Related
I have a file which stores many JavaScript objects in JSON form and I need to read the file, create each of the objects, and do something with them (insert them into a db in my case). The JavaScript objects can be represented a format:
Format A:
[{name: 'thing1'},
....
{name: 'thing999999999'}]
or Format B:
{name: 'thing1'} // <== My choice.
...
{name: 'thing999999999'}
Note that the ... indicates a lot of JSON objects. I am aware I could read the entire file into memory and then use JSON.parse() like this:
fs.readFile(filePath, 'utf-8', function (err, fileContents) {
if (err) throw err;
console.log(JSON.parse(fileContents));
});
However, the file could be really large, I would prefer to use a stream to accomplish this. The problem I see with a stream is that the file contents could be broken into data chunks at any point, so how can I use JSON.parse() on such objects?
Ideally, each object would be read as a separate data chunk, but I am not sure on how to do that.
var importStream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
importStream.on('data', function(chunk) {
var pleaseBeAJSObject = JSON.parse(chunk);
// insert pleaseBeAJSObject in a database
});
importStream.on('end', function(item) {
console.log("Woot, imported objects into the database!");
});*/
Note, I wish to prevent reading the entire file into memory. Time efficiency does not matter to me. Yes, I could try to read a number of objects at once and insert them all at once, but that's a performance tweak - I need a way that is guaranteed not to cause a memory overload, not matter how many objects are contained in the file.
I can choose to use FormatA or FormatB or maybe something else, just please specify in your answer. Thanks!
To process a file line-by-line, you simply need to decouple the reading of the file and the code that acts upon that input. You can accomplish this by buffering your input until you hit a newline. Assuming we have one JSON object per line (basically, format B):
var stream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
var buf = '';
stream.on('data', function(d) {
buf += d.toString(); // when data is read, stash it in a string buffer
pump(); // then process the buffer
});
function pump() {
var pos;
while ((pos = buf.indexOf('\n')) >= 0) { // keep going while there's a newline somewhere in the buffer
if (pos == 0) { // if there's more than one newline in a row, the buffer will now start with a newline
buf = buf.slice(1); // discard it
continue; // so that the next iteration will start with data
}
processLine(buf.slice(0,pos)); // hand off the line
buf = buf.slice(pos+1); // and slice the processed data off the buffer
}
}
function processLine(line) { // here's where we do something with a line
if (line[line.length-1] == '\r') line=line.substr(0,line.length-1); // discard CR (0x0D)
if (line.length > 0) { // ignore empty lines
var obj = JSON.parse(line); // parse the JSON
console.log(obj); // do something with the data here!
}
}
Each time the file stream receives data from the file system, it's stashed in a buffer, and then pump is called.
If there's no newline in the buffer, pump simply returns without doing anything. More data (and potentially a newline) will be added to the buffer the next time the stream gets data, and then we'll have a complete object.
If there is a newline, pump slices off the buffer from the beginning to the newline and hands it off to process. It then checks again if there's another newline in the buffer (the while loop). In this way, we can process all of the lines that were read in the current chunk.
Finally, process is called once per input line. If present, it strips off the carriage return character (to avoid issues with line endings – LF vs CRLF), and then calls JSON.parse one the line. At this point, you can do whatever you need to with your object.
Note that JSON.parse is strict about what it accepts as input; you must quote your identifiers and string values with double quotes. In other words, {name:'thing1'} will throw an error; you must use {"name":"thing1"}.
Because no more than a chunk of data will ever be in memory at a time, this will be extremely memory efficient. It will also be extremely fast. A quick test showed I processed 10,000 rows in under 15ms.
Just as I was thinking that it would be fun to write a streaming JSON parser, I also thought that maybe I should do a quick search to see if there's one already available.
Turns out there is.
JSONStream "streaming JSON.parse and stringify"
Since I just found it, I've obviously not used it, so I can't comment on its quality, but I'll be interested to hear if it works.
It does work consider the following Javascript and _.isString:
stream.pipe(JSONStream.parse('*'))
.on('data', (d) => {
console.log(typeof d);
console.log("isString: " + _.isString(d))
});
This will log objects as they come in if the stream is an array of objects. Therefore the only thing being buffered is one object at a time.
As of October 2014, you can just do something like the following (using JSONStream) - https://www.npmjs.org/package/JSONStream
var fs = require('fs'),
JSONStream = require('JSONStream'),
var getStream() = function () {
var jsonData = 'myData.json',
stream = fs.createReadStream(jsonData, { encoding: 'utf8' }),
parser = JSONStream.parse('*');
return stream.pipe(parser);
}
getStream().pipe(MyTransformToDoWhateverProcessingAsNeeded).on('error', function (err) {
// handle any errors
});
To demonstrate with a working example:
npm install JSONStream event-stream
data.json:
{
"greeting": "hello world"
}
hello.js:
var fs = require('fs'),
JSONStream = require('JSONStream'),
es = require('event-stream');
var getStream = function () {
var jsonData = 'data.json',
stream = fs.createReadStream(jsonData, { encoding: 'utf8' }),
parser = JSONStream.parse('*');
return stream.pipe(parser);
};
getStream()
.pipe(es.mapSync(function (data) {
console.log(data);
}));
$ node hello.js
// hello world
I had similar requirement, i need to read a large json file in node js and process data in chunks and call a api and save in mongodb.
inputFile.json is like:
{
"customers":[
{ /*customer data*/},
{ /*customer data*/},
{ /*customer data*/}....
]
}
Now i used JsonStream and EventStream to achieve this synchronously.
var JSONStream = require("JSONStream");
var es = require("event-stream");
fileStream = fs.createReadStream(filePath, { encoding: "utf8" });
fileStream.pipe(JSONStream.parse("customers.*")).pipe(
es.through(function(data) {
console.log("printing one customer object read from file ::");
console.log(data);
this.pause();
processOneCustomer(data, this);
return data;
}),
function end() {
console.log("stream reading ended");
this.emit("end");
}
);
function processOneCustomer(data, es) {
DataModel.save(function(err, dataModel) {
es.resume();
});
}
I realize that you want to avoid reading the whole JSON file into memory if possible, however if you have the memory available it may not be a bad idea performance-wise. Using node.js's require() on a json file loads the data into memory really fast.
I ran two tests to see what the performance looked like on printing out an attribute from each feature from a 81MB geojson file.
In the 1st test, I read the entire geojson file into memory using var data = require('./geo.json'). That took 3330 milliseconds and then printing out an attribute from each feature took 804 milliseconds for a grand total of 4134 milliseconds. However, it appeared that node.js was using 411MB of memory.
In the second test, I used #arcseldon's answer with JSONStream + event-stream. I modified the JSONPath query to select only what I needed. This time the memory never went higher than 82MB, however, the whole thing now took 70 seconds to complete!
I wrote a module that can do this, called BFJ. Specifically, the method bfj.match can be used to break up a large stream into discrete chunks of JSON:
const bfj = require('bfj');
const fs = require('fs');
const stream = fs.createReadStream(filePath);
bfj.match(stream, (key, value, depth) => depth === 0, { ndjson: true })
.on('data', object => {
// do whatever you need to do with object
})
.on('dataError', error => {
// a syntax error was found in the JSON
})
.on('error', error => {
// some kind of operational error occurred
})
.on('end', error => {
// finished processing the stream
});
Here, bfj.match returns a readable, object-mode stream that will receive the parsed data items, and is passed 3 arguments:
A readable stream containing the input JSON.
A predicate that indicates which items from the parsed JSON will be pushed to the result stream.
An options object indicating that the input is newline-delimited JSON (this is to process format B from the question, it's not required for format A).
Upon being called, bfj.match will parse JSON from the input stream depth-first, calling the predicate with each value to determine whether or not to push that item to the result stream. The predicate is passed three arguments:
The property key or array index (this will be undefined for top-level items).
The value itself.
The depth of the item in the JSON structure (zero for top-level items).
Of course a more complex predicate can also be used as necessary according to requirements. You can also pass a string or a regular expression instead of a predicate function, if you want to perform simple matches against property keys.
If you have control over the input file, and it's an array of objects, you can solve this more easily. Arrange to output the file with each record on one line, like this:
[
{"key": value},
{"key": value},
...
This is still valid JSON.
Then, use the node.js readline module to process them one line at a time.
var fs = require("fs");
var lineReader = require('readline').createInterface({
input: fs.createReadStream("input.txt")
});
lineReader.on('line', function (line) {
line = line.trim();
if (line.charAt(line.length-1) === ',') {
line = line.substr(0, line.length-1);
}
if (line.charAt(0) === '{') {
processRecord(JSON.parse(line));
}
});
function processRecord(record) {
// Process the records one at a time here!
}
I solved this problem using the split npm module. Pipe your stream into split, and it will "Break up a stream and reassemble it so that each line is a chunk".
Sample code:
var fs = require('fs')
, split = require('split')
;
var stream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
var lineStream = stream.pipe(split());
linestream.on('data', function(chunk) {
var json = JSON.parse(chunk);
// ...
});
Using the #josh3736 answer, but for ES2021 and Node.js 16+ with async/await + AirBnb rules:
import fs from 'node:fs';
const file = 'file.json';
/**
* #callback itemProcessorCb
* #param {object} item The current item
*/
/**
* Process each data chunk in a stream.
*
* #param {import('fs').ReadStream} readable The readable stream
* #param {itemProcessorCb} itemProcessor A function to process each item
*/
async function processChunk(readable, itemProcessor) {
let data = '';
let total = 0;
// eslint-disable-next-line no-restricted-syntax
for await (const chunk of readable) {
// join with last result, remove CR and get lines
const lines = (data + chunk).replace('\r', '').split('\n');
// clear last result
data = '';
// process lines
let line = lines.shift();
const items = [];
while (line) {
// check if isn't a empty line or an array definition
if (line !== '' && !/[\[\]]+/.test(line)) {
try {
// remove the last comma and parse json
const json = JSON.parse(line.replace(/\s?(,)+\s?$/, ''));
items.push(json);
} catch (error) {
// last line gets only a partial line from chunk
// so we add this to join at next loop
data += line;
}
}
// continue
line = lines.shift();
}
total += items.length;
// Process items in parallel
await Promise.all(items.map(itemProcessor));
}
console.log(`${total} items processed.`);
}
// Process each item
async function processItem(item) {
console.log(item);
}
// Init
try {
const readable = fs.createReadStream(file, {
flags: 'r',
encoding: 'utf-8',
});
processChunk(readable, processItem);
} catch (error) {
console.error(error.message);
}
For a JSON like:
[
{ "name": "A", "active": true },
{ "name": "B", "active": false },
...
]
https.get(url1 , function(response) {
var data = "";
response.on('data', function(chunk) {
data += chunk.toString();
})
.on('end', function() {
console.log(data)
});
});
I think you need to use a database. MongoDB is a good choice in this case because it is JSON compatible.
UPDATE:
You can use mongoimport tool to import JSON data into MongoDB.
mongoimport --collection collection --file collection.json
Hi I am stuck on this issue,
I reconstructed a few protobuf object in order to seed data to my miragejs instance,
const unknownObject = new core.UnknownObject()
const unknownEntity = new message.UnknownObjectEntity()
const unknownEntityRepo = new message.UnknownObjectEntityRepository()
const unknownObjectNotification = new message.UnknownObjectNotification()
const date = new google_protobuf_timestamp_pb.Timestamp()
unknownObject.setImage(Base64())
unknownObject.setTimestamp(date)
unknownObject.setWaypoint(new core.Waypoint())
console.log('unknownObject:', unknownObject.getTimestamp())
unknownEntity.setId('1')
unknownEntity.setUnknownobject(unknownObject)
console.log('unknownEntity:', unknownEntity)
const endBuffer = unknownEntityRepo.addEntity(unknownEntity).serializeBinary()
the end result (endBuffer) is a byte array. I want to reconstruct this byte array into a readablestream in order to seed the data. This is the final result needed.
ReadableStream {locked: false}
I can only fund resources made to be used to read the stream but never the transform a byte array into the ReadableStream.
Does anyone have experience with this. Thanks
You need to implement ReadableStream and add your content to the controller. Mozilla's MDN Web Docs provide some guidance in their simple random string example.
Here is a simplified example:
function buildStream(data)
{
//depending on what data is and how it will be used, you may need to convert it
//data = Uint8Array.from(data);
return new ReadableStream({
start(controller) {
// Add the data to the stream
controller.enqueue(data);
},
pull(controller) {
// nothing left to pull, so close
controller.close();
},
cancel() {
//nothing to do
}
});
}
I have a file which stores many JavaScript objects in JSON form and I need to read the file, create each of the objects, and do something with them (insert them into a db in my case). The JavaScript objects can be represented a format:
Format A:
[{name: 'thing1'},
....
{name: 'thing999999999'}]
or Format B:
{name: 'thing1'} // <== My choice.
...
{name: 'thing999999999'}
Note that the ... indicates a lot of JSON objects. I am aware I could read the entire file into memory and then use JSON.parse() like this:
fs.readFile(filePath, 'utf-8', function (err, fileContents) {
if (err) throw err;
console.log(JSON.parse(fileContents));
});
However, the file could be really large, I would prefer to use a stream to accomplish this. The problem I see with a stream is that the file contents could be broken into data chunks at any point, so how can I use JSON.parse() on such objects?
Ideally, each object would be read as a separate data chunk, but I am not sure on how to do that.
var importStream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
importStream.on('data', function(chunk) {
var pleaseBeAJSObject = JSON.parse(chunk);
// insert pleaseBeAJSObject in a database
});
importStream.on('end', function(item) {
console.log("Woot, imported objects into the database!");
});*/
Note, I wish to prevent reading the entire file into memory. Time efficiency does not matter to me. Yes, I could try to read a number of objects at once and insert them all at once, but that's a performance tweak - I need a way that is guaranteed not to cause a memory overload, not matter how many objects are contained in the file.
I can choose to use FormatA or FormatB or maybe something else, just please specify in your answer. Thanks!
To process a file line-by-line, you simply need to decouple the reading of the file and the code that acts upon that input. You can accomplish this by buffering your input until you hit a newline. Assuming we have one JSON object per line (basically, format B):
var stream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
var buf = '';
stream.on('data', function(d) {
buf += d.toString(); // when data is read, stash it in a string buffer
pump(); // then process the buffer
});
function pump() {
var pos;
while ((pos = buf.indexOf('\n')) >= 0) { // keep going while there's a newline somewhere in the buffer
if (pos == 0) { // if there's more than one newline in a row, the buffer will now start with a newline
buf = buf.slice(1); // discard it
continue; // so that the next iteration will start with data
}
processLine(buf.slice(0,pos)); // hand off the line
buf = buf.slice(pos+1); // and slice the processed data off the buffer
}
}
function processLine(line) { // here's where we do something with a line
if (line[line.length-1] == '\r') line=line.substr(0,line.length-1); // discard CR (0x0D)
if (line.length > 0) { // ignore empty lines
var obj = JSON.parse(line); // parse the JSON
console.log(obj); // do something with the data here!
}
}
Each time the file stream receives data from the file system, it's stashed in a buffer, and then pump is called.
If there's no newline in the buffer, pump simply returns without doing anything. More data (and potentially a newline) will be added to the buffer the next time the stream gets data, and then we'll have a complete object.
If there is a newline, pump slices off the buffer from the beginning to the newline and hands it off to process. It then checks again if there's another newline in the buffer (the while loop). In this way, we can process all of the lines that were read in the current chunk.
Finally, process is called once per input line. If present, it strips off the carriage return character (to avoid issues with line endings – LF vs CRLF), and then calls JSON.parse one the line. At this point, you can do whatever you need to with your object.
Note that JSON.parse is strict about what it accepts as input; you must quote your identifiers and string values with double quotes. In other words, {name:'thing1'} will throw an error; you must use {"name":"thing1"}.
Because no more than a chunk of data will ever be in memory at a time, this will be extremely memory efficient. It will also be extremely fast. A quick test showed I processed 10,000 rows in under 15ms.
Just as I was thinking that it would be fun to write a streaming JSON parser, I also thought that maybe I should do a quick search to see if there's one already available.
Turns out there is.
JSONStream "streaming JSON.parse and stringify"
Since I just found it, I've obviously not used it, so I can't comment on its quality, but I'll be interested to hear if it works.
It does work consider the following Javascript and _.isString:
stream.pipe(JSONStream.parse('*'))
.on('data', (d) => {
console.log(typeof d);
console.log("isString: " + _.isString(d))
});
This will log objects as they come in if the stream is an array of objects. Therefore the only thing being buffered is one object at a time.
As of October 2014, you can just do something like the following (using JSONStream) - https://www.npmjs.org/package/JSONStream
var fs = require('fs'),
JSONStream = require('JSONStream'),
var getStream() = function () {
var jsonData = 'myData.json',
stream = fs.createReadStream(jsonData, { encoding: 'utf8' }),
parser = JSONStream.parse('*');
return stream.pipe(parser);
}
getStream().pipe(MyTransformToDoWhateverProcessingAsNeeded).on('error', function (err) {
// handle any errors
});
To demonstrate with a working example:
npm install JSONStream event-stream
data.json:
{
"greeting": "hello world"
}
hello.js:
var fs = require('fs'),
JSONStream = require('JSONStream'),
es = require('event-stream');
var getStream = function () {
var jsonData = 'data.json',
stream = fs.createReadStream(jsonData, { encoding: 'utf8' }),
parser = JSONStream.parse('*');
return stream.pipe(parser);
};
getStream()
.pipe(es.mapSync(function (data) {
console.log(data);
}));
$ node hello.js
// hello world
I had similar requirement, i need to read a large json file in node js and process data in chunks and call a api and save in mongodb.
inputFile.json is like:
{
"customers":[
{ /*customer data*/},
{ /*customer data*/},
{ /*customer data*/}....
]
}
Now i used JsonStream and EventStream to achieve this synchronously.
var JSONStream = require("JSONStream");
var es = require("event-stream");
fileStream = fs.createReadStream(filePath, { encoding: "utf8" });
fileStream.pipe(JSONStream.parse("customers.*")).pipe(
es.through(function(data) {
console.log("printing one customer object read from file ::");
console.log(data);
this.pause();
processOneCustomer(data, this);
return data;
}),
function end() {
console.log("stream reading ended");
this.emit("end");
}
);
function processOneCustomer(data, es) {
DataModel.save(function(err, dataModel) {
es.resume();
});
}
I realize that you want to avoid reading the whole JSON file into memory if possible, however if you have the memory available it may not be a bad idea performance-wise. Using node.js's require() on a json file loads the data into memory really fast.
I ran two tests to see what the performance looked like on printing out an attribute from each feature from a 81MB geojson file.
In the 1st test, I read the entire geojson file into memory using var data = require('./geo.json'). That took 3330 milliseconds and then printing out an attribute from each feature took 804 milliseconds for a grand total of 4134 milliseconds. However, it appeared that node.js was using 411MB of memory.
In the second test, I used #arcseldon's answer with JSONStream + event-stream. I modified the JSONPath query to select only what I needed. This time the memory never went higher than 82MB, however, the whole thing now took 70 seconds to complete!
I wrote a module that can do this, called BFJ. Specifically, the method bfj.match can be used to break up a large stream into discrete chunks of JSON:
const bfj = require('bfj');
const fs = require('fs');
const stream = fs.createReadStream(filePath);
bfj.match(stream, (key, value, depth) => depth === 0, { ndjson: true })
.on('data', object => {
// do whatever you need to do with object
})
.on('dataError', error => {
// a syntax error was found in the JSON
})
.on('error', error => {
// some kind of operational error occurred
})
.on('end', error => {
// finished processing the stream
});
Here, bfj.match returns a readable, object-mode stream that will receive the parsed data items, and is passed 3 arguments:
A readable stream containing the input JSON.
A predicate that indicates which items from the parsed JSON will be pushed to the result stream.
An options object indicating that the input is newline-delimited JSON (this is to process format B from the question, it's not required for format A).
Upon being called, bfj.match will parse JSON from the input stream depth-first, calling the predicate with each value to determine whether or not to push that item to the result stream. The predicate is passed three arguments:
The property key or array index (this will be undefined for top-level items).
The value itself.
The depth of the item in the JSON structure (zero for top-level items).
Of course a more complex predicate can also be used as necessary according to requirements. You can also pass a string or a regular expression instead of a predicate function, if you want to perform simple matches against property keys.
If you have control over the input file, and it's an array of objects, you can solve this more easily. Arrange to output the file with each record on one line, like this:
[
{"key": value},
{"key": value},
...
This is still valid JSON.
Then, use the node.js readline module to process them one line at a time.
var fs = require("fs");
var lineReader = require('readline').createInterface({
input: fs.createReadStream("input.txt")
});
lineReader.on('line', function (line) {
line = line.trim();
if (line.charAt(line.length-1) === ',') {
line = line.substr(0, line.length-1);
}
if (line.charAt(0) === '{') {
processRecord(JSON.parse(line));
}
});
function processRecord(record) {
// Process the records one at a time here!
}
I solved this problem using the split npm module. Pipe your stream into split, and it will "Break up a stream and reassemble it so that each line is a chunk".
Sample code:
var fs = require('fs')
, split = require('split')
;
var stream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
var lineStream = stream.pipe(split());
linestream.on('data', function(chunk) {
var json = JSON.parse(chunk);
// ...
});
Using the #josh3736 answer, but for ES2021 and Node.js 16+ with async/await + AirBnb rules:
import fs from 'node:fs';
const file = 'file.json';
/**
* #callback itemProcessorCb
* #param {object} item The current item
*/
/**
* Process each data chunk in a stream.
*
* #param {import('fs').ReadStream} readable The readable stream
* #param {itemProcessorCb} itemProcessor A function to process each item
*/
async function processChunk(readable, itemProcessor) {
let data = '';
let total = 0;
// eslint-disable-next-line no-restricted-syntax
for await (const chunk of readable) {
// join with last result, remove CR and get lines
const lines = (data + chunk).replace('\r', '').split('\n');
// clear last result
data = '';
// process lines
let line = lines.shift();
const items = [];
while (line) {
// check if isn't a empty line or an array definition
if (line !== '' && !/[\[\]]+/.test(line)) {
try {
// remove the last comma and parse json
const json = JSON.parse(line.replace(/\s?(,)+\s?$/, ''));
items.push(json);
} catch (error) {
// last line gets only a partial line from chunk
// so we add this to join at next loop
data += line;
}
}
// continue
line = lines.shift();
}
total += items.length;
// Process items in parallel
await Promise.all(items.map(itemProcessor));
}
console.log(`${total} items processed.`);
}
// Process each item
async function processItem(item) {
console.log(item);
}
// Init
try {
const readable = fs.createReadStream(file, {
flags: 'r',
encoding: 'utf-8',
});
processChunk(readable, processItem);
} catch (error) {
console.error(error.message);
}
For a JSON like:
[
{ "name": "A", "active": true },
{ "name": "B", "active": false },
...
]
https.get(url1 , function(response) {
var data = "";
response.on('data', function(chunk) {
data += chunk.toString();
})
.on('end', function() {
console.log(data)
});
});
I think you need to use a database. MongoDB is a good choice in this case because it is JSON compatible.
UPDATE:
You can use mongoimport tool to import JSON data into MongoDB.
mongoimport --collection collection --file collection.json
There are two things I want to do:
I want to create a new array of objects from an existing object,
And increment the object so each object can have a count id of 1,2,3 etc
My issue is that when I write to the file it writes only 1 random object to the file and the rest don't show. There are so errors and all the objects have the same increment value. Please explain what I am doing wrong. Thanks.
Code:
data.json:
{
"users":[
{
"name":"mike",
"category":[
{
"title":"cook",
}
],
"store":{
"location":"uptown",
"city":"ulis"
},
"account":{
"type":"regular",
"payment":[
"active":false
]
}
}
]
}
index.js:
const appData = ('./data.json')
const fs = require('fs');
let newObject = {}
appData.forEach(function(items){
let x = items
let numincrement = 1++
newObject.name = x.name
newObject.count = numincrement
newObject.categories = x.categories
newObject.store = x.store
newObject.account = x.account
fs.writeFile('./temp.json', JSON.stringify(newObject, null, 2),'utf8' , function(err, data) {
// console.log(data)
if(err) {
console.log(err)
return
} else{
console.log('created')
}
})
})
There are a whole bunch of problems here:
You're just rewriting the same object over and over to the file. fs.writeFile() rewrites the entire file. It does not append to the file. In addition, you cannot append to the JSON format either. So, this code will only every write one object to the file.
To append new JSON data to what's in the existing file, you would have to read in the existing JSON, parse it to convert it to a Javascript array, then add new items onto the array, then convert back to JSON and write out the file again. For more efficient appending, you would need a different data format (perhaps comma delimited lines).
Your loop has all sorts of problems. You're assigning to the same newObject over and over again.
Your numincrement is inside the loop so it will have the same value on every invocation of the loop. You can also just use the index parameter passed to the forEach() callback instead of using your own variable.
If what you're trying to iterate over is the users array in your data, then you may need to be iterating over appData.users, not just appData.
If you really just want to append data to a text file, the JSON is not the easiest format to use. It might be easier to just use comma delimited lines. Then, you can just append new lines to the file. Can't really do that with JSON.
If you're willing to just overwrite the file with the current data, you can do this:
const appData = ('./data.json').users;
const fs = require('fs');
// create an array of custom objects
let newData = appData.map((item, index) => {
return {
name: item.name,
count: index + 1,
categories: item.categoies,
store: item.store,
account: item.account
};
});
// write out that data to a file as JSON (overwriting existing file)
fs.writeFile('./temp.json', JSON.stringify(newData, null, 2),'utf8' , function(err, data) {
if (err) {
console.log(err);
} else {
console.log("data written");
}
});
I have a file which stores many JavaScript objects in JSON form and I need to read the file, create each of the objects, and do something with them (insert them into a db in my case). The JavaScript objects can be represented a format:
Format A:
[{name: 'thing1'},
....
{name: 'thing999999999'}]
or Format B:
{name: 'thing1'} // <== My choice.
...
{name: 'thing999999999'}
Note that the ... indicates a lot of JSON objects. I am aware I could read the entire file into memory and then use JSON.parse() like this:
fs.readFile(filePath, 'utf-8', function (err, fileContents) {
if (err) throw err;
console.log(JSON.parse(fileContents));
});
However, the file could be really large, I would prefer to use a stream to accomplish this. The problem I see with a stream is that the file contents could be broken into data chunks at any point, so how can I use JSON.parse() on such objects?
Ideally, each object would be read as a separate data chunk, but I am not sure on how to do that.
var importStream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
importStream.on('data', function(chunk) {
var pleaseBeAJSObject = JSON.parse(chunk);
// insert pleaseBeAJSObject in a database
});
importStream.on('end', function(item) {
console.log("Woot, imported objects into the database!");
});*/
Note, I wish to prevent reading the entire file into memory. Time efficiency does not matter to me. Yes, I could try to read a number of objects at once and insert them all at once, but that's a performance tweak - I need a way that is guaranteed not to cause a memory overload, not matter how many objects are contained in the file.
I can choose to use FormatA or FormatB or maybe something else, just please specify in your answer. Thanks!
To process a file line-by-line, you simply need to decouple the reading of the file and the code that acts upon that input. You can accomplish this by buffering your input until you hit a newline. Assuming we have one JSON object per line (basically, format B):
var stream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
var buf = '';
stream.on('data', function(d) {
buf += d.toString(); // when data is read, stash it in a string buffer
pump(); // then process the buffer
});
function pump() {
var pos;
while ((pos = buf.indexOf('\n')) >= 0) { // keep going while there's a newline somewhere in the buffer
if (pos == 0) { // if there's more than one newline in a row, the buffer will now start with a newline
buf = buf.slice(1); // discard it
continue; // so that the next iteration will start with data
}
processLine(buf.slice(0,pos)); // hand off the line
buf = buf.slice(pos+1); // and slice the processed data off the buffer
}
}
function processLine(line) { // here's where we do something with a line
if (line[line.length-1] == '\r') line=line.substr(0,line.length-1); // discard CR (0x0D)
if (line.length > 0) { // ignore empty lines
var obj = JSON.parse(line); // parse the JSON
console.log(obj); // do something with the data here!
}
}
Each time the file stream receives data from the file system, it's stashed in a buffer, and then pump is called.
If there's no newline in the buffer, pump simply returns without doing anything. More data (and potentially a newline) will be added to the buffer the next time the stream gets data, and then we'll have a complete object.
If there is a newline, pump slices off the buffer from the beginning to the newline and hands it off to process. It then checks again if there's another newline in the buffer (the while loop). In this way, we can process all of the lines that were read in the current chunk.
Finally, process is called once per input line. If present, it strips off the carriage return character (to avoid issues with line endings – LF vs CRLF), and then calls JSON.parse one the line. At this point, you can do whatever you need to with your object.
Note that JSON.parse is strict about what it accepts as input; you must quote your identifiers and string values with double quotes. In other words, {name:'thing1'} will throw an error; you must use {"name":"thing1"}.
Because no more than a chunk of data will ever be in memory at a time, this will be extremely memory efficient. It will also be extremely fast. A quick test showed I processed 10,000 rows in under 15ms.
Just as I was thinking that it would be fun to write a streaming JSON parser, I also thought that maybe I should do a quick search to see if there's one already available.
Turns out there is.
JSONStream "streaming JSON.parse and stringify"
Since I just found it, I've obviously not used it, so I can't comment on its quality, but I'll be interested to hear if it works.
It does work consider the following Javascript and _.isString:
stream.pipe(JSONStream.parse('*'))
.on('data', (d) => {
console.log(typeof d);
console.log("isString: " + _.isString(d))
});
This will log objects as they come in if the stream is an array of objects. Therefore the only thing being buffered is one object at a time.
As of October 2014, you can just do something like the following (using JSONStream) - https://www.npmjs.org/package/JSONStream
var fs = require('fs'),
JSONStream = require('JSONStream'),
var getStream() = function () {
var jsonData = 'myData.json',
stream = fs.createReadStream(jsonData, { encoding: 'utf8' }),
parser = JSONStream.parse('*');
return stream.pipe(parser);
}
getStream().pipe(MyTransformToDoWhateverProcessingAsNeeded).on('error', function (err) {
// handle any errors
});
To demonstrate with a working example:
npm install JSONStream event-stream
data.json:
{
"greeting": "hello world"
}
hello.js:
var fs = require('fs'),
JSONStream = require('JSONStream'),
es = require('event-stream');
var getStream = function () {
var jsonData = 'data.json',
stream = fs.createReadStream(jsonData, { encoding: 'utf8' }),
parser = JSONStream.parse('*');
return stream.pipe(parser);
};
getStream()
.pipe(es.mapSync(function (data) {
console.log(data);
}));
$ node hello.js
// hello world
I had similar requirement, i need to read a large json file in node js and process data in chunks and call a api and save in mongodb.
inputFile.json is like:
{
"customers":[
{ /*customer data*/},
{ /*customer data*/},
{ /*customer data*/}....
]
}
Now i used JsonStream and EventStream to achieve this synchronously.
var JSONStream = require("JSONStream");
var es = require("event-stream");
fileStream = fs.createReadStream(filePath, { encoding: "utf8" });
fileStream.pipe(JSONStream.parse("customers.*")).pipe(
es.through(function(data) {
console.log("printing one customer object read from file ::");
console.log(data);
this.pause();
processOneCustomer(data, this);
return data;
}),
function end() {
console.log("stream reading ended");
this.emit("end");
}
);
function processOneCustomer(data, es) {
DataModel.save(function(err, dataModel) {
es.resume();
});
}
I realize that you want to avoid reading the whole JSON file into memory if possible, however if you have the memory available it may not be a bad idea performance-wise. Using node.js's require() on a json file loads the data into memory really fast.
I ran two tests to see what the performance looked like on printing out an attribute from each feature from a 81MB geojson file.
In the 1st test, I read the entire geojson file into memory using var data = require('./geo.json'). That took 3330 milliseconds and then printing out an attribute from each feature took 804 milliseconds for a grand total of 4134 milliseconds. However, it appeared that node.js was using 411MB of memory.
In the second test, I used #arcseldon's answer with JSONStream + event-stream. I modified the JSONPath query to select only what I needed. This time the memory never went higher than 82MB, however, the whole thing now took 70 seconds to complete!
I wrote a module that can do this, called BFJ. Specifically, the method bfj.match can be used to break up a large stream into discrete chunks of JSON:
const bfj = require('bfj');
const fs = require('fs');
const stream = fs.createReadStream(filePath);
bfj.match(stream, (key, value, depth) => depth === 0, { ndjson: true })
.on('data', object => {
// do whatever you need to do with object
})
.on('dataError', error => {
// a syntax error was found in the JSON
})
.on('error', error => {
// some kind of operational error occurred
})
.on('end', error => {
// finished processing the stream
});
Here, bfj.match returns a readable, object-mode stream that will receive the parsed data items, and is passed 3 arguments:
A readable stream containing the input JSON.
A predicate that indicates which items from the parsed JSON will be pushed to the result stream.
An options object indicating that the input is newline-delimited JSON (this is to process format B from the question, it's not required for format A).
Upon being called, bfj.match will parse JSON from the input stream depth-first, calling the predicate with each value to determine whether or not to push that item to the result stream. The predicate is passed three arguments:
The property key or array index (this will be undefined for top-level items).
The value itself.
The depth of the item in the JSON structure (zero for top-level items).
Of course a more complex predicate can also be used as necessary according to requirements. You can also pass a string or a regular expression instead of a predicate function, if you want to perform simple matches against property keys.
If you have control over the input file, and it's an array of objects, you can solve this more easily. Arrange to output the file with each record on one line, like this:
[
{"key": value},
{"key": value},
...
This is still valid JSON.
Then, use the node.js readline module to process them one line at a time.
var fs = require("fs");
var lineReader = require('readline').createInterface({
input: fs.createReadStream("input.txt")
});
lineReader.on('line', function (line) {
line = line.trim();
if (line.charAt(line.length-1) === ',') {
line = line.substr(0, line.length-1);
}
if (line.charAt(0) === '{') {
processRecord(JSON.parse(line));
}
});
function processRecord(record) {
// Process the records one at a time here!
}
I solved this problem using the split npm module. Pipe your stream into split, and it will "Break up a stream and reassemble it so that each line is a chunk".
Sample code:
var fs = require('fs')
, split = require('split')
;
var stream = fs.createReadStream(filePath, {flags: 'r', encoding: 'utf-8'});
var lineStream = stream.pipe(split());
linestream.on('data', function(chunk) {
var json = JSON.parse(chunk);
// ...
});
Using the #josh3736 answer, but for ES2021 and Node.js 16+ with async/await + AirBnb rules:
import fs from 'node:fs';
const file = 'file.json';
/**
* #callback itemProcessorCb
* #param {object} item The current item
*/
/**
* Process each data chunk in a stream.
*
* #param {import('fs').ReadStream} readable The readable stream
* #param {itemProcessorCb} itemProcessor A function to process each item
*/
async function processChunk(readable, itemProcessor) {
let data = '';
let total = 0;
// eslint-disable-next-line no-restricted-syntax
for await (const chunk of readable) {
// join with last result, remove CR and get lines
const lines = (data + chunk).replace('\r', '').split('\n');
// clear last result
data = '';
// process lines
let line = lines.shift();
const items = [];
while (line) {
// check if isn't a empty line or an array definition
if (line !== '' && !/[\[\]]+/.test(line)) {
try {
// remove the last comma and parse json
const json = JSON.parse(line.replace(/\s?(,)+\s?$/, ''));
items.push(json);
} catch (error) {
// last line gets only a partial line from chunk
// so we add this to join at next loop
data += line;
}
}
// continue
line = lines.shift();
}
total += items.length;
// Process items in parallel
await Promise.all(items.map(itemProcessor));
}
console.log(`${total} items processed.`);
}
// Process each item
async function processItem(item) {
console.log(item);
}
// Init
try {
const readable = fs.createReadStream(file, {
flags: 'r',
encoding: 'utf-8',
});
processChunk(readable, processItem);
} catch (error) {
console.error(error.message);
}
For a JSON like:
[
{ "name": "A", "active": true },
{ "name": "B", "active": false },
...
]
https.get(url1 , function(response) {
var data = "";
response.on('data', function(chunk) {
data += chunk.toString();
})
.on('end', function() {
console.log(data)
});
});
I think you need to use a database. MongoDB is a good choice in this case because it is JSON compatible.
UPDATE:
You can use mongoimport tool to import JSON data into MongoDB.
mongoimport --collection collection --file collection.json