How to do a "for" loop with asynchronous condition in Javascript?

When dealing with asynchronous code, you need to make use of callbacks. That is, if you want to do a() and b() in order but a() does something asynchronously, then you need to call b() from within a() once a() has a result. So not:

a(); // does something asynchronously b(); // tries to use a()'s result but it isn't available yet 

... but rather

a(b); // pass b to a() and a() will call it when ready function a(callback) { triggerAsyncFunction(function(result) { if (result === something) callback("a just finished"); }); } 

Note that a() doesn't refer to b() by name, it just calls whatever function is passed in as an argument.

So applying that to your code, maybe something like this:

waitForFreeAccnt.prototype.isMemberFree = function (cbf) { var self = this; self.api.getMemberInfo(function () { cbf(self.api.connect.accountType === 'FREE'); }); }; waitForFreeAccnt.prototype.testMemberXTimes = function(maxAttempts, callback) { var attempts = 0; var self = this; (function attempt() { self.isMemberFree(function(free) { if (free) callback(true); else if (++attempts < maxAttempts) setTimeout(attempt, 1000); else callback(false); }); )(); }; this.testMemberXTimes(10, function(isFree) { // the next part of your code here, or called from here // because at this point we know we've tested up to // ten times and isFree tells us the result }); 

Note that the way I coded getMemberInfo() it is basically doing the same thing yours was, but instead of returning a boolean it is calling the callback function and passing the same boolean value that you were returning. (I've removed the console.log()s to make the code shorter.)

Note also that you could structure the above to use promises, but the end result will be the same.

You could return a Promise

waitForFreeAccnt.prototype.isMemberFree = function () { return new Promise((reject, resolve)=> // set a timeout if api call takes too long var timeout = setTimeout(()=> reject(Error('API timeout')), 10000); // make api call this.api.getMemberInfo(()=> { clearTimeout(timeout); resolve(this.api.connect.accountType === 'FREE'); }); ); }; 

Then use it like this

whatever.isMemberFree().then(isFree=> { if (isFree) console.log('it is free'); else console.log('it is not free'); }) // handle timeout or other errors .catch(err=> { console.log(err.message); }); 

Building on naomik's answer, if you do it that way you can pretty easily use a for loop with it, using the (most likely) upcoming async/await feature - though it's not part of ES2015.

// Note "async" here! That will make "await" work. It makes the function // return a promise, which you'll be able to either "await" or // "test().then" later. var test = async function () { for (var start = 1; start < 10; start++) { // Right here we're using "await" - it makes JavaScript *wait* for // the promise that comes from self.isMemberFree() to be finished. // It's really handy because you can use it in loops like "for" and // "while" without changing the flow of your program! var result = await self.isMemberFree(); console.log(result); if (result) { break; } else { self.api.pause(1000); console.log('waiting'); } } }; 

For now you'll need to use a transpiler like Babel or Traceur before you can really use async/await, though. It's only supported in Microsoft Edge 14 right now.

And a big emphasis that what is returned from test() isn't whatever you directly return from inside it. If I do this:

var test = async function() { return 15; }; var result = test(); 

I'm not going to get 15 - I'll get a promise that will resolve as 15:

result.then(function(res) { console.log(res); // 15 }); // or, use an async function again: var main = async function() { console.log(await res); // 15 }; main();