Get the array index of duplicates

Question

In a JavaScript array how can I get the index of duplicate strings?

Example:

MyArray = ["abc","def","abc"]; //----> return 0,2("abc");

Another example:

My Array = ["abc","def","abc","xyz","def","abc"] 
//----> return 0,2,5("abc") and 1,4("def");

I have no idea how to do this. Thanks in advance for your help!

Answer 1

Update 01/2022: It's not 2013 anymore, and many things have changed. I neither recommend modifying the prototype, nor is the approach in this answer the "best" as it requires several iterations over the array.

Here's an updated version of the original answer, retaining its spirit, as well as the original answer below.

function getDuplicates<T>(input: T[]): Map<T, number[]> {
    return input.reduce((output, element, idx) => {
        const recordedDuplicates = output.get(element);
        if (recordedDuplicates) {
            output.set(element, [...recordedDuplicates, idx]);
        } else if (input.lastIndexOf(element) !== idx) {
            output.set(element, [idx]);
        }

        return output;
    }, new Map<T, number[]>());
}

---

Yet another approach:

Array.prototype.getDuplicates = function () {
    var duplicates = {};
    for (var i = 0; i < this.length; i++) {
        if(duplicates.hasOwnProperty(this[i])) {
            duplicates[this[i]].push(i);
        } else if (this.lastIndexOf(this[i]) !== i) {
            duplicates[this[i]] = [i];
        }
    }

    return duplicates;
};

It returns an object where the keys are the duplicate entries and the values are an array with their indices, i.e.

["abc","def","abc"].getDuplicates() -> { "abc": [0, 2] }