Published Feb 11, 2020
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You should use promises to turn this:
fs.readFile("file.json", function (err, val) {
if (err) {
console.error("unable to read file");
}
else {
try {
val = JSON.parse(val);
console.log(val.success);
}
catch (e) {
console.error("invalid json in file");
}
}
});
Into this:
fs.readFileAsync("file.json").then(JSON.parse).then(function (val) {
console.log(val.success);
})
.catch(SyntaxError, function (e) {
console.error("invalid json in file");
})
.catch(function (e) {
console.error("unable to read file");
});
You might notice that the promise approach looks very similar to using synchronous I/O:
try {
var val = JSON.parse(fs.readFileSync("file.json"));
console.log(val.success);
}
// Gecko-only syntax; used for illustrative purposes
catch (e if e instanceof SyntaxError) {
console.error("invalid json in file");
}
catch (e) {
console.error("unable to read file");
}
This is the point - to have something that works like the return and throw
in synchronous code.
You can also use promises to improve code that was written with callbacks:
//Copyright Plato https://stackoverflow.com/a/19385911/995876
//CC BY-SA 2.5
mapSeries(URLs, function (URL, done) {
var options = {};
needle.get(URL, options, function (error, response, body) {
if (error) {
return done(error);
}
try {
var ret = JSON.parse(body);
return done(null, ret);
}
catch (e) {
done(e);
}
});
}, function (err, results) {
if (err) {
console.log(err);
} else {
console.log('All Needle requests successful');
// results is a 1 to 1 mapping in order of URLs > needle.body
processAndSaveAllInDB(results, function (err) {
if (err) {
return done(err);
}
console.log('All Needle requests saved');
done(null);
});
}
});
This is far more readable when done with promise:
Promise.promisifyAll(needle);
var options = {};
var current = Promise.resolve();
Promise.map(URLs, function (URL) {
current = current.then(function () {
return needle.getAsync(URL, options);
});
return current;
}).map(function (responseAndBody) {
return JSON.parse(responseAndBody[1]);
}).then(function (results) {
return processAndSaveAllInDB(results);
}).then(function () {
console.log('All Needle requests saved');
}).catch(function (e) {
console.log(e);
});