javascript - read and write a json file in node.js - Stack Overflow

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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;
            }
        });
    });
});

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;
            }
        });
    });
});

• javascript
• json
• node.js

Share Improve this question Follow asked Sep 14, 2014 at 9:20 McsMcs 56411 gold badge66 silver badges1515 bronze badges

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2 Answers 2

Sorted by: Reset to default 8

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');
  })  
})

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.

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