CODEWITHSUNDEEP
javascriptnode.js
Human Cyborg Relations
Human Cyborg Relations

Reputation: 1244

Call next iteration after each callback

Lets say I've got the following code,

function someFunction(){
    var i = 0;
    while (i < 10){
        someAsyncProcess(someField, function(err, data){
            i++;
            // i want the next iteration of the while loop to occur after this
        }
    }
}

The someAsyncProcess runs, and increments 'i'.

However, due to the async nature of JavaScript, the while loop runs thousands of times before i = 10. What if I want the while loop to run exactly 10 times. What if I want the while loop to execute the code inside only after the callback function has finished executing.

Is it possible to do this without a setTimeout function?

Please note that I am still relatively new to JavaScript so if I incorrectly used any jargon, correct me.

Upvotes: 2

Views: 1687

Answers (3)

Felix Kling
Felix Kling

Reputation: 816232

while is synchronous. You cannot make it wait until an asynchronous process is done. You cannot use a while loop in this case.

Instead you can put your code in a function and call the function itself again if the condition is met.

Example:

function someFunction() {
    var i = 0;
    function step() {
        if (i < 10) {
            someAsyncProcess(someField, function(err, data) {
                i++;
                step();
            });
        }
    }
    step();
}

There are quite a few libraries out there that provide ready-made solutions for this. You might also want to look into promises in general.

Upvotes: 5

Sajan
Sajan

Reputation: 821

You need to use replace while loop Iteration with function recursion 

function someFunction() {
    var i = 0;
    (function asyncWhile() { 
        if (i < 10) {
            someAsyncProcess(someField, function(err, data) {
                //You code here
                i++;
                asyncWhile(); 
            });
        }
    })(); //auto invoke
}

Upvotes: 0

XCEPTION
XCEPTION

Reputation: 1753

Use node package q which will help you return promises. You can achieve the same in the following manner using q.

var q = require('q');
function someFunction(){

    var promises = []    
    var i = 0;
    while (i < 10) {
        promises.push(function() {
            i++;
            //write code here for using current value of i
        })
    }
    return q.all(promises);
}

You can call someFunction() as below

someFunction().then(function(arrayOfResponseFromEachPromiseFunction) {
    console.log(JSON.stringify(arrayOfResponseFromEachPromiseFunction, null, 4));
}).catch(function(err){
    console.log(err);
}).done();

Please rectify if you find any syntax error. Hope it helps.

Upvotes: 0

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