How to fix creating a new object in JSON file?

SadlerJ Published at Dev

SadlerJ

I'm trying to create a function that when called will update a specific object in json file. However, it updates the object as well as creating a new one.

I've tried many different methods in trying to get this to work, but all have failed. The closest I've got to it working is the code shown below, but it still doesn't do what is required.

This is my function:

var fs = require('fs');
var _ = require("underscore");

module.exports = {
  personalUpdate: function (id, forename, surname, dob, gender, callback) {    
    let rawdata = fs.readFileSync('data.json');
    let data = JSON.parse(rawdata);
    let filtered = _.where(data['students'], { id: id });
    let all = filtered[0];

    all.forename = forename;
    all.surname = surname;
    all.dob = dob;
    all.gender = gender;

    data["students"].push(all);

    fs.writeFileSync('data.json', JSON.stringify(data, null, 2), (err) => {
      if (err) throw err;
    });
    callback("success");

  }
}

And this is the JSON file that I want to update:

{
  "teachers": [
    {
      "name": "",
      "email": "",
      "password": "",
      "formGroup": "",
      "id": ""
    }
  ],
  "students": [
    {
      "surname": "test",
      "forename": "test",
      "dob": "",
      "homeAddress": "",
      "homePhone": "",
      "gender": "",
      "tutorGroup": "",
      "schoolEmail": "",
      "grades": [
        {
          "french": 8,
          "maths": 7
        }
      ],
      "id": ""
    },
    {
      "surname": "test2",
      "forename": "test2",
      "dob": "",
      "homeAddress": "test2",
      "homePhone": "",
      "gender": "",
      "tutorGroup": "",
      "schoolEmail": "",
      "grades": [
        {
          "french": 9,
          "maths": 8
        }
      ],
      "id": ""
    }

  ]
}

I had to remove and change the objects and info inside them, as it contained confidential information.

When running this function, it finds the object that is specified in the parameter. It then updates that object, but it then creates another object at the bottom of the original JSON object, which it is not supposed to.

Also, is there a better way to update the specific objects in the JSON file?

Daniel Abrão

tl;dr

The result set is duplicating because you are pushing it into the array
The change is being applied due to the variables holding the same object reference, so they are being mirrored across objects that share the same pointer.

Explanation

It creates a new one due to the data["students"].push(all); instruction.

When you manipulate objects in javascript you need to be aware of how the reference between them work, so you can avoid bugs and use them in your benefit.

For example, take this set of instructions:

let a = {"x": 1};
let b = a;
b.x = 3;
console.log(a) // it will output {"x": 3}

Notice that we:

Create an object with the prop x equal 1 and assign it to the variable a
Initialize a variable b with the value of a
Change the property x on the variable/object b
Then we can observe that the change was also reflected in the variable a, due to the object reference.

So, basically this is exactly what is happening with your instructions when you do all.forename = forename; it changes the variable all, but also the original object which it derives from.

Here is a nice reference that explains this concept more in-depth

@EDIT

I strongly advise you not using the sync version of functions like readFileSync since this blocks the event loop. Here is the official guidelines about it

Collected from the Internet

Please contact [email protected] to delete if infringement.

edited at2020-12-8

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