeScript, Heroku, and MySQL based on Tania Rascia's guide.

Try it out. Leave a comment at the bottom of this post. Be nice, please?

The Use Cases that I wanted the general Public to be able to fulfill were to:

  • Post a comment => /api/comments
  • GET all the comments for a blog post => /api/comments?url=<slug>

So I quickly threw together an Express app and hooked up the API calls.

The Express server

Here's the main app.ts file. It has two functions getComments and postComment

import express from 'express'
import bodyParser from 'body-parser'
import cors from 'cors'
import { commentRepo } from './repos';
import { stripTrailingSlash } from './utils';

const app = express()

app.use(bodyParser.urlencoded({ extended: true }))

const getComments = async (req, res) => {
  const url = stripTrailingSlash(req.query.url);
  if (!!url === false || url.length < 2) {
    return res.status(500).json({ message: "not a valid request" })
  try {
    const comments = await commentRepo.getComments(url);
    return res.status(201).json({ comments: comments ? comments : [] });
  } catch (err) {
    return res.status(500).json({ message: 'Error', error: err.toString() })

const postComment = async (req, res) => {
  try {
    const { name, id, url, comment } = req.body;

    if (name.length < 2 || name.length >= 100) {
      return res.status(400).json({ message: 'Name needs to be within 2 and 100 chars' })

    if (comment.length < 2 || comment.length >= 3000) {
      return res.status(400).json({ message: 'Comment needs to be less than 3000' })

    if (!!id === false) {
      return res.status(400).json({ message: 'Id needs to be provided' })

    if (!!url === false) {
      return res.status(400).json({ message: 'Url needs to be provided' })

    await commentRepo.saveComment({ name, id, url, comment });

    return res.status(200).json({ message: 'Comment posted' });
  } catch (err) {
    return res.status(500).json({ message: 'Error', error: err.toString() })

app.get('/comments', getComments);'/comments', postComment);

app.listen(process.env.PORT || 9021, () => {
  console.log(`Server listening on 9021`)
import { Comment } from '../models/Comment';
import { CommentMap } from '../mappers/CommentMap';

export interface ICommentRepo {
  getComments (url: string): Promise<Comment[]>;
  saveComment (comment: Comment): Promise<any>;
  approveComment (commentId: string): Promise<any>;
  declineComment (commentId: string): Promise<any>;

export class CommentRepo implements ICommentRepo {
  private conn: any;

  constructor (conn: any) {
    this.conn = conn;

  getComments (url: string): Promise<Comment[]> {
    const query = `SELECT * FROM comments WHERE url = ?;`;
    return new Promise((resolve, reject) => {
      this.conn.query(query, [url], (err, result) => {
        if (err) return reject(err);
        return resolve( => CommentMap.toDomain(r)));

  saveComment (comment: Comment): Promise<any> {
    const query = `INSERT INTO comments 
    (id, name, comment, created_at, url, approved) 
    (?, ?, ?, ?, ?, ?)`;

    const values = [,, 
      new Date(),

    return new Promise((resolve, reject) => {
      this.conn.query(query, values, (err, result) => {
        if (err) return reject(err);
        return resolve();


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About the author

Khalil Stemmler

Khalil Stemmler is a Developer / Designer and co-founder of Univjobs. He frequently publishes articles about Domain-Driven Design and Advanced TypeScript & Node.js best practices for large-scale applications.

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