Execute multiple tasks asynchronously and return first successful result in JavaScript function

Question

I have to write a javaScript function that return some data to the caller.

In that function I have multiple ways to retrieve data i.e.,

1. Lookup from ca

Answer 1

The problem in your example getOrder lies in that if the 3 lookup functions are going to be asynchronous, you won't get the order back from them right away and as they are not blocking, the getOrder would return null; You would be better off defining a callback function which takes action on the first returned order data and simply ignores the rest of them.

var doSomethingWithTheOrder = function CallBackOnce (yourResult) {
    if (!CallBackOnce.returned) {
        CallBackOnce.returned = true;
        // Handle the returned data
        console.log('handle', yourResult);
    } else {
        // Ignore the rest
        console.log('you are too late');
    }
}

Make your data lookup functions accept a callback

function cacheLookUp(id, callback) {
    // Make a real lookup here
    setTimeout(function () {
        callback('order data from cache');
    }, 3000);    
}

function localeStorageLookUp(id, callback) {
    // Make a real lookup here
    setTimeout(function () {
        callback('order data from locale storage');
    }, 1500);    
}

function restLookUp(id, callback) {
    // Make a real lookup here
    setTimeout(function () {
        callback('order data from rest');
    }, 5000);    
}

And pass the callback function to each of them

function getOrder(id) {
    cacheLookUp(id, doSomethingWithTheOrder);
    localeStorageLookUp(id, doSomethingWithTheOrder);
    restLookUp(id, doSomethingWithTheOrder);
}