Below is an example array:

const attD = [1, 2, 3, 4, 3, 3]

How can we get the index of the duplicate values from this array? In this example, 3 is repeated thrice. So, I want to get the index 4, index 5 as these are duplicated at that positions (ignoring the first occurrence).

I have tried the below code, but it is giving me values. Instead, I need the index.

const findDuplicates = attD => attD.filter((item, index) => attD.indexOf(item) !== index);
const duplicates = findDuplicates(attD);
console.log(duplicates);

CodePudding user response：

1. With .map(), add the index of duplicated values into the array. For non-duplicate value, will be added with undefined value [Will be filtered in Step 2].
2. With .filter(), remove the value which is undefined from the array.

const attD = [1, 2, 3, 4, 3, 3];

const findDuplicates = attD => attD.map((item, index) => {
  if (attD.indexOf(item) !== index)
    return index;
}).filter(x => x);  // Equivalent to .filter(x => x != undefined || x != null)

const duplicates = findDuplicates(attD);
console.log(duplicates);

CodePudding user response：

You can use this solution:

const attD = [1, 2, 3, 4, 3, 3]
const findDuplicates = attD => attD.map((item, index) => {
  if (attD.indexOf(item) !== index)
    return index;
}).filter(Boolean);

and you can use set:

const attD = [1, 2, 3, 4, 3, 3];
const setAttd = new Set(attD);
const findDuplicates = attD => attD.map((item, index) => {
 if (setAttd.has(item)) {
        setAttd.delete(item);
    } else {
        return index;
    }
}).filter(Boolean);