synchronous and asynchronous loops in javascript

Question

Are loops synchronous or asynchronous in JavaScript? (for, while, etc)

Supposing I have:

for(let i=0; i<10; i++){
    // A (nested stuff...)
}

//         


        

Answer 1

let items = YourArray;
let i = 0;
await new Promise(async (resolve, reject) => {
    try {
        if (items.length == 0) return resolve();
        let funSync = async () => {
            await yourASyncFunctions(items[i].doAnything);
            i++;
            if (i == items.length) resolve();
            else funSync();
        }
        funSync();
    } catch (e) {
        reject(e);
    }
});

Answer 2

The for loop runs immediately to completion while all your asynchronous operations are started.

Well, here we have some nested loops. Notice, "BBB" always fires after.

for(let i=0; i<10; i++){
   for(let i=0; i<10; i++){
     for(let i=0; i<10; i++){
       console.log("AA")
     }
   }
}

console.log('BBB')

now, look at this

for(let i=0; i<10; i++){
   setTimeout(function() {console.log("AA")}, 2000)
}

console.log('BBB')

This is because of something called the "event loop". And the fact that with that setTimeout we are simulating an async operation. It could be an ajax call or some other async process.

Check this out: http://latentflip.com/loupe

This will really help you understand these sorts of async/sync loop topics.

updated to show how promises might work here (given comments below):

var stringValues = ['yeah', 'noooo', 'rush', 'RP'];
var P = function(val, idx){
    return new Promise(resolve => setTimeout(() => resolve(val), 1000 * idx));
};


// We now have an array of promises waiting to be resolved.
// The Promise.all basically will resolve once ALL promises are 
// resolved. Keep in mind, that if at any time something rejects
// it stops

// we iterator over our stringValues array mapping over the P function, 
// passing in the value of our array.
var results = Promise.all(stringValues.map(P));

// once all are resolved, the ".then" now fires. It is here we would do 
results.then(data => 
    console.log(data) //["yeah", "noooo", "rush", "RP"]
);

let me know if I am not clear enough.

Answer 3

for(const elment of arrayElements) {
            await yourFunc(elment)
            await yourOtherFunc('somePatameter')
}

Answer 4

If you place asynchronous loops inside a for...loop and want to stop the loop until each operation ends, you must use the async/await syntax like this.

async function foo() {
    var array = [/* some data that will be used async*/]

    //This loop will wait for each next() to pass the next iteration
    for (var i = 0; i < array.length; i++) { 
        await new Promise(next=> {
            someAsyncTask(array[i], function(err, data){
                /*.... code here and when you finish...*/
                next()
            })
        })        
    }
}

foo().then(() => { /*After foo execution*/ })

Answer 5

First of all, your statement about "Using for the execution of B will start before A sometimes... (so asynchronous)" is wrong.

The loop function (like while, for, .forEach or .map) in Javascript will be run synchronously (blocking), whether you run it in a Browser or Runtime Environment like NodeJS. We can prove it by running the code below (maybe the process will take a few seconds):

let counter1 = 0
let counter2 = 0
let counter3 = 0

console.log("Start iteration")
console.time("Time required")

// First heavy iteration
for (let i = 0; i < 1000; i++) {
  counter1 += 1

  // Second heavy iteration
  for (let i2 = 0; i2 < 1000; i2++) {
    counter2 += 1

    // Third heavy iteration
    for (let i3 = 0; i3 < 1000; i3++) {
      counter3 += 1
    }
  }
}

console.log("Iteration was successful")
console.timeEnd("Time required")
console.log('The value of `counter1` is: ' + counter1)
console.log('The value of `counter2` is: ' + counter2)
console.log('The value of `counter3` is: ' + counter3)

And then what kind of looping causes your code to run asynchronously (non blocking)?

The answer is:

The code that is placed inside the Promise callback or the function with the async keyword or some native functions with callback (not all) like setTimeout, setInterval and etc will be run asynchronously.

Example:

setTimeout(() => {
  console.log('A')
})

console.log('B')

In code, setTimeout function is declared first. However, the output of the code shows that the console.log('B') function run earlier than the setTimeout function.