8

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 } ]
Emile Bergeron
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Stick-With-SQL
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4 Answers4

16

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);
.as-console-wrapper { max-height: 100% !important; top: 0; }

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);
.as-console-wrapper { max-height: 100% !important; top: 0; }
Nina Scholz
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5

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;
});
Wolfie
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4

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));
Roman Yakoviv
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  • explanation: This successfully strips the reference to original Object when creating the JSON string. Strings are stored by value so it has no reference. – NicoLA Jan 30 '19 at 22:19
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    This fails with any value that's unsupported in JSON, like Date objects, Functions, etc. See [this answer which explains the downsides](https://stackoverflow.com/a/122704/1218980) – Emile Bergeron Sep 05 '19 at 20:22
0

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()

jarbaspsf
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