Asynchronously delay JS until a condition is met

Question

I have a class, ChatRoom, that can only render after it receives a long-running HTTP request (it could take 1 second or 30 seconds). So I need to delay rendering un

Answer 1

Consider this:

(function wait() {
    if ( chatroom.json ) {
        chatroom.render();
    } else {
        setTimeout( wait, 500 );
    }
})();

This will check every half second.

Live demo: http://jsfiddle.net/kBgTx/

Answer 2

You could also achieve this using lodash's debouncer with recursion.

import _debounce from 'lodash/debounce';

const wait = (() => {
    if ( chatroom.json ) {
        chatroom.render();
    } else {
      _debounce(wait, 500)();
    }
})();

Answer 3

The answer I came up with is like this:

var count = 0;
// Number of functions that need to run. This can be dynamically generated
// In this case I call check(data, cb) a total of 3 times
var functionNum = 3; 
function toCallAfter(){
    console.log('I am a delayed function');
}

I had this for a check function that ran once regularly and twice in a loop:

check(data, function(err){ // check is my asynchronous function to check data integrity
    if (err){
        return cb(null, { // cb() is the return function for the containing function
            errCode: 'MISSINGINFO',
            statusCode: 403,
            message : 'All mandatory fields must be filled in'
        });
    } // This part here is an implicit else
    count++; // Increment count each time required functions complete to
             // keep track of how many function have completed
    if (count === functionNum) {
        return anon();
    }
    return;
});
// Run twice more in a loop
for(var i = 0; i < 2; i++) {
    check(data, function(err) { // calls check again in a loop
        if (err){
            return cb(null, {
                errCode: 'MISSINGINFO',
                statusCode: 403,
                message : 'All mandatory fields must be filled in'
            });
        }
        count++;
        if (count === functionNum) {
            return toCallAfter();
        }
        return;
    });
}

Lastly, I'd like to point out a significant performance fault in the alternate (and extraordinarily common) answer:

(function wait() {
    if ( chatroom.json ) {
        chatroom.render();
    } else {
        setTimeout( wait, 500 );
    }
})();

In this case, you are essentially holding the browser or server (If using node.js) hostage for 500 milliseconds for every check which is an incredibly long time for a computer. Meaning a huge performance hit. My solution of directly keeping track of the required completed functions is free from time restraints and will run instantly as soon as all functions complete.