read and write a json file in node.js

Question

okay I have this json file:

{   
    "joe": {
        "name": "joe",
        "lastName": "black"
    },

    "matt": {
        "name": "matt",
        "lastName": "damon"
    }
}

and I want to add a person with node.js:

{   
    "joe": {
        "name": "joe",
        "lastName": "black"
    },

    "matt": {
        "name": "matt",
        "lastName": "damon"
    },

    "brad": {
        "name": "brad",
        "lastName": "pitt"
    }
}

With the below code I am trying to read the json document, parse it, add the person, and then write again to file. However, the parsed object (jsonObj) is not recognized in the write function. I know it has to do with the event loop, but I am not sure how to solve it. I tried using process.nextTick, but can't get it to work.

var jsonObj;

fs.open('files/users.json', 'r+', function opened(err, fd) {
    if (err) {
        throw err;
    }
    var readBuffer = new Buffer(1024),
        bufferOffset = 0,
        readLength = readBuffer.length,
        startRead = 0; 
    fs.read(fd, readBuffer, bufferOffset, readLength, startRead, function read(err, readBytes) {
        if (err) {
            throw err;
        }
        if (readBytes > 0) {
            jsonObj = JSON.parse(readBuffer.slice(0, readBytes).toString());
            jsonObj["brad"] = {};
            jsonObj["brad"].name = "brad";
            jsonObj["brad"].lastName = "pitt";
        **//Until here it works fine and the 'jsonObj' is properly updated**
        }
    });
 });

process.nextTick(function () {
    var writeBuffer, 
        startWrite = 0,
        bufferPosition = 0,
        writeLength;
    fs.open('files/users.json', 'r+', function opened(err, fd) {
        if (err) {
            throw err;
        }
        ***//Below I get the 'jsonObj is undefined' error***
        writeBuffer = new Buffer(JSON.stringify(jsonObj.toString(), null, '/\t/'));
        writeLength = writeBuffer.length;
        fs.write(fd, writeBuffer, bufferPosition, writeLength, startWrite, function wrote(err, written) {
            if (err) {
                throw err;
            }
        });
    });
});

score 8 · Accepted Answer · answered Sep 14 '14 at 11:17

In node you can require json files:

var fs = require('fs');
var users = require('./names');

users.brad = {
  name: 'brad',
  lastName: 'pitt'
}

var string = JSON.stringify(users,null,'\t');

fs.writeFile('./names.json',string,function(err) {
  if(err) return console.error(err);
  console.log('done');
})

Optional async version without requiring:

var fs = require('fs');

fs.readFile('./names.json',{encoding: 'utf8'},function(err,data) {
  var users = JSON.parse(data);
   users.brad2 = { name: 'brad', lastName: 'pitt' };
  var string = JSON.stringify(users,null,'\t');

  fs.writeFile('./names.json',string,function(err) {
    if(err) return console.error(err);
    console.log('done');
  })  
})

score 2 · Answer 2 · answered Sep 14 '14 at 09:25

2

The simple hint is: if you need to use process.nextTick, something is wrong. Leave that function to the library programmers! :)

Just move the function you call with nextTick to Until here...

nextTick does not wait until you read that file (that might take a few hundret ticks!), it just waits for the next tick. And that is available the nanosecond you call fs.read - because after fs.read the nodejs main loop is idle until either the kernel returns some information about that file or someone tells nodejs what to do on the next tick.

answered Sep 14 '14 at 09:25

CFrei

3,552
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29

well a more helpful answer would be setTimeOut, which works very well – Mcs Sep 14 '14 at 11:05
1

Lol... no don't use setTimeout either – david Sep 14 '14 at 12:02
why not if I go setTimeOut(theWriteFunction, 100), after the read function, it works – Mcs Sep 14 '14 at 12:20
2

What if that file grows bigger? What if your production env is one day with a network storage and that becomes slow? (Good luck debugging on that day...) And: why waste 997 ms? And from theoretical point of view: the software does just not what you want and works by accident. – CFrei Sep 14 '14 at 12:23

read and write a json file in node.js

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