foreach loop with regular timeout in nodejs

Question

I am trying to send the values from server to the client-side. Here, I have lot of values in my database so what I need is after fetching all the values I want to loop it send each value to client with regular pause after each value.. I have tried setTimeout() but still i cant get desired result...

server.js :

var express = require("express");
var app = express();
var http = require('http').Server(app);
var io = require('socket.io')(http);

connection.query("select * from base_table", function (err, rows) {
    if (err) {  err; } else {
       rows.forEach(function(index){
          setTimeout(function(){
              io.emit('Fetched Values', index);
          }, 1000);
       }); 
    }
});

Client.js:

socket.on('Fetched Values', function(data) {
    console.log(data);
});

You can use a promise chain that sets the next timeout after the previous one is invoked — elvis_ferns
– elvis_ferns, Commented Feb 23, 2018 at 5:34

elvis_ferns · Accepted Answer · 2018-02-23 05:39:31Z

1

connection.query("select * from base_table", function (err, rows) {
  if (err) {  err; } else {
   let promise = Promise.resolve();
   rows.forEach(function(index){
      promise = promise
       .then(() => {
          return new Promise((resolve) => {
            setTimeout(function(){
             resolve(io.emit('Fetched Values', index));
            }, 1000); 
          })
       })

    }); 
  }
});

answered Feb 23, 2018 at 5:39

elvis_ferns

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1 Comment

gubera Over a year ago

Yeah this works ! Being newbie , I never have used Promise function in nodejs... I will look deep into this ! Thanks ... It saved my time

Jake Holzinger · Accepted Answer · 2018-02-23 05:43:43Z

0

The array.forEach() method is synchronous so it is not ideal for asynchronous looping. You can create your own method for looping asynchronously, or use a module like async which has a lot of nice features to support a variety of use cases rather than just looping. Promises are also very powerful for performing asynchronous tasks, but there is a bit of learning overhead to use them properly.

Here's a simple method that can perform a for each loop asynchronously.

function asyncForEach(array, timeout, callback, index) {
    index = index || 0;
    setTimeout(function() {
        callback(array[index], index);
        if (array.length > index + 1) {
            asyncForEach(array, timeout, callback, index + 1);
        }
    }, timeout);
}

asyncForEach([1, 2, 3], 1000, function(number, index) {
    console.log(number + ' ' + index);
});

answered Feb 23, 2018 at 5:43

Jake Holzinger

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1 Comment

Jake Holzinger Over a year ago

It was originally designed for use in node, but it works in the browser.

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