Copy array of objects and make changes without modifying original array

Question

I have an array of objects. I would like to deep copy the array of objects and make some changes to each object. I want to do this without modifying the original array or original objects that were in that array.

This is the way I have done it. However, being new to JavaScript I want to make sure this is a good approach.

Is there a better way to do this?

const users = 
[
    {
        id       : 1,    
        name     : 'Jack',
        approved : false
    },
    {
        id       : 2,    
        name     : 'Bill',
        approved : true
    },
    {
        id       : 3,    
        name     : 'Rick',
        approved : false
    },
    {
        id       : 4,    
        name     : 'Rick',
        approved : true
    }
];


const users2 = 
    users
        .map(
            (u) => 
            {
                return Object.assign({}, u);
            }
        )    
        .map(
            (u) => 
            {
                u.approved = true;
                return u;
            }
        );    


console.log('New users2 array of objects:')
console.log(users2);

console.log('This was original users array is untouched:')
console.log(users);

Output:

New users2 array of objects:
[ { id: 1, name: 'Jack', approved: true },
  { id: 2, name: 'Bill', approved: true },
  { id: 3, name: 'Rick', approved: true },
  { id: 4, name: 'Rick', approved: true } ]
This was original users array is untouched:
[ { id: 1, name: 'Jack', approved: false },
  { id: 2, name: 'Bill', approved: true },
  { id: 3, name: 'Rick', approved: false },
  { id: 4, name: 'Rick', approved: true } ]

Answer 1

For a single pass, you could use Object.assign with the changed property as well.

const users = [{ id: 1, name: 'Jack', approved: false }, { id: 2, name: 'Bill', approved: true }, { id: 3, name: 'Rick', approved: false }, { id: 4, name: 'Rick', approved: true }];
const users2 = users.map(u => Object.assign({}, u, { approved: true }));

console.log(users2);
console.log(users);

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UPDATE with spreading properties.

const users = [{ id: 1, name: 'Jack', approved: false }, { id: 2, name: 'Bill', approved: true }, { id: 3, name: 'Rick', approved: false }, { id: 4, name: 'Rick', approved: true }];
const users2 = users.map(u => ({ ...u, approved: true }));

console.log(users2);
console.log(users);

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Answer 2

Yes that looks good. You could also perform the modification when you are cloning, in order to avoid mapping over the array twice.

const users2 = users.map((u) => {
    const copiedUser = Object.assign({}, u);
    copiedUser.approved = true;
    return copiedUser;
}); 

Answer 3

I prefer JSON.stringify and JSON.parse

var users = [ { id: 1, name: 'Jack', approved: false },
{ id: 2, name: 'Bill', approved: true },
{ id: 3, name: 'Rick', approved: false },
{ id: 4, name: 'Rick', approved: true } ];

// user2 will be copy of array users without reference
var users2 = JSON.parse(JSON.stringify(users));

Answer 4

There are a few ways to copy a array in javascript, i believed that the most used are:

• slice()
• Array.from()

The slice function will return a portion (or all content) of a given array as a new array, based at a begin and end index (begin and end index are optional):

const a = [1,2,3,4,5,6,7,8,9]
/*
* Only begin index
*/
const b = a.slice(2)
console.log(b) //Will Print [3,4,5,6,7,8,9]
/*
* Begin index and end index
*/
const c = a.slice(5,8)
console.log(c) //Will Print [6,7,8]
/*
* No indexes provided
*/
const d = a.slice()
console.log(d) //Will print [1,2,3,4,5,6,7,8,9]

Array.from() is a function that will create a new array from an array-like or iterable parameters.

const a = Array.from('bar');
console.log(a) //Will Print ["b","a","r"]
const b = ["for","bar"];
const c = Array.from(b);
console.log(c) //Will print  ["for","bar"]

More about slice

More about Array.from()

Answer 5

In React JS I tried most ways of copying and modifying the newly copied array but it still modified the original array. The methods I tried are slice Array.from for loop spread operator, all this ended up modifying the original array. The solution was, stringify the original array and assign it to a new variable, then parse the variable to get a fresh copy without reference as stated by @Roman Yakoviv above.