JavaScript Discord bot array iteration issue with async callbacks

Other Questions Discord bot
1

I’m building a Discord bot in JavaScript and running into a weird problem with array iteration. When I try to loop through an array and make API calls, the loop variable seems to get messed up inside the callback function.

Here’s my code:

const Discord = require('discord.js');
const fetch = require('node-fetch');
const TOKEN = "your_bot_token";
const API_KEY = "?key=your_api_key";
const STREAM_API = "https://api.example.com/status/";
const client = new Discord.Client();
var streamers = ["streamer_a", "streamer_b", "streamer_c"];

client.login('bot_token_here');

client.on("message", function(msg) {
    if (msg.content === "!status") {
        for (var j = 0; j < streamers.length; j++) {
            fetch(STREAM_API + streamers[j] + API_KEY)
                .then(response => response.json())
                .then(data => {
                    if (data.online == false) {
                        msg.reply(streamers[j] + ' is offline');
                    } else {
                        msg.reply(streamers[j] + ' is online');
                    }
                });
        }
    }
});

The strange thing is that when I log the loop counter before the API call, it shows 0, 1, 2 correctly. But inside the callback function, it always shows 3. This makes the bot display “undefined is online/offline” instead of the actual streamer names. The loop itself works fine, but something happens with the variable inside the async callback. Anyone know what’s causing this behavior?

2

The problem is that var has function scope, not block scope. When the fetch promises resolve, the loop’s already finished and j equals 3 - so all closures reference the same variable. Switch to let instead since it creates a new binding each iteration. You could also use forEach directly on the streamers array or wrap your fetch in an IIFE to capture the current value. I prefer forEach because it’s more readable.

3

Had this exact same issue with my first Discord bot. It’s JavaScript’s event loop timing - your for loop finishes instantly while the fetch requests are still running. When those promises finally resolve, the loop variable j has already hit 3, so you’re getting undefined since streamers[3] doesn’t exist. Don’t rely on the loop variable - pass the streamer name directly into your promise chain. Try fetch(STREAM_API + streamers[j] + API_KEY).then(response => response.json()).then(data => { /* use streamers[j] here becomes the actual streamer name */ }) or even better, use streamers.forEach(streamer => { fetch(STREAM_API + streamer + API_KEY)... }) to skip the closure problem completely.

4

classic closure problem! the loop finishes before your promises resolve, so j hits 3 (out of bounds). use let instead of var in your for loop - it creates block scope for each iteration. that’ll fix the undefined issue ur seeing.