node.js - How to not duplicate data when using fs.writeFile?

Question

I'm trying to get some data from a simple form and write a JSON file using node JS and FS. I need this:

[{"id":1,"answer1":"C","answer2":"C"},{"id":2,"answer1":"A","answer2":"A"}]

This is the code I wrote:

const express = require("express");
const fs = require('fs');

const app = express();

const bodyParser = require("body-parser");
app.use(bodyParser.urlencoded({
     extended: true
}));

app.get("/", function(req, res) {
     res.sendFile(__dirname + "/form.html")
});

app.post("/", function(req, res) {

     let result1 = req.body.question1;
     let result2 = req.body.question2;

     const dataFile = require("./data.json");
     const userAnswers = {
          id: dataFile.length + 1,
          answer1: result1,
          answer2: result2,
     };

     dataFile.push(userAnswers);


     fs.writeFile("data.json", JSON.stringify(dataFile), 'utf8', function(err) {
          if (err) {
               console.log("An error occured while writing JSON Object to File.");
               return console.log(err);
          }

          console.log("JSON file has been saved.");

     });

     res.sendFile(__dirname + "/form.html");
});


app.listen(3000, function() {
     console.log("The server is running on 3000")
});

It actually works, the JSON file gets created and when I click on the submit button the inputs disappear, then I can continue putting new data.

The problem is that when I refresh the page, the last input gets duplicated in my JSON file. I don't know how to reset it. May someone help me?

Answer 1

It sounds like your refresh is resubmitting the previous data (some browsers will prompt you before doing that).

You could keep it from doing that by replacing:

res.sendFile(__dirname + "/form.html"); 

in your app.post() handler with this:

res.redirect("/");

The end-user display will still be the same, but a refresh on that page can't resubmit the previous form POST any more. It will just reload the form without any other side effects.