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How to change useState for nested array of objects

I have a state:

const obj = [{id: "1", obj_of_names: [{
                             id: "a1",
                             name: "abc",
                             school: "xyz" },
                            { id:"b1",
                             name: "def",
                             school: "zyx" },
                            { id:"c1",
                             name: "ghi",
                             school: "pqr" }]
             },
             {id: "2", obj_of_names: [{
                             id: "d1",
                             name: "abc123",
                             school: "xyz123" }]
             }]

const [newobj, setObj] = useState("obj")

Now I want to update school for the id:d1 inside id:2. Suppose the old value is school: xyz123 now I want to update it to school: newxyz123. How can I achieve this using the spread operator or by another method?

I tried the below code:

setObj((prevObjects) => {
            const tempIndex = prevObjects.findIndex(
                (item) => item.id === reqdImgID
            );
const newObject = obj[tempIndex].obj_of_names.map((obj) => { 
if (obj.id == reqdID) { 
return {  ...obj, 
school: newSchool,};}
return obj; }););

But I am getting this as an ouput: { id:"c1", name: "ghi", school: "newxyz123" }

about 4 years ago · Juan Pablo Isaza

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Answer 1 · 2022-07-09T01:39:18.880Z

I suggest creating a function that receives the original array, a matcher function, and a replace function. The purpose of this function is to return a new array with the value changed.

function mutateObject(array, matcher, replacer) {
    // returns a new array, with all elements untouched, except for those that match the matcher function, which are replaced with new objects created by the replacer function
    return array.map(function (item) {
        return matcher(item) ? {...replacer(item)} : item;
    });
}

Then, use the function to replace the values.

mutateObject(obj, group => group.id == "2", group => {
    const newNames = mutateObject(group.obj_of_names, name => name.id === "d1", name => ({ ...name, school: "newxyz123" }));
    return { ...group, obj_of_names: newNames }
});

What this will do:

Using the original array, find the object where the id (in the root object) is "2".
Apply the replacer function, which will:
1. Use the obj_of_names to find the object where the id is "d1".
2. Apply the replacer function to return a new object with the new school value, using the spread operator.
3. Return a new object cloned by the spread operator with replaced obj_of_names object, which contains the replaced school value.

Of course, you can improve this code and its readability by using various libraries to clone the objects, mutate them, and return new objects, but it is a workaround if you want to get started. Also, if you want to change the school name, I suggest using the answer @aweimon referenced, but if you want to change multiple objects on various criteria, you can improve the function above.

const obj = [{
    id: "1",
    obj_of_names: [{
        id: "a1",
        name: "abc",
        school: "xyz"
      },
      {
        id: "b1",
        name: "def",
        school: "zyx"
      },
      {
        id: "c1",
        name: "ghi",
        school: "pqr"
      }
    ]
  },
  {
    id: "2",
    obj_of_names: [{
      id: "d1",
      name: "abc123",
      school: "xyz123"
    }]
  }
]

function mutateObject(array, matcher, replacer) {
  // returns a new array, with all elements untouched, except for those that match the matcher function, which are replaced with new objects created by the replacer function
  return array.map(function(item) {
    return matcher(item) ? { ...replacer(item)
    } : item;
  });
}

const updatedObj = mutateObject(obj, group => group.id == "2", group => {
  const newNames = mutateObject(group.obj_of_names, name => name.id === "d1", name => ({ ...name,
    school: "newxyz123"
  }));
  return { ...group,
    obj_of_names: newNames
  }
});

console.log(updatedObj);

Answer 2 · 2022-07-09T01:39:18.890Z

const arr = [{id: "1", obj_of_names: [{
                             id: "a1",
                             name: "abc",
                             school: "xyz" },
                            { id:"b1",
                             name: "def",
                             school: "zyx" },
                            { id:"c1",
                             name: "ghi",
                             school: "pqr" }]
             },
             {id: "2", obj_of_names: [{
                             id: "d1",
                             name: "abc123",
                             school: "xyz123" }]
             }]

You can call a map inside another map and change the array whitin the object like so:

const newArr =  arr.map((item)=>{
        if(item.id == 2){
            return {...item, obj_of_names: item.obj_of_names.map((item2)=>{
    
                if(item2.id === 'd1'){
                    return {...item2, school: 'newxyz123'}
                }
    
                return item2
            }) }
        }
    
        return item
    })

    const [state, setState] = useState(arr)

Then you can call setState and update it with the newArr

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How to change useState for nested array of objects

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Andres GPT