Node.js, 'event loop`, setTimeout does not work as I expect

2022-05-04 10:16 Jacek Wojcik imported from Stackoverflow

Please have a look at code below:

setTimeout(() => {console.log('A')}, 3000);
setTimeout(() => {console.log('B')}, 1000);
setTimeout(() => {console.log('C')}, 500);

since setTimeout adds message to message queue and messages are FIFO queue, and one message needs to finish before next one may start, I expect to see A B C order but I see C B A.

It means that I do not understand something here. What is it that I do not understand?

2 answers

answered 2022-05-04 10:23 Yogev D.
According to this article

The function setTimeout is called with 2 arguments: a message to add to the queue, and a time value (optional; defaults to 0). The time value represents the (minimum) delay after which the message will be pushed into the queue. If there is no other message in the queue, and the stack is empty, the message is processed right after the delay.

That means the message queue also knows how to handle the delays.
answered 2022-05-04 10:41 Fiodorov Andrei
The setTimeout is provided to us by the Web API: it lets us delay tasks without blocking the main thread. The callback doesn’t immediately get added to the call stack. It simply gets added to the queue after setTimeout time.

In this article you can have more details and visualization about event loop.

See also questions close to this topic

Error message : MongoServerError: bad auth :Authentication failed

What is the reason behind this error? this code I am using to connect to DB.

const uri =`mongodb+srv://${process.env.DB_USER}:${process.env.DB_PASSWORD}@cluster0.xft2s.mongodb.net/myFirstDatabase?retryWrites=true&w=majority`;

In Mongo, If a document I'm saving "Prateek" then I don't want on the next create operation even the "prateek" or "praTEEK", etc is saved

//** If I'm adding a new document with the name: "India", then I don't want that the DB allow another name with the name: "INDIA", "india", "indIA", etc. I'm new and learning, help would be great!!**

// Controller

var Dinosaur = require('../models/dinosaurs');

//addDino
module.exports.addDino = (req, res) => {
    var name = req.body.name;
    var type = req.body.type;
    var height = req.body.height;
    var weight = req.body.weight;
    var Period = req.body.Period;

    req.checkBody('name', 'Name is required').notEmpty();

    var errors = req.validationErrors();
    if (errors)
        return res.status(400).send({
            message: 'Name is Required'
        });
    else {
        let newDino = {
            name: name,
            type: type,
            height: height,
            weight: weight,
            Period: Period
        }
        Dinosaur.addDino(newDino, (err, result) => {
            if (err) {
                if (err.name) return res.status(409).send({
                    message: name + ' Already Exist'
                });
                else if (err.url) return res.json({ status: false, error: { url: "Url already exist" }, message: err.url });
                else return res.json(err, "Server Error");
            }
            else {
                return res.status(200).send({
                    message: "Done"
                });
            }
        });
    }
}

// Model

var mongoose = require('mongoose');

//dinosaur schema
var DinosaurSchema = mongoose.Schema({
    name: {
        type: String,
        unique: true
    },
    type: {
        type: String
    },
    height: {
        type: Number
    },
    weight: {
        type: Number
    },
    Period: {
        type: String
    }
});

var Dinosaur = mongoose.model('dinosaur', DinosaurSchema);

//add
module.exports.addDino = (query, callback) => {
    Dinosaur.create(query, callback);
}

// GetAll, Already Created a new document with the name "Brachiosaurus"

// > Create, a new create with the first letter lower case "brachiosaurus", Don't want it to be pushed.

//Get All, Got pushed.

i am trying to run the node of ganache but getting this error

I am trying to run the node of ganache but getting this error.please resolve my problem.

Related questions

Node.js, 'event loop`, setTimeout does not work as I expect

2 answers

See also questions close to this topic