Asynchronous callbacks can make errors hard to catch. A try-catch around a call that starts a task may not catch an error raised later, when the callback runs. Node.js-style callbacks commonly report failure through an `err` argument. section 6.3.1
try {
getJSON("data/ninjas.json", function() {
//Handle results
});
} catch(e) {/*Handle errors*/}
Compare the surrounding try-catch with the callback’s later execution: the catch does not automatically handle errors from that asynchronous work. section 6.3.1
With chained promises, one catch can handle a failure from any earlier step. This keeps whole-sequence error handling out of each individual success step. section 6.3.5
Listing 6.16. Chaining promises with then
getJSON("data/ninjas.json")
.then(ninjas => getJSON(ninjas[0].missionsUrl)) #A
.then(missions => getJSON(missions[0].detailsUrl)) #A
.then(mission => assert(mission !== null, "Ninja mission obtained!")) #A
.catch(error => fail("An error has occurred")); #B
The chain fetches ninja and mission data in sequence, then uses one catch for failures along the way. section 6.3.5
