Writing array to JSON file

Question

I need to store an array of URLs associated with number in json file. This is what I got so far:

json[message.guild.id] = {
  songs: []
};
fs.readFile("./settings.json", "utf8", (err, data) => {
  if (err) {
    throw err;
  } else {
    json[message.guild.id] = JSON.parse(data);
  }
});
json[message.guild.id].songs.push(url); // adding a new URL
fs.writeFile("./settings.json", JSON.stringify(json, null, 4), err => {
  if (err) throw err;
});

And this does work, but if I try to save next URL it just overrides the existing one. What is wrong and how I can remove one of the saved URLs?

You are writing over the file, that's why. What you want to do is to append to it. check this out : stackoverflow.com/a/11267583/6212957 — Feelsbadman
– Feelsbadman, Commented Jan 5, 2019 at 19:19
It's not really clear what you are saving. It might help if you showed an example of the settings.json — Mark
– Mark, Commented Jan 5, 2019 at 19:31
Example: { "498208636234104846": { "songs": [ "https://www.youtube.com/watch?v=S81bLqpstUE" ] } } — AnB
– AnB, Commented Jan 5, 2019 at 19:34

GenericUser · Accepted Answer · 2019-01-05 19:41:48Z

1

Update 2

My first attempt overlooked the JSON issue, which I've left for posterity but you should not do because it will create an incorrect JSON object here.

However, I'm curious how you're kicking off this script. If it's the same block as you provided, you might be resetting the array for the message.guild.id every time. You could try protecting against it with something like this:

if(!json.hasOwnProperty[message.guild.id]){
    json[message.guild.id] = {};
} else if (!json[message.guild.id].hasOwnProperty('songs')){
    json[message.guild.id].songs = [];
}

Again this is just an assumption since we don't see the implementation.

Update fixed typos from paste

You need to add an options object to the writeFile method and set the flag key to 'a'. This tells writeFile to write in append mode. Documentation here and here.

json[message.guild.id] = {
  songs: []
};
fs.readFile("./settings.json", (err, data) => {
  if (err) {
    throw err;
  } else {
    json[message.guild.id] = JSON.parse(data);
  }
});
json[message.guild.id].songs.push(url); // adding a new URL
fs.writeFile("./settings.json", {encoding:"utf8", flag:'a'}, JSON.stringify(json, null, 4), err => {
  if (err) throw err;
});

edited Jan 5, 2019 at 19:41

answered Jan 5, 2019 at 19:23

GenericUser

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3 Comments

Mark Over a year ago

How does appending to a JSON file work? Won't this create malformed JSON?

GenericUser Over a year ago

You're absolutely correct. I was focusing on correcting the overwrite instead of the data format.

AnB Over a year ago

@Joe I still can't understand that, I want the structure of this file to be like:

{     "498208636234104846": {         "songs": [             "https://www.youtube.com/watch?v=S81bLqpstUE",             "https://www.youtube.com/watch?v=5lJhX6PGmMo"         ]     },     "457965143612194829": {         "songs": [             "https://www.youtube.com/watch?v=2YeDOa2XoeF",             "https://www.youtube.com/watch?v=cPOvNhET5gI"         ]     } }

This script is fired up when user executes a command to add a song URL for a given guild ID. I hope you can understand me.

Pedro Silva · Accepted Answer · 2019-01-05 19:20:36Z

0

You are using callback functions, so you can't use it like that

json[message.guild.id] = {
  songs: []
};
fs.readFile("./settings.json", "utf8", (err, data) => {
  if (err) {
    throw err;
  } else {
    json[message.guild.id] = JSON.parse(data);
    json[message.guild.id].songs.push(url); // adding a new URL
    fs.writeFile("./settings.json", JSON.stringify(json, null, 4), err => {
      if (err) throw err;
    });
  }
});

This should work however it seems to not make sense.

Take a look at async functions, callbacks and promises. Specially how callback works, your code has to be executed inside each function

answered Jan 5, 2019 at 19:20

Pedro Silva

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2 Comments

Feelsbadman Over a year ago

OP mentioned if I try to save next URL it just overrides the existing one - meaning that it's removing the existing content. So shouldn't he use the appendFile function instead of writeFile?

Pedro Silva Over a year ago

Yes, he should or use a flag the @joe answer stated. however the question has a problem with the callbacks

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