Building Yarncom Phase 2: Automation, Word Counts, and the "Arrow Function" Trap

I’m currently on Day 2 of building Yarncom, a community-driven blogging API. Yesterday, I handled the Security Handshake. Today, I faced the Logic Layer.
The goal was simple; Whenever a user saves a blog post, the API should automatically calculate the Estimated Reading Time based on a 200-words-per-minute average.
Here is the technical breakdown of the challenges I faced and the code that solved them.

1. The Automation Logic (The Pre-Save Hook)
Instead of forcing the user to tell us how long their post is, I used a Mongoose pre-save hook. This is a function that runs after the data is sent but before it’s persisted in the database.
blogSchema.pre('save', async function() {
if (this.body) {
const wordsPerMinute = 200;
const words = this.body.split(/\s+/).length;
this.reading_time = Math.ceil(words / wordsPerMinute);
console.log( Calculated: ${this.reading_time} min);
}
});

2. The Trap: Arrow Functions vs. Regular Functions
This is where I got stuck. I initially used an Arrow Function: async () => { ... }.
The Problem: Arrow functions do not bind their own this. In Mongoose, this needs to point to the Document being saved. Because I used an arrow, this.body was undefined, and my terminal was silent.
The Fix: Always use a regular function() in Mongoose hooks if you need to access the data.

3. Cleaning up the "Next" Callback
I encountered the "next is not a function" error. This usually happens when you try to call a callback that isn't provided in an async context. By switching to a pure async function for my hook, I removed the need for the next() entirely. Mongoose is smart enough to wait for the Promise to resolve before moving to the save.

4. Result in Postman
After a few "failed renders," I finally got that beautiful 201 Created status. The response now includes:
"reading_time": 1"message": "Yarn successfully spun!"

What I Learned

Backend engineering is as much about Environment Awareness as it is about syntax. You have to know where your this is pointing and who is holding the "Baton" in the middleware chain.

node #javascript #mongodb #webdev #tutorial