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问题:
I need some help with my code. I'm new at nodejs and have a lot of trouble with it.
What i'm trying to do:
My code is not working. I think i messed up with this asynchronous-synchronous stuff - help me!
var amazon = require('amazon-product-api');
var fs = require('fs');
var client = amazon.createClient({
awsId: "XXX",
awsSecret: "XXX",
awsTag: "888"
});
var array = fs.readFileSync('./test.txt').toString().split('\n');
for (var i = 1; i < array.length; i++) {
var ASIN = array[i];
return client.itemLookup({
domain: 'webservices.amazon.de',
responseGroup: 'Large',
idType: 'ASIN',
itemId: ASIN
})
.then(function(results) {
fs.writeFile(ASIN + '.json', JSON.stringify(results), function(err) {
if (err) {
console.log(err);
} else {
console.log("JSON saved");
}
})
return results;
}).catch(function(err) {
console.log(err);
});
};
回答1:
Because fs.writefile
is a traditional asynchronous callback - you need to follow the promise spec and return a new promise wrapping it with a resolve and rejection handler like so:
return new Promise(function(resolve, reject) {
fs.writeFile("<filename.type>", data, '<file-encoding>', function(err) {
if (err) reject(err);
else resolve(data);
});
});
So in your code you would use it like so right after your call to .then()
.then(function(results) {
return new Promise(function(resolve, reject) {
fs.writeFile(ASIN + '.json', JSON.stringify(results), function(err) {
if (err) reject(err);
else resolve(data);
});
});
}).then(function(results) {
console.log("results here: " + results)
}).catch(function(err) {
console.log("error here: " + err);
});
回答2:
say
const util = require('util')
const fs_writeFile = util.promisify(fs.writeFile)
https://nodejs.org/api/util.html#util_util_promisify_original
this is less prone to bugs than the top-voted answer
回答3:
As of 2018...
...the correct answer is to use async/await with the native fs
module. Upgrade to Node.js 10 (already supported by major cloud providers) and do this:
const fs = require('fs').promises;
// This must run inside a function marked `async`:
const file = await fs.readFile('filename.txt', 'utf8');
await fs.writeFile('filename.txt', 'test');
Do not use third-party packages and do not write your own wrappers, that's not necessary anymore.
As of Node 11.1.0
, you will get a warning that this feature is experimental, but it works just fine and it's the way to go in the future.
回答4:
Finally, the latest node.js release v10.3.0 has natively supported fs promises.
const fsPromises = require('fs').promises; // or require('fs/promises') in v10.0.0
fsPromises.writeFile(ASIN + '.json', JSON.stringify(results))
.then(() => {
console.log('JSON saved');
})
.catch(er => {
console.log(er);
});
You can check the official documentation for more details.
https://nodejs.org/api/fs.html#fs_fs_promises_api
回答5:
Update Sept 2017: fs-promise
has been deprecated in favour of fs-extra
.
I haven't used it, but you could look into fs-promise. It's a node module that:
Proxies all async fs methods exposing them as Promises/A+ compatible
promises (when, Q, etc). Passes all sync methods through as values.
回答6:
const util = require('util')
const fs = require('fs');
const fs_writeFile = util.promisify(fs.writeFile)
fs_writeFile('message.txt', 'Hello Node.js')
.catch((error) => {
console.log(error)
});
回答7:
For easy to use asynchronous convert all callback to promise use some library like "bluebird" .
.then(function(results) {
fs.writeFile(ASIN + '.json', JSON.stringify(results), function(err) {
if (err) {
console.log(err);
} else {
console.log("JSON saved");
return results;
}
})
}).catch(function(err) {
console.log(err);
});
Try solution with promise (bluebird)
var amazon = require('amazon-product-api');
var fs = require('fs');
var Promise = require('bluebird');
var client = amazon.createClient({
awsId: "XXX",
awsSecret: "XXX",
awsTag: "888"
});
var array = fs.readFileSync('./test.txt').toString().split('\n');
Promise.map(array, function (ASIN) {
client.itemLookup({
domain: 'webservices.amazon.de',
responseGroup: 'Large',
idType: 'ASIN',
itemId: ASIN
}).then(function(results) {
fs.writeFile(ASIN + '.json', JSON.stringify(results), function(err) {
if (err) {
console.log(err);
} else {
console.log("JSON saved");
return results;
}
})
}).catch(function(err) {
console.log(err);
});
});