处理错误

One sometimes-unintuive aspect of promises is that if you throw an exception in the fulfillment handler, it will not be caught by the error handler.

return foo()
.then(function (value) {
    throw new Error("Can't bar.");
}, function (error) {
    // We only get here if "foo" fails
});

To see why this is, consider the parallel between promises and try/catch. We are try-ing to execute foo(): the error handler represents a catch for foo(), while the fulfillment handler represents code that happens after the try/catch block. That code then needs its own try/catch block.

In terms of promises, this means chaining your rejection handler:

return foo()
.then(function (value) {
    throw new Error("Can't bar.");
})
.fail(function (error) {
    // We get here with either foo's error or bar's error
});