Nodejs Synchronous For each loop

Question

I want to do a for each loop but have it run synchronously. Each iteration of the loop will do an http.get call and that will return json for it to insert the values into a

Answer 1

"use strict";

var Promise = require("bluebird");
var some = require('promise-sequence/lib/some');

var pinger = function(wht) {
    return new Promise(function(resolve, reject) {
        setTimeout(function () { 

            console.log('I`ll Be Waiting: ' + wht);
            resolve(wht);

        }, Math.random() * (2000 - 1500) + 1500);
    });
}

var result = [];
for (var i = 0; i <= 12; i++) {
    result.push(i);
}

some(result, pinger).then(function(result){
  console.log(result);
});

Answer 2

I found out that I wasn't releasing my mysql connections after I was done with each call and this tied up the connections causing it to fail and appear to be an issue with synchronization.

After explicitly calling connection.release(); it caused my code to work 100% correctly even in an asynchronous fashion.

Thanks for those who posted to this question.

Answer 3

To loop and synchronously chain asynchronous actions, the cleanest solution is probably to use a promise library (promises are being introduced in ES6, this is the way to go).

Using Bluebird, this could be

Var p = Promise.resolve();
forEach(sets, function(item, index, arr) {
    p.then(new Promise(function(resolve, reject) {
         http.get(theUrl, function(res) {
         ....
         res.on('end', function() {
              ...
              resolve();
         }
    }));
});
p.then(function(){
   // all tasks launched in the loop are finished
});

Answer 4

var urls = ['http://stackoverflow.com/', 'http://security.stackexchange.com/', 'http://unix.stackexchange.com/'];

for (i = 0; i < urls.length; i++){
   http.get(urls[i], function(res) {
       // add some code here to process the response
   });
}

Answer 5

With recursion the code is pretty clean. Wait for the http response to come back then fire off next attempt. This will work in all versions of node.

var urls = ['http://stackoverflow.com/', 'http://security.stackexchange.com/', 'http://unix.stackexchange.com/'];

var processItems = function(x){
  if( x < urls.length ) {
    http.get(urls[x], function(res) {

      // add some code here to process the response

      processItems(x+1);
    });
  }
};

processItems(0);

A solution using promises would also work well, and is more terse. For example, if you have a version of get that returns a promise and Node v7.6+, you could write an async/await function like this example, which uses some new JS features.

const urls = ['http://stackoverflow.com/', 'http://security.stackexchange.com/', 'http://unix.stackexchange.com/'];

async function processItems(urls){
  for(const url of urls) {
    const response = await promisifiedHttpGet(url);    
    // add some code here to process the response.
  }
};

processItems(urls);

Note: both of these examples skip over error handling, but you should probably have that in a production app.

Answer 6

Just wrap the loop in an async function. This example illustrates what I mean:

const oneSecond = async () => 
    new Promise((res, _) => setTimeout(res, 1000));

This function completes after just 1 second:

const syncFun = () => {
    for (let i = 0; i < 5; i++) {
        oneSecond().then(() => console.log(`${i}`));
    }
}

syncFun(); // Completes after 1 second ❌

This one works as expected, finishing after 5 seconds:

const asyncFun = async () => {
    for (let i = 0; i < 5; i++) {
        await oneSecond();
        console.log(`${i}`);
    }
}

asyncFun(); // Completes after 5 seconds ✅