Write / add data in JSON file using Node.js

问题

I am trying to write JSON file using node from loop data, e.g.:

let jsonFile = require('jsonfile');

for (i = 0; i < 11; i++) {
    jsonFile.writeFile('loop.json', "id :" + i + " square :" + i * i);
}

outPut in loop.json is:

id :1 square : 1

but I want output file like this (below) and also if I run that code again it should add that new output as elements in same existing JSON file:

{
   "table":[
      {
         "Id ":1,
         "square ":1
      },
      {
         "Id ":2,
         "square ":3
      },
      {
         "Id ":3,
         "square ":9
      },
      {
         "Id ":4,
         "square ":16
      },
      {
         "Id ":5,
         "square ":25
      },
      {
         "Id ":6,
         "square ":36
      },
      {
         "Id ":7,
         "square ":49
      },
      {
         "Id ":8,
         "square ":64
      },
      {
         "Id ":9,
         "square ":81
      },
      {
         "Id ":10,
         "square ":100
      }
   ]
}

I want to use same file that I created 1st time but whenever I run that code new elements should add in that same file

const fs = require('fs');

let obj = {
    table: []
};

fs.exists('myjsonfile.json', function(exists) {

    if (exists) {

        console.log("yes file exists");

        fs.readFile('myjsonfile.json', function readFileCallback(err, data) {

            if (err) {
                console.log(err);
            } else {
                obj = JSON.parse(data);

                for (i = 0; i < 5; i++) {
                    obj.table.push({
                        id: i,
                        square: i * i
                    });
                }

                let json = JSON.stringify(obj);
                fs.writeFile('myjsonfile.json', json);
            }
        });
    } else {

        console.log("file not exists");

        for (i = 0; i < 5; i++) {
            obj.table.push({
                id: i,
                square: i * i
            });
        }

        let json = JSON.stringify(obj);
        fs.writeFile('myjsonfile.json', json);
    }
});

回答1:

If this json file won't become too big over the time you should try:

Create a javascript object with the table array in it
```
var obj = {
   table: []
};
```
Add some data to it like
```
obj.table.push({id: 1, square:2});
```
Convert it from an object to string with stringify
```
var json = JSON.stringify(obj);
```

use fs to write the file to disk

var fs = require('fs');
fs.writeFile('myjsonfile.json', json, 'utf8', callback);

if you want to append it read the json file and convert it back to an object

fs.readFile('myjsonfile.json', 'utf8', function readFileCallback(err, data){
    if (err){
        console.log(err);
    } else {
    obj = JSON.parse(data); //now it an object
    obj.table.push({id: 2, square:3}); //add some data
    json = JSON.stringify(obj); //convert it back to json
    fs.writeFile('myjsonfile.json', json, 'utf8', callback); // write it back 
}});

This will work for data big as 100 MB max effectively. Over this limit, you should use a database engine.

UPDATE:

Create a function which returns the current date (year+month+day) as a string. Create the file named this string + .json. the fs module has a function which can check for file existence named fs.stat(path, callback). With this, you can check if the file exists. If it exists, use the read function if it's not, use the create function. Use the date string as the path cuz the file will be named as the today date + .json. the callback will contain a stats object which will be null if the file does not exist.

回答2:

Please try the following program. You might be expecting this output.

var fs = require('fs');

var data = {}
data.table = []
for (i=0; i <26 ; i++){
   var obj = {
       id: i,
       square: i * i
   }
   data.table.push(obj)
}
fs.writeFile ("input.json", JSON.stringify(data), function(err) {
    if (err) throw err;
    console.log('complete');
    }
);

Save this program in a javascript file, say, square.js.

Then run the program from command prompt using the command node square.js

What it does is, simply overwriting the existing file with new set of data, every time you execute the command.

Happy Coding.

回答3:

you should read the file, every time you want to add a new property to the json, and then add the the new properties

var fs = require('fs');
fs.readFile('data.json',function(err,content){
  if(err) throw err;
  var parseJson = JSON.parse(content);
  for (i=0; i <11 ; i++){
   parseJson.table.push({id:i, square:i*i})
  }
  fs.writeFile('data.json',JSON.stringify(parseJson),function(err){
    if(err) throw err;
  })
})

回答4:

Above example is also correct, but i provide simple example:

var fs = require("fs");
var sampleObject = {
    name: 'pankaj',
    member: 'stack',
    type: {
        x: 11,
        y: 22
    }
};

fs.writeFile("./object.json", JSON.stringify(sampleObject, null, 4), (err) => {
    if (err) {
        console.error(err);
        return;
    };
    console.log("File has been created");
});

回答5:

For formatting jsonfile gives spaces option which you can pass as a parameter:

   jsonfile.writeFile(file, obj, {spaces: 2}, function (err) {
         console.error(err);
   })

Or use jsonfile.spaces = 4. Read details here.

I would not suggest writing to file each time in the loop, instead construct the JSON object in the loop and write to file outside the loop.

var jsonfile = require('jsonfile');
var obj={
     'table':[]
    };

for (i=0; i <11 ; i++){
       obj.table.push({"id":i,"square":i*i});
}
jsonfile.writeFile('loop.json', obj, {spaces:2}, function(err){
      console.log(err);
});

回答6:

try

var fs = require("fs");
var sampleObject = { your data };

fs.writeFile("./object.json", JSON.stringify(sampleObject, null, 4), (err) => {
    if (err) {  console.error(err);  return; };
    console.log("File has been created");
});

回答7:

For synchronous approach

const fs = require('fs')
fs.writeFileSync('file.json', JSON.stringify(jsonVariable));

来源：https://stackoverflow.com/questions/36856232/write-add-data-in-json-file-using-node-js

标签

javascript

json

node.js