Javascript

Promises

@tkssharma

UnitedHealth Group

Delhi,India

loadSomething().then(function(something) {
    loadAnotherthing().then(function(another) {
    DoSomethingOnThem(something, another);
    });
});

Nested promise

q.all([loadSomething(), loadAnotherThing()])
    .spread(function(something, another) {
        DoSomethingOnThem(something, another);
});

Let's Fix Nested promise

function anAsyncCall() {
    var promise = doSomethingAsync();
    promise.then(function() {
        somethingComplicated();
    });
    
    return promise;
}

Broken Promise Chain

function anAsyncCall() {
    var promise = doSomethingAsync();
    return promise.then(function() {
        somethingComplicated()
    });   
}

Fix for Broken Promise Chain

function workMyCollection(arr) {
    var resultArr = [];
    function _recursive(idx) {
        if (idx >= resultArr.length) return resultArr;
            
        return doSomethingAsync(arr[idx]).then(function(res) {
            resultArr.push(res);
            return _recursive(idx + 1);
        });
    }

    return _recursive(0);
}

Async Operation on Array

function workMyCollection(arr) {
    return q.all(arr.map(function(item) {
        return doSomethingAsync(item);
    }));    
}

Fix code for Async Operation on Array Using Map

var promise;
if (asyncCallNeeded) 
    promise = doSomethingAsync();
else
    promise = Q.resolve(42);
        
promise.then(function() {
    doSomethingCool();
});

Ghost promise

Q(asyncCallNeeded ? doSomethingAsync() : 42)
    .then(
        function(value){
            doSomethingGood();
        })
    .catch( 
        function(err) {
            handleTheError();
        });

Fix for Ghost promise