Adding property using Object assign

Question

I have the following function:

addHrefs = (list) => {
    const listWithHrefs = list.map((item) => {
        item.href = customFunction(item.name);
        return item;
    });
    return listWithHrefs;
}

The problem here is I am mutating the list object. I want to know how to add a particular (key, value) pair without mutating list . Is it possible with Object.assign ? How can I do it?

Answer 1

It is possible with Object.assign .

The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object.

You could assign the property value to a new object and return it.

addHrefs = list =>
    list.map(item => Objects.assign({}, item, { href: customFunction(item.name) }));

Answer 2

You can pass plain object as first parameter to Object.assign() , referenced object at second parameter. Perform action on variable assigned to Object.assign() , return variable

 let list = [{ a: 1 }, { a: 2 }, { a: 3 }]; let res = list.map(item => { let o = Object.assign({}, item); // do stuff with `o` oa = oa + 10; // return `o` return o }); console.log(list, res);