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I learned two ways of downloading files in node js and express, but I couldn't find out what the difference between them is: res.download(path) and createReadStream(path).
I know that createReadStream() is creating a stream to know overflow the server and it's good way, but what about the other one?
These are the two examples:
const orderId = req.params.orderId;
const invoiceName = 'invoice-' + orderId + '.pdf';
const invoicePath = path.join('data', 'invoices', invoiceName)
res.download(invoicePath, (err) => {
if (err) {
return next(err);
}
});
and
const readStream = fs.createReadStream(invoicePath);
res.setHeader('Content-type', 'application/pdf');
res.setHeader('Content-Dispoition', 'attachment; filename=' + invoiceName);
readStream
.on('open', function () {
// This just pipes the read stream to the response object (which goes to the client)
readStream.pipe(res);
})
.on('end', function () {
readStream.unpipe(res);
console.log('All the data in the file has been read');
})
.on('close', function (err) {
console.log('Stream has been Closed');
next(err)
});
Briefly looking at express res.download() source code, it essentially automates what you are doing manually with the alternative example. This includes relying on streams for efficient transfer and minimizing memory footprint.
https://github.com/expressjs/express/blob/master/lib/response.js
It actually ends up calling the sendfile() function defined at line 1016 - which ends up calling file.pipe(res).
I'm hacking on a Node program that uses smtp-protocol to capture SMTP emails and act on the mail data. The library provides the mail data as a stream, and I don't know how to get that into a string.
I'm currently writing it to stdout with stream.pipe(process.stdout, { end: false }), but as I said, I need the stream data in a string instead, which I can use once the stream has ended.
How do I collect all the data from a Node.js stream into a string?
Another way would be to convert the stream to a promise (refer to the example below) and use then (or await) to assign the resolved value to a variable.
function streamToString (stream) {
const chunks = [];
return new Promise((resolve, reject) => {
stream.on('data', (chunk) => chunks.push(Buffer.from(chunk)));
stream.on('error', (err) => reject(err));
stream.on('end', () => resolve(Buffer.concat(chunks).toString('utf8')));
})
}
const result = await streamToString(stream)
What do you think about this ?
async function streamToString(stream) {
// lets have a ReadableStream as a stream variable
const chunks = [];
for await (const chunk of stream) {
chunks.push(Buffer.from(chunk));
}
return Buffer.concat(chunks).toString("utf-8");
}
None of the above worked for me. I needed to use the Buffer object:
const chunks = [];
readStream.on("data", function (chunk) {
chunks.push(chunk);
});
// Send the buffer or you can put it into a var
readStream.on("end", function () {
res.send(Buffer.concat(chunks));
});
Hope this is more useful than the above answer:
var string = '';
stream.on('data',function(data){
string += data.toString();
console.log('stream data ' + part);
});
stream.on('end',function(){
console.log('final output ' + string);
});
Note that string concatenation is not the most efficient way to collect the string parts, but it is used for simplicity (and perhaps your code does not care about efficiency).
Also, this code may produce unpredictable failures for non-ASCII text (it assumes that every character fits in a byte), but perhaps you do not care about that, either.
(This answer is from years ago, when it was the best answer. There is now a better answer below this. I haven't kept up with node.js, and I cannot delete this answer because it is marked "correct on this question". If you are thinking of down clicking, what do you want me to do?)
The key is to use the data and end events of a Readable Stream. Listen to these events:
stream.on('data', (chunk) => { ... });
stream.on('end', () => { ... });
When you receive the data event, add the new chunk of data to a Buffer created to collect the data.
When you receive the end event, convert the completed Buffer into a string, if necessary. Then do what you need to do with it.
I'm using usually this simple function to transform a stream into a string:
function streamToString(stream, cb) {
const chunks = [];
stream.on('data', (chunk) => {
chunks.push(chunk.toString());
});
stream.on('end', () => {
cb(chunks.join(''));
});
}
Usage example:
let stream = fs.createReadStream('./myFile.foo');
streamToString(stream, (data) => {
console.log(data); // data is now my string variable
});
And yet another one for strings using promises:
function getStream(stream) {
return new Promise(resolve => {
const chunks = [];
# Buffer.from is required if chunk is a String, see comments
stream.on("data", chunk => chunks.push(Buffer.from(chunk)));
stream.on("end", () => resolve(Buffer.concat(chunks).toString()));
});
}
Usage:
const stream = fs.createReadStream(__filename);
getStream(stream).then(r=>console.log(r));
remove the .toString() to use with binary Data if required.
update: #AndreiLED correctly pointed out this has problems with strings. I couldn't get a stream returning strings with the version of node I have, but the api notes this is possible.
From the nodejs documentation you should do this - always remember a string without knowing the encoding is just a bunch of bytes:
var readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', function(chunk) {
assert.equal(typeof chunk, 'string');
console.log('got %d characters of string data', chunk.length);
})
Easy way with the popular (over 5m weekly downloads) and lightweight get-stream library:
https://www.npmjs.com/package/get-stream
const fs = require('fs');
const getStream = require('get-stream');
(async () => {
const stream = fs.createReadStream('unicorn.txt');
console.log(await getStream(stream)); //output is string
})();
Streams don't have a simple .toString() function (which I understand) nor something like a .toStringAsync(cb) function (which I don't understand).
So I created my own helper function:
var streamToString = function(stream, callback) {
var str = '';
stream.on('data', function(chunk) {
str += chunk;
});
stream.on('end', function() {
callback(str);
});
}
// how to use:
streamToString(myStream, function(myStr) {
console.log(myStr);
});
I had more luck using like that :
let string = '';
readstream
.on('data', (buf) => string += buf.toString())
.on('end', () => console.log(string));
I use node v9.11.1 and the readstream is the response from a http.get callback.
The cleanest solution may be to use the "string-stream" package, which converts a stream to a string with a promise.
const streamString = require('stream-string')
streamString(myStream).then(string_variable => {
// myStream was converted to a string, and that string is stored in string_variable
console.log(string_variable)
}).catch(err => {
// myStream emitted an error event (err), so the promise from stream-string was rejected
throw err
})
What about something like a stream reducer ?
Here is an example using ES6 classes how to use one.
var stream = require('stream')
class StreamReducer extends stream.Writable {
constructor(chunkReducer, initialvalue, cb) {
super();
this.reducer = chunkReducer;
this.accumulator = initialvalue;
this.cb = cb;
}
_write(chunk, enc, next) {
this.accumulator = this.reducer(this.accumulator, chunk);
next();
}
end() {
this.cb(null, this.accumulator)
}
}
// just a test stream
class EmitterStream extends stream.Readable {
constructor(chunks) {
super();
this.chunks = chunks;
}
_read() {
this.chunks.forEach(function (chunk) {
this.push(chunk);
}.bind(this));
this.push(null);
}
}
// just transform the strings into buffer as we would get from fs stream or http request stream
(new EmitterStream(
["hello ", "world !"]
.map(function(str) {
return Buffer.from(str, 'utf8');
})
)).pipe(new StreamReducer(
function (acc, v) {
acc.push(v);
return acc;
},
[],
function(err, chunks) {
console.log(Buffer.concat(chunks).toString('utf8'));
})
);
All the answers listed appear to open the Readable Stream in flowing mode which is not the default in NodeJS and can have limitations since it lacks backpressure support that NodeJS provides in Paused Readable Stream Mode.
Here is an implementation using Just Buffers, Native Stream and Native Stream Transforms and support for Object Mode
import {Transform} from 'stream';
let buffer =null;
function objectifyStream() {
return new Transform({
objectMode: true,
transform: function(chunk, encoding, next) {
if (!buffer) {
buffer = Buffer.from([...chunk]);
} else {
buffer = Buffer.from([...buffer, ...chunk]);
}
next(null, buffer);
}
});
}
process.stdin.pipe(objectifyStream()).process.stdout
This worked for me and is based on Node v6.7.0 docs:
let output = '';
stream.on('readable', function() {
let read = stream.read();
if (read !== null) {
// New stream data is available
output += read.toString();
} else {
// Stream is now finished when read is null.
// You can callback here e.g.:
callback(null, output);
}
});
stream.on('error', function(err) {
callback(err, null);
})
Using the quite popular stream-buffers package which you probably already have in your project dependencies, this is pretty straightforward:
// imports
const { WritableStreamBuffer } = require('stream-buffers');
const { promisify } = require('util');
const { createReadStream } = require('fs');
const pipeline = promisify(require('stream').pipeline);
// sample stream
let stream = createReadStream('/etc/hosts');
// pipeline the stream into a buffer, and print the contents when done
let buf = new WritableStreamBuffer();
pipeline(stream, buf).then(() => console.log(buf.getContents().toString()));
setEncoding('utf8');
Well done Sebastian J above.
I had the "buffer problem" with a few lines of test code I had, and added the encoding information and it solved it, see below.
Demonstrate the problem
software
// process.stdin.setEncoding('utf8');
process.stdin.on('data', (data) => {
console.log(typeof(data), data);
});
input
hello world
output
object <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64 0d 0a>
Demonstrate the solution
software
process.stdin.setEncoding('utf8'); // <- Activate!
process.stdin.on('data', (data) => {
console.log(typeof(data), data);
});
input
hello world
output
string hello world
In my case, the content type response headers was Content-Type: text/plain. So, I've read the data from Buffer like:
let data = [];
stream.on('data', (chunk) => {
console.log(Buffer.from(chunk).toString())
data.push(Buffer.from(chunk).toString())
});
I've been using node-oracledb for a few months and I've managed to achieve what I have needed to so far.
I'm currently working on a search app that could potentially return about 2m rows of data from a single call. To ensure I don't get a disconnect from the browser and the server, I thought I would try queryStream so that there is a constant flow of data back to the client.
I implemented the queryStream example as-is, and this worked fine for a few hundred thousand rows. However, when the returned rows is greater than one million, Node runs out of memory. By logging and watching both client and server log events, I can see that client is way behind the server in terms of rows sent and received. So, it looks like Node is falling over because it's buffering so much data.
It's worth noting that at this point, my selectstream implementation is within a req/res function called via Express.
To return the data, I do something like....
stream.on('data', function (data) {
rowcount++;
let obj = new myObjectConstructor(data);
res.write(JSON.stringify(obj.getJson());
});
I've been reading about how streams and pipe can help with flow, so what I'd like to be able to do is to be able to pipe the results from the query to a) help with flow and b) to be able to pipe the results to other functions before sending back to the client.
E.g.
function getData(req, res){
var stream = myQueryStream(connection, query);
stream
.pipe(toSomeOtherFunction)
.pipe(yetAnotherFunction)
.pipe(res);
}
I'm spent a few hours trying to find a solution or example that allows me to pipe results, but I'm stuck and need some help.
Apologies if I'm missing something obvious, but I'm still getting to grips with Node and especially streams.
Thanks in advance.
There's a bit of an impedance mismatch here. The queryStream API emits rows of JavaScript objects, but what you want to stream to the client is a JSON array. You basically have to add an open bracket to the beginning, a comma after each row, and a close bracket to the end.
I'll show you how to do this in a controller that uses the driver directly as you have done, instead of using separate database modules as I advocate in this series.
const oracledb = require('oracledb');
async function get(req, res, next) {
try {
const conn = await oracledb.getConnection();
const stream = await conn.queryStream('select * from employees', [], {outFormat: oracledb.OBJECT});
res.writeHead(200, {'Content-Type': 'application/json'});
res.write('[');
stream.on('data', (row) => {
res.write(JSON.stringify(row));
res.write(',');
});
stream.on('end', () => {
res.end(']');
});
stream.on('close', async () => {
try {
await conn.close();
} catch (err) {
console.log(err);
}
});
stream.on('error', async (err) => {
next(err);
try {
await conn.close();
} catch (err) {
console.log(err);
}
});
} catch (err) {
next(err);
}
}
module.exports.get = get;
Once you get the concepts, you can simplify things a bit with a reusable Transform class which allows you to use pipe in the controller logic:
const oracledb = require('oracledb');
const { Transform } = require('stream');
class ToJSONArray extends Transform {
constructor() {
super({objectMode: true});
this.push('[');
}
_transform (row, encoding, callback) {
if (this._prevRow) {
this.push(JSON.stringify(this._prevRow));
this.push(',');
}
this._prevRow = row;
callback(null);
}
_flush (done) {
if (this._prevRow) {
this.push(JSON.stringify(this._prevRow));
}
this.push(']');
delete this._prevRow;
done();
}
}
async function get(req, res, next) {
try {
const toJSONArray = new ToJSONArray();
const conn = await oracledb.getConnection();
const stream = await conn.queryStream('select * from employees', [], {outFormat: oracledb.OBJECT});
res.writeHead(200, {'Content-Type': 'application/json'});
stream.pipe(toJSONArray).pipe(res);
stream.on('close', async () => {
try {
await conn.close();
} catch (err) {
console.log(err);
}
});
stream.on('error', async (err) => {
next(err);
try {
await conn.close();
} catch (err) {
console.log(err);
}
});
} catch (err) {
next(err);
}
}
module.exports.get = get;
Rather than writing your own logic to create a JSON stream, you can use JSONStream to convert an object stream to (stringified) JSON, before piping it to its destination (res, process.stdout etc) This saves the need to muck around with .on('data',...) events.
In the example below, I've used pipeline from node's stream module rather than the .pipe method: the effect is similar (with better error handling I think). To get objects from oracledb.queryStream, you can specify option {outFormat: oracledb.OUT_FORMAT_OBJECT} (docs). Then you can make arbitrary modifications to the stream of objects produced. This can be done using a transform stream, made perhaps using through2-map, or if you need to drop or split rows, through2. Below the stream is sent to process.stdout after being stringified as JSON, but you could equally send to it express's res.
require('dotenv').config() // config from .env file
const JSONStream = require('JSONStream')
const oracledb = require('oracledb')
const { pipeline } = require('stream')
const map = require('through2-map') // see https://www.npmjs.com/package/through2-map
oracledb.getConnection({
user: process.env.DB_USER,
password: process.env.DB_PASSWORD,
connectString: process.env.CONNECT_STRING
}).then(connection => {
pipeline(
connection.queryStream(`
select dual.*,'test' as col1 from dual
union select dual.*, :someboundvalue as col1 from dual
`
,{"someboundvalue":"test5"} // binds
,{
prefetchRows: 150, // for tuning
fetchArraySize: 150, // for tuning
outFormat: oracledb.OUT_FORMAT_OBJECT
}
)
,map.obj((row,index) => {
row.arbitraryModification = index
return row
})
,JSONStream.stringify() // false gives ndjson
,process.stdout // or send to express's res
,(err) => { if(err) console.error(err) }
)
})
// [
// {"DUMMY":"X","COL1":"test","arbitraryModification":0}
// ,
// {"DUMMY":"X","COL1":"test5","arbitraryModification":1}
// ]
This feels like an obvious question but it's perplexing me: I want a Node function that downloads a resource at a URI. I need it to work for several different content types without the user needing to specify which type it is.
I know how to pipe request to fs.createWriteStream when you know it's going to be an image, but not how to handle it when you've already invoked the callback from request. Here's where I am:
var request = require('request'),
fs = require('graceful-fs');
function cacheURI(uri, cache_path, cb) {
request(uri, function(err, resp, body) {
var content_type = resp.headers['content-type'].toLowerCase().split("; ")[0],
type = content_type.split("/")[0],
sub_type = content_type.split("/")[1];
if (sub_type == "json") {
body = JSON.parse(body);
}
if (type == "image") {
// this is where the trouble starts
var ws = fs.createWriteStream(cache_path);
ws.write(body);
ws.on('close', function() {
console.log('image done');
console.log(resp.socket.bytesRead);
ws.end();
cb()
});
} else {
// this works fine for text resources
fs.writeFile(cache_path, body, cb);
}
});
}
This answer to a previous question suggests the following:
request.get({url: 'https://someurl/somefile.torrent', encoding: 'binary'}, function (err, response, body) {
fs.writeFile("/tmp/test.torrent", body, 'binary', function(err) {
if(err)
console.log(err);
else
console.log("The file was saved!");
});
});
But I can't pass "binary" to request if I don't yet know the type of response I'll get.
UPDATE
Per the suggested answer, changing "close" to "finish" in the event handler does fire the callback:
if (opts.image) {
var ws = fs.createWriteStream(opts.path);
ws.on('finish', function() {
console.log('image done');
console.log(resp.socket.bytesRead);
});
//tried as buffer as well
//ws.write(new Buffer(body));
ws.write(body);
ws.end();
}
This does write the image file, but not correctly:
As suggested in here, try using the finish event (if you have node >= v0.10)
ws.on('finish', function() {
console.log('image done');
console.log(resp.socket.bytesRead);
ws.end();
cb()
});
I wrote a pretty simple function that downloads an image from a given URL, resize it and upload to S3 (using 'gm' and 'knox'), I have no idea if I'm doing the reading of a stream to a buffer correctly. (everything is working, but is it the correct way?)
also, I want to understand something about the event loop, how do I know that one invocation of the function won't leak anything or change the 'buf' variable to another already running invocation (or this scenario is impossible because the callbacks are anonymous functions?)
var http = require('http');
var https = require('https');
var s3 = require('./s3');
var gm = require('gm');
module.exports.processImageUrl = function(imageUrl, filename, callback) {
var client = http;
if (imageUrl.substr(0, 5) == 'https') { client = https; }
client.get(imageUrl, function(res) {
if (res.statusCode != 200) {
return callback(new Error('HTTP Response code ' + res.statusCode));
}
gm(res)
.geometry(1024, 768, '>')
.stream('jpg', function(err, stdout, stderr) {
if (!err) {
var buf = new Buffer(0);
stdout.on('data', function(d) {
buf = Buffer.concat([buf, d]);
});
stdout.on('end', function() {
var headers = {
'Content-Length': buf.length
, 'Content-Type': 'Image/jpeg'
, 'x-amz-acl': 'public-read'
};
s3.putBuffer(buf, '/img/d/' + filename + '.jpg', headers, function(err, res) {
if(err) {
return callback(err);
} else {
return callback(null, res.client._httpMessage.url);
}
});
});
} else {
callback(err);
}
});
}).on('error', function(err) {
callback(err);
});
};
Overall I don't see anything that would break in your code.
Two suggestions:
The way you are combining Buffer objects is a suboptimal because it has to copy all the pre-existing data on every 'data' event. It would be better to put the chunks in an array and concat them all at the end.
var bufs = [];
stdout.on('data', function(d){ bufs.push(d); });
stdout.on('end', function(){
var buf = Buffer.concat(bufs);
})
For performance, I would look into if the S3 library you are using supports streams. Ideally you wouldn't need to create one large buffer at all, and instead just pass the stdout stream directly to the S3 library.
As for the second part of your question, that isn't possible. When a function is called, it is allocated its own private context, and everything defined inside of that will only be accessible from other items defined inside that function.
Update
Dumping the file to the filesystem would probably mean less memory usage per request, but file IO can be pretty slow so it might not be worth it. I'd say that you shouldn't optimize too much until you can profile and stress-test this function. If the garbage collector is doing its job you may be overoptimizing.
With all that said, there are better ways anyway, so don't use files. Since all you want is the length, you can calculate that without needing to append all of the buffers together, so then you don't need to allocate a new Buffer at all.
var pause_stream = require('pause-stream');
// Your other code.
var bufs = [];
stdout.on('data', function(d){ bufs.push(d); });
stdout.on('end', function(){
var contentLength = bufs.reduce(function(sum, buf){
return sum + buf.length;
}, 0);
// Create a stream that will emit your chunks when resumed.
var stream = pause_stream();
stream.pause();
while (bufs.length) stream.write(bufs.shift());
stream.end();
var headers = {
'Content-Length': contentLength,
// ...
};
s3.putStream(stream, ....);
Javascript snippet
function stream2buffer(stream) {
return new Promise((resolve, reject) => {
const _buf = [];
stream.on("data", (chunk) => _buf.push(chunk));
stream.on("end", () => resolve(Buffer.concat(_buf)));
stream.on("error", (err) => reject(err));
});
}
Typescript snippet
async function stream2buffer(stream: Stream): Promise<Buffer> {
return new Promise < Buffer > ((resolve, reject) => {
const _buf = Array < any > ();
stream.on("data", chunk => _buf.push(chunk));
stream.on("end", () => resolve(Buffer.concat(_buf)));
stream.on("error", err => reject(`error converting stream - ${err}`));
});
}
You can easily do this using node-fetch if you are pulling from http(s) URIs.
From the readme:
fetch('https://assets-cdn.github.com/images/modules/logos_page/Octocat.png')
.then(res => res.buffer())
.then(buffer => console.log)
Note: this solely answers "How to read a stream into a buffer?" and ignores the context of the original question.
ES2018 Answer
Since Node 11.14.0, readable streams support async iterators.
const buffers = [];
// node.js readable streams implement the async iterator protocol
for await (const data of readableStream) {
buffers.push(data);
}
const finalBuffer = Buffer.concat(buffers);
Bonus: In the future, this could get better with the stage 2 Array.fromAsync proposal.
// 🛑 DOES NOT WORK (yet!)
const finalBuffer = Buffer.concat(await Array.fromAsync(readableStream));
You can convert your readable stream to a buffer and integrate it in your code in an asynchronous way like this.
async streamToBuffer (stream) {
return new Promise((resolve, reject) => {
const data = [];
stream.on('data', (chunk) => {
data.push(chunk);
});
stream.on('end', () => {
resolve(Buffer.concat(data))
})
stream.on('error', (err) => {
reject(err)
})
})
}
the usage would be as simple as:
// usage
const myStream // your stream
const buffer = await streamToBuffer(myStream) // this is a buffer
I suggest loganfsmyths method, using an array to hold the data.
var bufs = [];
stdout.on('data', function(d){ bufs.push(d); });
stdout.on('end', function(){
var buf = Buffer.concat(bufs);
}
IN my current working example, i am working with GRIDfs and npm's Jimp.
var bucket = new GridFSBucket(getDBReference(), { bucketName: 'images' } );
var dwnldStream = bucket.openDownloadStream(info[0]._id);// original size
dwnldStream.on('data', function(chunk) {
data.push(chunk);
});
dwnldStream.on('end', function() {
var buff =Buffer.concat(data);
console.log("buffer: ", buff);
jimp.read(buff)
.then(image => {
console.log("read the image!");
IMAGE_SIZES.forEach( (size)=>{
resize(image,size);
});
});
I did some other research
with a string method but that did not work, per haps because i was reading from an image file, but the array method did work.
const DISCLAIMER = "DONT DO THIS";
var data = "";
stdout.on('data', function(d){
bufs+=d;
});
stdout.on('end', function(){
var buf = Buffer.from(bufs);
//// do work with the buffer here
});
When i did the string method i got this error from npm jimp
buffer: <Buffer 00 00 00 00 00>
{ Error: Could not find MIME for Buffer <null>
basically i think the type coersion from binary to string didnt work so well.
I suggest to have array of buffers and concat to resulting buffer only once at the end. Its easy to do manually, or one could use node-buffers
I just want to post my solution. Previous answers was pretty helpful for my research. I use length-stream to get the size of the stream, but the problem here is that the callback is fired near the end of the stream, so i also use stream-cache to cache the stream and pipe it to res object once i know the content-length. In case on an error,
var StreamCache = require('stream-cache');
var lengthStream = require('length-stream');
var _streamFile = function(res , stream , cb){
var cache = new StreamCache();
var lstream = lengthStream(function(length) {
res.header("Content-Length", length);
cache.pipe(res);
});
stream.on('error', function(err){
return cb(err);
});
stream.on('end', function(){
return cb(null , true);
});
return stream.pipe(lstream).pipe(cache);
}
in ts, [].push(bufferPart) is not compatible;
so:
getBufferFromStream(stream: Part | null): Promise<Buffer> {
if (!stream) {
throw 'FILE_STREAM_EMPTY';
}
return new Promise(
(r, j) => {
let buffer = Buffer.from([]);
stream.on('data', buf => {
buffer = Buffer.concat([buffer, buf]);
});
stream.on('end', () => r(buffer));
stream.on('error', j);
}
);
}
You can do this by:
async function toBuffer(stream: ReadableStream<Uint8Array>) {
const list = []
const reader = stream.getReader()
while (true) {
const { value, done } = await reader.read()
if (value)
list.push(value)
if (done)
break
}
return Buffer.concat(list)
}
or using buffer consumer
const buf = buffer(stream)
You can check the "content-length" header at res.headers. It will give you the length of the content you will receive (how many bytes of data it will send)