Can't dynamically assign JS object property

user_14422715 0
Viewed 85

I'm trying to make a web app that takes in the names of several people and assigns each one a friend but am running into an issue assigning the friends.

The issue I'm running into is that it properly assigns the first few people friends, but always crashes on the second last person in the array and displays the error message

TypeError: Cannot set property 'friend' of undefined.

Here is a snippet from my app.js code.

const express = require("express");
const bodyParser = require("body-parser");

const app = express();
app.use(bodyParser.urlencoded({extended: true}));

let participantsObject = {};
let participantsArray = [];
let randomNumber;

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

app.post("/new", function(req, res) {
  participantsObject[req.body.person] = {};
  participantsArray.push(req.body.person);
  res.redirect("/new");
});

app.get("/setup", function() {
  let size = participantsArray.length;
  let participantsWithoutAGifter = participantsArray;

  for (var i = 0; i < size; i++) {
    randomNumber = Math.floor(Math.random() * participantsWithoutAGifter.length)
    participantsObject[participantsArray[i]]["friend"] = participantsWithoutAGifter[randomNumber];
    participantsWithoutAGifter.splice(randomNumber, 1);
  }

  console.log(participantsObject);

});

app.listen(3000, function() {
  console.log("Server started on port 3000");
});

And here is my new.html

<!DOCTYPE html>
<html lang="en" dir="ltr">
  <head>
    <meta charset="utf-8">
    <title></title>
  </head>
  <body>
    <form method="post">
      <input type="text" name="person" placeholder="Enter participant name" autofocus>
      <button type="submit">Add</button>
      <button type="button"><a href="/setup">Finished</a></button>
    </form>
  </body>
</html>

Ideally, it would be able to assign all participants a friend and then log the entire object.

Any help would be much appreciated, thanks in advance.

express node.js javascript
2 Answers

The property the loop is trying to select does not yet exist on the participantsObject.

Add a check if the property is there. If it is, then assign the friend property on the object. And if not, then assign a new object with a friend property.

for (var i = 0; i < size; i++) {
  randomNumber = Math.floor(Math.random() * participantsWithoutAGifter.length);
  let participantsIndex = participantsArray[i];
  if (participantsObject.hasOwnProperty(participantsIndex)) {
    participantsObject[participantsIndex]['friend'] = participantsWithoutAGifter[randomNumber]
  } else {
    participantsObject[participantsIndex] = {
      friend: participantsWithoutAGifter[randomNumber]
    }
  }
  participantsWithoutAGifter.splice(randomNumber, 1);
}
user_116196470 
// participantsObject[participantsArray[i]]["friend"] = participantsWithoutAGifter[randomNumber]; // <-- this will not work

const newProperty = 'friend'
participantsObject[participantsArray[i]] = {
  ...participantsObject[participantsArray[i]],  // <-- so that you don't lose other perperties
  [newProperty]: participantsWithoutAGifter[randomNumber]
}

Alternately you can use Object.assign

const newProperty = 'friend'
const source = {}
source[newProperty] = participantsWithoutAGifter[randomNumber]
Object.assign(participantsObject[participantsArray[i]], source )

Here is link to code: https://github.com/ApolloTang/stackoverflow-soln--cannot-dyn-assign-js-obj-props/blob/main/app.js

user_31368610
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