setTimeout inside Promise object

Question

I am beginning journey on JS and NodeJS. While writing a hello-world app encountered different behavior of setTimeout. It would be great if I could know the cause of this difference.

Scenario 1: response waits for 5 sec
Scenario 2: Request get resolved immediately though timeout is set for 5 second

Code difference in two scenario is; in Scenario-1 setTimeout has anonymous function and in scenario-2 it calls another function in the same module.

calculator.js

Scenario-1:

module.exports.addNumbers = function(numberArray){

    return new Promise(function(fullfil, reject){
        setTimeout(() => {
            if(!(typeof numberArray === 'Array'))
                reject('Not number elements found!');
        },5000);
        //fullfil(10000);
    });

}

Scenario-2:

module.exports.addNumbers = function(numberArray){
    return new Promise(function(fullfil, reject){
        setTimeout(add(numberArray, fullfil, reject),5000);
        //fullfil(10000);
    });
}

function add(numberArray, fullfil, reject){
    if(!(typeof numberArray === 'Array'))
        reject('Not number elements found!');
}

Router (Same in both scenario):

var express = require('express');
var router = express.Router();
var calculator = require('../services/calculator');

router.get('/',function(req,res,next){

    //res.writeHeader(200,{'content-type':'text/plain'});

    res.send('Hello World - ' + JSON.stringify(res));
    console.log(res);
});

router.get('/testpromise',function(req,res,next){

    calculator.addNumbers('First Try')
        .then(function(result){
            res.send(' Final Result ## '+result);
        })
        .catch(function(err){
            res.send(' ERROR MSG ## '+err);
            console.error(' ERROR MSG ##' +err);
        });

});

module.exports = router;

score 1 · Accepted Answer · answered May 31 '18 at 19:40

The first argument to setTimeout needs to be a function to call.

In your first example you are providing such a function (() => {}):

  setTimeout(() => {
        if(!(typeof numberArray === 'Array'))
            reject('Not number elements found!');
    },5000);

In the second example however you are not passing in a function as the first argument, instead you are calling it straight away (hence it gets evaluated there and then).

setTimeout(add(numberArray, fullfil, reject),5000);

As far as I can see add(numberArray, fullfil, reject) does not return a function.

You could do this to wrap it into a function:

setTimeout(() => add(numberArray, fullfil, reject),5000);

or make add return a function:

function add(numberArray, fullfil, reject){
    return () => {
        if(!(typeof numberArray === 'Array'))
            reject('Not number elements found!');
    }
}
// or 
function add(numberArray, fullfil, reject){
    return function() {
        if(!(typeof numberArray === 'Array'))
            reject('Not number elements found!');
    }
}

Thanks for the explanation. It helped indeed! – Saumya Ranjan Satapathy May 31 '18 at 19:47 — Saumya Ranjan Satapathy, May 31 '18 at 19:47

score 1 · Answer 2 · answered May 31 '18 at 19:59

1

function foo(varOne, varTwo) {
  console.log(varOne, varTwo);
}

setTimeout(foo, 5000, 1, 2); // you pass the parameters after the time parameter

setTimeout(()=> {foo(1, 2)}, 5000); // you call the function inside an anonymous function

You can check this for more details : How can I pass a parameter to a setTimeout() callback?

answered May 31 '18 at 19:59

Alfred DSouza

328
2
12

Thanks for the answer. – Saumya Ranjan Satapathy Jun 04 '18 at 17:37

setTimeout inside Promise object

2 Answers2