Mixing
How to make sure all lines are executed before function return
Calling the readCSV from index.js
const productIds = await readCSV();
in another file:
async function readCSV() {
const filepath = path.resolve('src/input_csv.csv');
const readstream = await fs.createReadStream(filepath);
const stream = await readstream.pipe(parser());
let productIds = ;
await stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
await stream.on('end', () => {
console.log(productIds);
});
if (productIds.length > 0) return productIds;
else return 'no products found';
});
}
it is giving the output:
in main:
SourceProductId 1000050429
SourceProductId 1132353
SourceProductId 999915195
SourceProductId 50162873
SourceProductId 1000661087
[ '1000050429', '1132353', '999915195', '50162873', '1000661087' ]
I'm expecting the function to return an array of all values read from CSV. but the seems like it is returning before executing the stream.on statement. How to make sure all lines are executed before returning. I placed await before every statement but no luck
node.js asynchronous async-await
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
add a comment |
Calling the readCSV from index.js
const productIds = await readCSV();
in another file:
async function readCSV() {
const filepath = path.resolve('src/input_csv.csv');
const readstream = await fs.createReadStream(filepath);
const stream = await readstream.pipe(parser());
let productIds = ;
await stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
await stream.on('end', () => {
console.log(productIds);
});
if (productIds.length > 0) return productIds;
else return 'no products found';
});
}
it is giving the output:
in main:
SourceProductId 1000050429
SourceProductId 1132353
SourceProductId 999915195
SourceProductId 50162873
SourceProductId 1000661087
[ '1000050429', '1132353', '999915195', '50162873', '1000661087' ]
I'm expecting the function to return an array of all values read from CSV. but the seems like it is returning before executing the stream.on statement. How to make sure all lines are executed before returning. I placed await before every statement but no luck
node.js asynchronous async-await
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
1
async/awaitonly works with Promise-like objects. Streams are not like that. You need to learn about Promises before scatteringawaitkeyword all over your code. For exampleawait fs.createReadStream(filepath)what are you trying to "await" for here?
– Yury Tarabanko
Nov 20 '18 at 12:59
how to make sure stream done before returning?
– m9m9m
Nov 20 '18 at 13:01
I have added those as part of debugging now I have removed those.
– m9m9m
Nov 20 '18 at 13:04
You need to create a Promise that will be resolved on stream end.
– Yury Tarabanko
Nov 20 '18 at 13:04
add a comment |
Calling the readCSV from index.js
const productIds = await readCSV();
in another file:
async function readCSV() {
const filepath = path.resolve('src/input_csv.csv');
const readstream = await fs.createReadStream(filepath);
const stream = await readstream.pipe(parser());
let productIds = ;
await stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
await stream.on('end', () => {
console.log(productIds);
});
if (productIds.length > 0) return productIds;
else return 'no products found';
});
}
it is giving the output:
in main:
SourceProductId 1000050429
SourceProductId 1132353
SourceProductId 999915195
SourceProductId 50162873
SourceProductId 1000661087
[ '1000050429', '1132353', '999915195', '50162873', '1000661087' ]
I'm expecting the function to return an array of all values read from CSV. but the seems like it is returning before executing the stream.on statement. How to make sure all lines are executed before returning. I placed await before every statement but no luck
node.js asynchronous async-await
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
Calling the readCSV from index.js
const productIds = await readCSV();
in another file:
async function readCSV() {
const filepath = path.resolve('src/input_csv.csv');
const readstream = await fs.createReadStream(filepath);
const stream = await readstream.pipe(parser());
let productIds = ;
await stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
await stream.on('end', () => {
console.log(productIds);
});
if (productIds.length > 0) return productIds;
else return 'no products found';
});
}
it is giving the output:
in main:
SourceProductId 1000050429
SourceProductId 1132353
SourceProductId 999915195
SourceProductId 50162873
SourceProductId 1000661087
[ '1000050429', '1132353', '999915195', '50162873', '1000661087' ]
I'm expecting the function to return an array of all values read from CSV. but the seems like it is returning before executing the stream.on statement. How to make sure all lines are executed before returning. I placed await before every statement but no luck
node.js asynchronous async-await
node.js asynchronous async-await
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
asked Nov 20 '18 at 12:53
m9m9mm9m9m
475512
475512
1
async/awaitonly works with Promise-like objects. Streams are not like that. You need to learn about Promises before scatteringawaitkeyword all over your code. For exampleawait fs.createReadStream(filepath)what are you trying to "await" for here?
– Yury Tarabanko
Nov 20 '18 at 12:59
how to make sure stream done before returning?
– m9m9m
Nov 20 '18 at 13:01
I have added those as part of debugging now I have removed those.
– m9m9m
Nov 20 '18 at 13:04
You need to create a Promise that will be resolved on stream end.
– Yury Tarabanko
Nov 20 '18 at 13:04
add a comment |
1
async/awaitonly works with Promise-like objects. Streams are not like that. You need to learn about Promises before scatteringawaitkeyword all over your code. For exampleawait fs.createReadStream(filepath)what are you trying to "await" for here?
– Yury Tarabanko
Nov 20 '18 at 12:59
how to make sure stream done before returning?
– m9m9m
Nov 20 '18 at 13:01
I have added those as part of debugging now I have removed those.
– m9m9m
Nov 20 '18 at 13:04
You need to create a Promise that will be resolved on stream end.
– Yury Tarabanko
Nov 20 '18 at 13:04
1
1
async/await only works with Promise-like objects. Streams are not like that. You need to learn about Promises before scattering await keyword all over your code. For example await fs.createReadStream(filepath) what are you trying to "await" for here?– Yury Tarabanko
Nov 20 '18 at 12:59
async/await only works with Promise-like objects. Streams are not like that. You need to learn about Promises before scattering await keyword all over your code. For example await fs.createReadStream(filepath) what are you trying to "await" for here?– Yury Tarabanko
Nov 20 '18 at 12:59
how to make sure stream done before returning?
– m9m9m
Nov 20 '18 at 13:01
how to make sure stream done before returning?
– m9m9m
Nov 20 '18 at 13:01
I have added those as part of debugging now I have removed those.
– m9m9m
Nov 20 '18 at 13:04
I have added those as part of debugging now I have removed those.
– m9m9m
Nov 20 '18 at 13:04
You need to create a Promise that will be resolved on stream end.
– Yury Tarabanko
Nov 20 '18 at 13:04
You need to create a Promise that will be resolved on stream end.
– Yury Tarabanko
Nov 20 '18 at 13:04
add a comment |
1 Answer
1
active
oldest
votes
stream.on doesn't return a promise, so you cannot await it.
You can work around this by returning a promise from your readCSV function:
function readCSV() {
return new Promise(resolve => {
const filepath = path.resolve('src/input_csv.csv');
const readstream = fs.createReadStream(filepath);
const stream = readstream.pipe(parser());
let productIds = ;
stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
stream.on('end', () => {
console.log(productIds);
if (productIds.length > 0) resolve(productIds);
else resolve('no products found');
});
});
}
You create a promise by passing a callback function as argument. That callback function gets an argument resolve, which is a another callback function that you call when your asynchronous operation is done, passing the result.
In the example above, we call this resolve callback with the product IDs after the file read stream has finished.
Since readCSV now returns a promise, you can await it like you did in your code example:
const productIds = await readCSV();
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
Sorry misread your answer.
– Yury Tarabanko
Nov 20 '18 at 13:03
It worked!! please explain.
– m9m9m
Nov 22 '18 at 9:50
I've added some explanation to make it clear what's going on
– Patrick Hund
Nov 22 '18 at 10:29
add a comment |
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1 Answer
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oldest
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oldest
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active
oldest
votes
stream.on doesn't return a promise, so you cannot await it.
You can work around this by returning a promise from your readCSV function:
function readCSV() {
return new Promise(resolve => {
const filepath = path.resolve('src/input_csv.csv');
const readstream = fs.createReadStream(filepath);
const stream = readstream.pipe(parser());
let productIds = ;
stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
stream.on('end', () => {
console.log(productIds);
if (productIds.length > 0) resolve(productIds);
else resolve('no products found');
});
});
}
You create a promise by passing a callback function as argument. That callback function gets an argument resolve, which is a another callback function that you call when your asynchronous operation is done, passing the result.
In the example above, we call this resolve callback with the product IDs after the file read stream has finished.
Since readCSV now returns a promise, you can await it like you did in your code example:
const productIds = await readCSV();
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
Sorry misread your answer.
– Yury Tarabanko
Nov 20 '18 at 13:03
It worked!! please explain.
– m9m9m
Nov 22 '18 at 9:50
I've added some explanation to make it clear what's going on
– Patrick Hund
Nov 22 '18 at 10:29
add a comment |
stream.on doesn't return a promise, so you cannot await it.
You can work around this by returning a promise from your readCSV function:
function readCSV() {
return new Promise(resolve => {
const filepath = path.resolve('src/input_csv.csv');
const readstream = fs.createReadStream(filepath);
const stream = readstream.pipe(parser());
let productIds = ;
stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
stream.on('end', () => {
console.log(productIds);
if (productIds.length > 0) resolve(productIds);
else resolve('no products found');
});
});
}
You create a promise by passing a callback function as argument. That callback function gets an argument resolve, which is a another callback function that you call when your asynchronous operation is done, passing the result.
In the example above, we call this resolve callback with the product IDs after the file read stream has finished.
Since readCSV now returns a promise, you can await it like you did in your code example:
const productIds = await readCSV();
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
Sorry misread your answer.
– Yury Tarabanko
Nov 20 '18 at 13:03
It worked!! please explain.
– m9m9m
Nov 22 '18 at 9:50
I've added some explanation to make it clear what's going on
– Patrick Hund
Nov 22 '18 at 10:29
add a comment |
stream.on doesn't return a promise, so you cannot await it.
You can work around this by returning a promise from your readCSV function:
function readCSV() {
return new Promise(resolve => {
const filepath = path.resolve('src/input_csv.csv');
const readstream = fs.createReadStream(filepath);
const stream = readstream.pipe(parser());
let productIds = ;
stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
stream.on('end', () => {
console.log(productIds);
if (productIds.length > 0) resolve(productIds);
else resolve('no products found');
});
});
}
You create a promise by passing a callback function as argument. That callback function gets an argument resolve, which is a another callback function that you call when your asynchronous operation is done, passing the result.
In the example above, we call this resolve callback with the product IDs after the file read stream has finished.
Since readCSV now returns a promise, you can await it like you did in your code example:
const productIds = await readCSV();
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
stream.on doesn't return a promise, so you cannot await it.
You can work around this by returning a promise from your readCSV function:
function readCSV() {
return new Promise(resolve => {
const filepath = path.resolve('src/input_csv.csv');
const readstream = fs.createReadStream(filepath);
const stream = readstream.pipe(parser());
let productIds = ;
stream.on('data', data => {
productIds.push(data.SourceProductId);
console.log('SourceProductId', data.SourceProductId);
});
stream.on('end', () => {
console.log(productIds);
if (productIds.length > 0) resolve(productIds);
else resolve('no products found');
});
});
}
You create a promise by passing a callback function as argument. That callback function gets an argument resolve, which is a another callback function that you call when your asynchronous operation is done, passing the result.
In the example above, we call this resolve callback with the product IDs after the file read stream has finished.
Since readCSV now returns a promise, you can await it like you did in your code example:
const productIds = await readCSV();
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
edited Nov 22 '18 at 10:28
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
answered Nov 20 '18 at 13:01
Patrick HundPatrick Hund
7,38052147
7,38052147
Sorry misread your answer.
– Yury Tarabanko
Nov 20 '18 at 13:03
It worked!! please explain.
– m9m9m
Nov 22 '18 at 9:50
I've added some explanation to make it clear what's going on
– Patrick Hund
Nov 22 '18 at 10:29
add a comment |
Sorry misread your answer.
– Yury Tarabanko
Nov 20 '18 at 13:03
It worked!! please explain.
– m9m9m
Nov 22 '18 at 9:50
I've added some explanation to make it clear what's going on
– Patrick Hund
Nov 22 '18 at 10:29
Sorry misread your answer.
– Yury Tarabanko
Nov 20 '18 at 13:03
Sorry misread your answer.
– Yury Tarabanko
Nov 20 '18 at 13:03
It worked!! please explain.
– m9m9m
Nov 22 '18 at 9:50
It worked!! please explain.
– m9m9m
Nov 22 '18 at 9:50
I've added some explanation to make it clear what's going on
– Patrick Hund
Nov 22 '18 at 10:29
I've added some explanation to make it clear what's going on
– Patrick Hund
Nov 22 '18 at 10:29
add a comment |
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1
async/awaitonly works with Promise-like objects. Streams are not like that. You need to learn about Promises before scatteringawaitkeyword all over your code. For exampleawait fs.createReadStream(filepath)what are you trying to "await" for here?– Yury Tarabanko
Nov 20 '18 at 12:59
how to make sure stream done before returning?
– m9m9m
Nov 20 '18 at 13:01
I have added those as part of debugging now I have removed those.
– m9m9m
Nov 20 '18 at 13:04
You need to create a Promise that will be resolved on stream end.
– Yury Tarabanko
Nov 20 '18 at 13:04