Parent object changes even if the copy of it is created while splice

I have a parent object categories, and a function that flattens and removes duplicate numbers inside the array.
But even though I make a copy the categories object (let newObj = {...categories}) and perform the above flatten function, it affects the parent categories object.

Code:

let categories = {
  Red: {
    Name: "Fire Flame",
    List: [
      ["1","5"],
      ["10","15"]
    ]
  },
  Blue: {
    Name: "Super Light",
    List: [
      ["1","5"],
      ["9","12"]
    ]
  },
  Custom: {
    List: [
      "Star Flash"
    ]
  }
}



  const submitData = () => {
    let obj = { ...categories };
    let types = Object.keys(obj);
    types.map((key) => {
      flattenAndUnique(key);
    });
    function flattenAndUnique(key) {
      let myList = obj[key].List;
      let resList = [];
      if (key !== "Custom") {
        myList.map(async (data, index) => {
          let min = Math.min(...data);
          let max = Math.max(...data);
          for (let i = min; i <= max; i++) {
            resList.push(String(i));
          }
          const spliced = [...myList.splice(index, 1, resList)];
          let flatLevelArray = [].concat.apply([], myList);
          let uniqueLevelArray = flatLevelArray.filter(
            (v, i, a) => a.indexOf(v) === i
          );
          obj = {
            ...obj,
            [key]: {
              ...obj[key],
              List: uniqueLevelArray,
            },
          };
          resList = [];
        });
      } else {
        let flatLevelArray = [].concat.apply([], myList);
        obj = {
          ...obj,
          Custom: {
            ...obj.Custom,
            List: flatLevelArray,
          },
        };
      }
    }
    console.log("final obj %% ===>", obj);
    console.log("Floors final &&&&", categories);
  };
  
  submitData()

sumbitData function actually flattend the array and removes duplicates. For eg,
If i have two array – [["1","4"] , ["11","13"]], the function transforms it into – ["1","2","3","4","11","12","13"]
which works fine,
My only issue is that my parent categories also changes.

>Solution :

First of all, you get the reference of the List property.

let myList = obj[key].List;

And then in the bottom, you splice it. ( splice will effect the origin array as it has the same referance )

const spliced = [...myList.splice(index, 1, resList)];

It should be

let myList = [...obj[key].List];