As a node programmer. I'm used to use "nodebacks" for handling errors in my code:
myFn(param, function(err, data) {
if (err){
//error handling logic
}
else {
// business logic
}
});
When writing that function, I can do something like:
var myFn = function(param, callback){
var calc = doSomeCalculation(param);
if(calc === null) { // or some other way to detect error
callback(new Error("error with calculation"), null);
}
...
someAsyncOp(calcN,function(err, finalResult){
if(err) return callback(err, null);
callback(null, finalResult); // the error is null to signal no error
});
};
How would I do this sort of error handling with promises?
Whenever you have a doubt about how to do something with promises - think about the synchronous version.
try{
var result = myFn(param);
// business logic with result
} catch(e) {
//error handling logic
}
This, at least to me looks a lot cleaner than a callback with a first parameter that is sometimes null.
The promises way is almost always very similar to the synchronous version of the problem:
myFn(param).then(function(result){
// business logic with result
}).catch(function(e){
//error handling logic
});
Where myFn would look something like when working with callbacks:
var myFn = function(param){
return new Promise(function(resolve, reject){
var calc = doSomeCalculation(param);
if(calc === null) { // or some other way to detect error
reject(new Error("error with calculation"), null);
}
someAsyncOp(calcN,function(err, finalResult){
if(err) reject(err);
resolve(finalResult);
})
});
};
This is only something you should have to do when working with callbacks, when working with promises it is a lot simpler, and you can do:
var myFn = function(param){
var calc = doSomeCalculation(param);
...
return someAsyncOp(calcN); // returning a promise.
}
Moreover, when working inside promise chains, you get throw safety:
myFn(param).then(function(calcN){
// here, you throw to raise an error and return to resolve
// new Promise should be used only when starting a chain.
}).catch(function(err){
// handle error
}).then(function(){
// ready to go again, we're out of the catch
});
Note, some libraries like Bluebird , RSVP and Q offer syntactic sugar and automatic promisification of methods so you rarely have to use new Promise yourself.
Also, consider reading this and that to learn more about promise error handling.
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