Parse large JSON file in Nodejs and handle each object independently

Question

I need to read a large JSON file (around 630MB) in Nodejs and insert each object to MongoDB.

I've read the answer here:Parse large JSON file in Nodejs.

However, answers there are handling the JSON file line by line, instead of handling it object by object. Thus, I still don't know how to get an object from this file and operate it.

I have about 100,000 this kind of objects in my JSON file.

Data Format:

[
  {
    "id": "0000000",
    "name": "Donna Blak",
    "livingSuburb": "Tingalpa",
    "age": 53,
    "nearestHospital": "Royal Children's Hospital",
    "treatments": {
        "19890803": {
            "medicine": "Stomach flu B",
            "disease": "Stomach flu"
        },
        "19740112": {
            "medicine": "Progeria C",
            "disease": "Progeria"
        },
        "19830206": {
            "medicine": "Poliomyelitis B",
            "disease": "Poliomyelitis"
        }
    },
    "class": "patient"
  },
 ...
]

Cheers,

Alex

Answer 1

There is a nice module named 'stream-json' that does exactly what you want.

It can parse JSON files far exceeding available memory.

and

StreamArray handles a frequent use case: a huge array of relatively small objects similar to Django-produced database dumps. It streams array components individually taking care of assembling them automatically.

Here is a modern example parsing an arbitrarily large JSON array. It uses import statements and for await...of, which supports returning or breaking from:

import parser from 'stream-json';
import StreamArray from 'stream-json/streamers/StreamArray';
import Chain from 'stream-chain';

async function importJson(filePath) {
  const pipeline = new Chain([
    fs.createReadStream(filePath),
    parser(),
    new StreamArray(),
  ]);

  for await (const { value } of pipeline) {
    await doSomethingWith(value);  // the JSON array element
  }
}

2018

Here is a very basic example using stream events:

const StreamArray = require('stream-json/streamers/StreamArray');
const path = require('path');
const fs = require('fs');

const jsonStream = StreamArray.withParser();

//You'll get json objects here
//Key is an array-index here
jsonStream.on('data', ({key, value}) => {
    console.log(key, value);
});

jsonStream.on('end', () => {
    console.log('All done');
});

const filename = path.join(__dirname, 'sample.json');
fs.createReadStream(filename).pipe(jsonStream.input);

If you'd like to do something more complex e.g. process one object after another sequentially (keeping the order) and apply some async operations for each of them then you could do the custom Writeable stream like this:

const StreamArray = require('stream-json/streamers/StreamArray');
const {Writable} = require('stream');
const path = require('path');
const fs = require('fs');

const fileStream = fs.createReadStream(path.join(__dirname, 'sample.json'));
const jsonStream = StreamArray.withParser();

const processingStream = new Writable({
    write({key, value}, encoding, callback) {
        //Save to mongo or do any other async actions

        setTimeout(() => {
            console.log(value);
            //Next record will be read only current one is fully processed
            callback();
        }, 1000);
    },
    //Don't skip this, as we need to operate with objects, not buffers
    objectMode: true
});

//Pipe the streams as follows
fileStream.pipe(jsonStream.input);
jsonStream.pipe(processingStream);

//So we're waiting for the 'finish' event when everything is done.
processingStream.on('finish', () => console.log('All done'));

Please note: The examples above are tested for 'stream-json@1.1.3'. For some previous versions (presumably proior to 1.0.0) you might have to:

const StreamArray = require('stream-json/utils/StreamArray');

and then

const jsonStream = StreamArray.make();