How can I change array value(inside array of objects) without mutating the container? javascript

Question

what I want to achieve is filling the array "presents" with the value of arr when name is "Mario" in this case, what I get is the array "present" modify, but what I want is an non mutative array, only change the value of the empty array with the value of another array(in this case I put arr as example)

let present= [
    {
      name: "Peter",
      presents: []
    },
    {
      name: "Mario",
      presents: []
    },
    {
      name: "Amanda",
      presents: []
    },
    {
      name: "David",
      presents: []
    }
]
arr = ["car","coal"]

const res= present.map(s =>s.name == "Mario" ? s.presents.concat(arr) : [])

console.log(res)

What I want to get:

let present= [
    {
      name: "Peter",
      presents: []
    },
    {
      name: "Mario",
      presents: ["car","coal"]
    },
    {
      name: "Amanda",
      presents: []
    },
    {
      name: "David",
      presents: []
    }
]

pd: sorry , I could not explain better, still learning English.

Md Sabbir Alam · Accepted Answer · 2021-01-08 18:49:44Z

1

You can do the following using reduce(),

const present = [{ name: "Peter", presents: [] }, { name: "Mario", presents: [] }, { name: "Amanda", presents: [] }, { name: "David", presents: [] } ],
arr = ["car", "coal"],

res = present.reduce((prev, curr) => {
  if(curr.name === 'Mario') {
    prev.push({...curr, presents: [...curr.presents, ...arr]})
  } else {
    prev.push(curr);
  }
  return prev;
}, []);
console.log(res);

answered Jan 8, 2021 at 18:49

Md Sabbir Alam

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Nina Scholz · Accepted Answer · 2021-01-08 19:04:12Z

1

You could spread the objects and add the wanted array.

const
    present = [{ name: "Peter", presents: [] }, { name: "Mario", presents: [] }, { name: "Amanda", presents: [] }, { name: "David", presents: [] } ],
    array = ["car", "coal"],
    result = present.map(({ presents, ...o }) => ({
        ...o,
        presents: [...presents, ...(o.name === "Mario" ? array : [])]
    }));

console.log(result);

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

edited Jan 8, 2021 at 19:04

answered Jan 8, 2021 at 18:46

Nina Scholz

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