Javascript – Add object to json file – Node.js

javascriptjsonjsonobjectnode.js

I'm trying to add an object to a very large JSON file in Node.js (but only if the id doesn't match an existing object). What I have so far:

example JSON file:

[
  {
    id:123,
    text: "some text"
  },
  {
    id:223,
    text: "some other text"
  }
]

app.js

var fs = require('fs');     
var jf = require('jsonfile')
var util = require('util')    
var file = 'example.json'

// Example new object
var newThing = {
  id: 324,
  text: 'more text'
}

// Read the file
jf.readFile(file, function(err, obj) {
  // Loop through all the objects in the array
  for (i=0;i < obj.length; i++) {
    // Check each id against the newThing
    if (obj[i].id !== newThing.id) {
      found = false;
      console.log('thing ' + obj[i].id + ' is different. keep going.');
    }else if (obj[i].id == newThing.id){
      found = true;
      console.log('found it. stopping.');
      break;
    }
  }
  // if we can't find it, append it to the file
  if(!found){
    console.log('could not find it so adding it...');
    fs.appendFile(file, ', ' + JSON.stringify(newTweet) + ']', function (err) {
      if (err) throw err;
      console.log('done!');
    });
  }
})

This is so close to what I want. The only problem is the trailing ] character at the end of the JSON file. Is there a way to delete it using the file system API or something? Or is there a much easier way to do exactly what I want?

Best Solution

The proper way to handle this is to parse the JSON file, modify the object, and output it again.

var obj = require('file.json');
obj.newThing = 'thing!';
fs.writeFile('file.json', JSON.stringify(obj), function (err) {
  console.log(err);
});

Native deep cloning

It's called "structured cloning", works experimentally in Node 11 and later, and hopefully will land in browsers. See this answer for more details.

Fast cloning with data loss - JSON.parse/stringify

If you do not use Dates, functions, undefined, Infinity, RegExps, Maps, Sets, Blobs, FileLists, ImageDatas, sparse Arrays, Typed Arrays or other complex types within your object, a very simple one liner to deep clone an object is:

JSON.parse(JSON.stringify(object))

const a = {
  string: 'string',
  number: 123,
  bool: false,
  nul: null,
  date: new Date(),  // stringified
  undef: undefined,  // lost
  inf: Infinity,  // forced to 'null'
  re: /.*/,  // lost
}
console.log(a);
console.log(typeof a.date);  // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date);  // result of .toISOString()

See Corban's answer for benchmarks.

Reliable cloning using a library

Since cloning objects is not trivial (complex types, circular references, function etc.), most major libraries provide function to clone objects. Don't reinvent the wheel - if you're already using a library, check if it has an object cloning function. For example,

lodash - cloneDeep; can be imported separately via the lodash.clonedeep module and is probably your best choice if you're not already using a library that provides a deep cloning function
AngularJS - angular.copy
jQuery - jQuery.extend(true, { }, oldObject); .clone() only clones DOM elements
just library - just-clone; Part of a library of zero-dependency npm modules that do just do one thing. Guilt-free utilities for every occasion.

ES6 (shallow copy)

For completeness, note that ES6 offers two shallow copy mechanisms: Object.assign() and the spread syntax. which copies values of all enumerable own properties from one object to another. For example:

var A1 = {a: "2"};
var A2 = Object.assign({}, A1);
var A3 = {...A1};  // Spread Syntax

Javascript – Remove properties from objects (JavaScript)

To remove a property from an object (mutating the object), you can do it like this:

delete myObject.regex;
// or,
delete myObject['regex'];
// or,
var prop = "regex";
delete myObject[prop];

Demo

var myObject = {
    "ircEvent": "PRIVMSG",
    "method": "newURI",
    "regex": "^http://.*"
};
delete myObject.regex;

console.log(myObject);

For anyone interested in reading more about it, Stack Overflow user kangax has written an incredibly in-depth blog post about the delete statement on their blog, Understanding delete. It is highly recommended.

If you'd like a new object with all the keys of the original except some, you could use the destructuring.