Where is "Unexpected end of JSON input" coming from?

Here's my code:

const fs = require("fs");
const path = require("path");

module.exports = class Product {
  constructor(t) {
    this.title = t;
  }

  save() {
    const p = path.join(
      path.dirname(process.mainModule.filename),
      "data",
      "products.json"
    );
    fs.readFile(p, (err, fileContent) => {
      let products = [];
      if (!err) {
        products = JSON.parse(fileContent);
      }
      products.push(this);
      fs.writeFile(p, JSON.stringify(products), err => {
        console.log(err);
      });
    });
  }

  static fetchAll() {
    return products;
  }
};

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The error that I'm getting is:

SyntaxError: Unexpected end of JSON input
    at JSON.parse (<anonymous>)
    at fs.readFile (C:\wamp\www\adm\node\express_tutorial\models\product.js:18:25)
    at FSReqWrap.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:53:3)
[nodemon] app crashed - waiting for file changes before starting...

This is line #18: products = JSON.parse(fileContent);

Two questions: First of all, what am I missing that would result in the error that I'm getting?

Secondly, what is "18:25?" I'm referring to the actual error message. "18," I'm assuming, is line #18, but what is "25?"

Thanks!