.push() isn't saving to my array - NodeJS

Question

I'm trying to code a Webserver in plain NodeJS with http. What i'm trying to do is when you have the url /add/ whatever is after the /add/ would be added to a array, and showen on / but its not saving the array, and resets it when i add something new. Can someone explain what i did wrong?

Sorry that my code is a bit messy.

Index.js:

const http = require('http');
const $conf = require('./config.json');

http.createServer((req, res) => {

    let $list = [];

    const url = req.url;
    const $req = url.split('/');
    const $req_q = url.split('?');

    if (req.url == "/"){
        res.write($list.toString());
    } else if (req.url == `/add/${$req[2]}`){

        $list.push($req[2])
        console.log($list)
        res.write(`ADDED ${$req[2]}`)

    } else {
        res.write('Error');
    }
    res.end();
}).listen($conf.port);

Config.json:

{
    "port": 80
}

When i goto /add/hi i get "ADDED hi".

Then go back to / and get nothing.

The console.log in the else if block does it print to console? — Tom Bombadil
– Tom Bombadil, Commented Apr 10, 2021 at 6:29

Drakmord2 · Accepted Answer · 2021-04-10 06:33:59Z

1

A NodeJS webserver executes a route (e.g '/' or '/add') on-demand. That means that everything inside the function is executed from scratch on every request.

Therefore your $list variable gets initialized everytime you call a route and you can't see the result on the screen.

You can fix it by moving the variable to the global scope.

const http = require('http');
const $conf = require('./config.json');

let $list = []; // Now this value is persisted between requests

http.createServer((req, res) => {
    const url = req.url;
    const $req = url.split('/');
    const $req_q = url.split('?');

    if (req.url == "/"){
        res.write($list.toString());
    } else if (req.url == `/add/${$req[2]}`){

        $list.push($req[2])
        console.log($list)
        res.write(`ADDED ${$req[2]}`)

    } else {
        res.write('Error');
    }
    res.end();
}).listen($conf.port);

answered Apr 10, 2021 at 6:33

Drakmord2

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1 Comment

Patrick Roberts Over a year ago

I'm not sure whether it would be obvious or not to OP but it would at least be beneficial to point out that this $list is shared between all clients accessing the webserver and is not separate for each client.

Apoorva Chikara · Accepted Answer · 2021-04-10 07:00:58Z

The main reason is the $list array is initialized every time there is a new request. So, there can be two ways of doing this at the moment.

Create a variable outside this route scope so you can access the previous value. (this variable will be shared for every request/client)
Another and more efficient way is you can store the value more efficiently using node-json-db, as there might be thousands of request for which you might need to save this data and it won't be efficient at that time. You can use node-json-db.

Sample code:

import { JsonDB } from 'node-json-db';
import { Config } from 'node-json-db/dist/lib/JsonDBConfig'

// The first argument is the database filename. If no extension, '.json' is assumed and automatically added.
// The second argument is used to tell the DB to save after each push
// If you put false, you'll have to call the save() method.
// The third argument is to ask JsonDB to save the database in an human readable format. (default false)
// The last argument is the separator. By default it's slash (/)
var db = new JsonDB(new Config("myDataBase", true, false, '/'));

// Pushing the data into the database
// With the wanted DataPath
// By default the push will override the old value
db.push("/test1","super test");

// Get the data from the root
var data = db.getData("/");

// From a particular DataPath
var data = db.getData("/test1");

// If you try to get some data from a DataPath that doesn't exists
// You'll get an Error
try {
    var data = db.getData("/test1/test/dont/work");
} catch(error) {
    // The error will tell you where the DataPath stopped. In this case test1
    // Since /test1/test does't exist.
    console.error(error);
};

There are several other operations you can perform for that you should check its documentation.

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