有条件的诺言链

Question

I want to chain my promises depending upon if the previous call was resolved or rejected. I am making a call to server in all promises. So, I am writing it like-

apiServices.patientsSearch(id)
.then(function(data){
    return callback(null,data);
},function(err){
    return apiServices.studiesSearch(id);
}).then(function(data){
    return callback(null,data);
},function(){
    return apiServices.seriesSearch(id); 
}).then(function(data){
    return callback(null,data);
})
.catch(function(err){
    return callback(false,err);
});

As every then returns a promise object, problem is that catch is always being called if any promise except the last one calls resolve. One way I am thinking is to check if err is empty and ignore it. Is it the right way to do it ?

I am using request module, if I set forever: true, I start getting-

{ [Error: socket hang up] code: 'ECONNRESET' }

With forever false, it works. Why my socket is still busy even after the request has ended ? As the next request will go only when reject is called, so socket should be free by that time.

Answer 1

You should only call the callback once. Don't pass it as the onfulfilled-handler after each promise, call it once in the end:

apiServices.patientsSearch(id).then(null, function(err){
    return apiServices.studiesSearch(id);
}).then(null, function(){
    return apiServices.seriesSearch(id); 
}).then(function(data){
    callback(null,data);
}, function(err){
    callback(false,err);
});

or

apiServices.patientsSearch(id).catch(function(err){
    return apiServices.studiesSearch(id);
}).catch(function(){
    return apiServices.seriesSearch(id); 
}).then(function(data){
    callback(null,data);
}, function(err){
    callback(false,err);
});

Of course, you shouldn't be calling any callbacks at all in promise-based code, so use this only if you have to interface with legacy code. Otherwise, don't take a callback parameter and just return the promise:

return apiServices.patientsSearch(id).catch(function(err){
    return apiServices.studiesSearch(id);
}).catch(function(){
    return apiServices.seriesSearch(id); 
});