passes and then the concatenated result is written to another file, we already have
what we want. On the other hand, what we may want is for the three files to be read,
and in no sooner than 60 seconds, the results to be written to a fourth file—in which
case, we would want to move the timeout into the
Promise.all
.
Don’t Write Your Own Generator Runner
While it’s a good exercise to write our own generator runner, as we have done with
grun
, there are many nuances and improvements we could make on it. It’s better not
to reinvent the wheel. The
is full-featured and robust. If you are
building websites, you may want to look into
, which is designed to work with
co
,
allowing you to write web handlers using
yield
, as we have in
theFutureIsNow
.
Exception Handling in Generator Runners
Another important benefit of generator runners is that they enable exception han‐
dling with
try/catch
. Remember that exception handling is problematic with call‐
backs and promises; throwing an exception inside a callback cannot be caught from
outside the callback. Generator runners, because they enable synchronous semantics
while still preserving asynchronous execution, have a side benefit of working with
try/catch
. Let’s add a couple of exception handlers to our
theFutureIsNow
function:
function*
theFutureIsNow
() {
let
data
;
try
{
data
=
yield
Promise
.
all
([
nfcall
(
fs
.
readFile
,
'a.txt'
),
nfcall
(
fs
.
readFile
,
'b.txt'
),
nfcall
(
fs
.
readFile
,
'c.txt'
),
]);
}
catch
(
err
) {
console
.
error
(
"Unable to read one or more input files: "
+
err
.
message
);
throw
err
;
}
yield
ptimeout
(
60
*
1000
);
try
{
yield
nfcall
(
fs
.
writeFile
,
'd.txt'
,
data
[
0
]
+
data
[
1
]
+
data
[
2
]);
}
catch
(
err
) {
console
.
error
(
"Unable to write output file: "
+
err
.
message
);
throw
err
;
}
}
I’m not claiming that
try...catch
exception handling is inherently superior to
catch
handlers on promises, or error-first callbacks, but it is a well-understood mechanism
for exception handling, and if you prefer synchronous semantics, then you will want
to be able to use it for exception handling.
216 | Chapter 14: Asynchronous Programming
