Should I deep copy when modifying nested object?

Question

I recently started learning functional programming in Javascript, and one thing that's valued is Immutability. One way to preserve immutability is to when we want to modify some object that we first create a copy and then modify that copy and return it, for example:

const person = {
    firstName: 'John',
    lastName: 'Smith',
}

const modifyFName = obj => {
    const objCopy = {...obj};
    objCopy.firstName = 'James';
    return objCopy;
}

console.log(modifyFName(person)) // {firstName: 'James', lastName: 'Smith'}
console.log(person) // {firstName: 'John', lastName: 'Smith'}

But if I want to modify some deeply nested object creating shallow copy like one above wouldn't make much of a difference, for example:

const person = {
    firstName: 'John',
    lastName: 'Smith',
    parents: {
        mom: {
            firstName: 'Jennifer',
            lastName: "Swift"
        },
        dad: {
            firstName: 'Tom',
            lastName: 'Cartman'
        }
    }
}

const modifyDadName = obj => {
    const objCopy = {...obj};
    objCopy.parents.dad.firstName = 'Aurelion';
    return objCopy;
}

console.log(modifyDadName(person)) // {... dad: "Aurelion"}
console.log(person) // {... dad: "Aurelion"}

Both objects have changed. So I am wondering in this situation should I perhaps use deep copy, or maybe use some third-party Immutable data structures, or is there some other solution for this?

T.J. Crowder · Accepted Answer · 2022-02-12 11:01:46Z

You're right that a shallow copy doesn't help, but you don't need a full deep copy, you only have to make copies of the things you want to change. In your case, that's person, parents, and dad, but not mom:

const modifyDadName = obj => {
    return {                            // Replacement `obj` (`person`)
        ...obj,
        parents: {                      // Replacement `obj.parents`
            ...obj.parents,
            dad: {                      // Replacement `obj.parents.dad`
                ...obj.parents.dad,
                firstName: "Aurelion",  // New name
            }
        }
    };
};

dad changes because, well, that's the operation we're doing (changing firstName). :-) parents changes because changing dad means there's a new object on its dad property. person (obj) changes because there's a new parents object. But mom doesn't change, so we can reuse the same object.

Collectives™ on Stack Overflow

Should I deep copy when modifying nested object?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related