I have an array to be like this
var arr = [0, 0, 0, 0, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 5, 5].

so it contains duplicate values. In here last index value of
'0' is '3',
'1' is '4',
'2' is '5',
'3' is '13',
and so on.
And i counted total duplicate values

var counts = {};
arr.forEach(function(x) { counts[x] = (counts[x] || 0)+1; })

but i want to know last duplicate value index only. please help me.
thanks in advance

I have an array to be like this
var arr = [0, 0, 0, 0, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 5, 5].

so it contains duplicate values. In here last index value of
'0' is '3',
'1' is '4',
'2' is '5',
'3' is '13',
and so on.
And i counted total duplicate values

var counts = {};
arr.forEach(function(x) { counts[x] = (counts[x] || 0)+1; })

but i want to know last duplicate value index only. please help me.
thanks in advance

• javascript
• arrays

• And how do you want the results? – epascarello Commented Feb 2, 2016 at 19:07
• i want the result in an array. – htoniv Commented Feb 2, 2016 at 19:08
• Seeing as counts is an object, there is no "last", as there is no order – adeneo Commented Feb 2, 2016 at 19:10
• Will the array always be sorted? – TimoStaudinger Commented Feb 2, 2016 at 19:10
• 1 Will the array always have values grouped together (ie. [0,0,1,1,2,2]) or can they be scattered (ie. [0,1,2,0,1,2])? – william.taylor.09 Commented Feb 2, 2016 at 19:16

4 Answers 4

var arr = [0, 0, 0, 0, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 5, 5];
var existingItems = {};

arr.forEach(function(value, index) {
    existingItems[value] = index;
});

console.log(existingItems);

Simple loop and check to see if next index is different

var arr = [0, 0, 0, 0, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 5, 5]
var indexes = arr.reduce(function(result, cur, ind, arr){
   if (ind+1===arr.length || cur != arr[ind+1]) {  //check to see if last or different
       result.push(ind);  //if different, store the index
   }
   return result;
},[]);
console.log(indexes);

Array.prototype.lastIndexOf() es in handy here:

var arr = [0, 0, 0, 0, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 5, 5];

var out = {};

arr.forEach(function(item) {
  if(!out.hasOwnProperty(item)) {
    out[item] = arr.lastIndexOf(item);
  }
});

console.log(out); // Object {0: 3, 1: 4, 2: 5, 3: 13, 4: 15, 5: 22}

This will also work for unsorted input arrays.

Well, what you can do is check if a value is a duplicate (count > 1) and then use the second parameter of forEach which is the index to remember it. Something like:

var arr = [0, 0, 0, 0, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 5, 5];
var counts = {};
var indexes = {};
arr.forEach(function(x, idx) {
  counts[x] = (counts[x] || 0) + 1;
  if(counts[x] > 1) { indexes[x] = idx; } // check if a value is a duplicate and update its index
});

// logs the last index of all duplicate values
console.log(indexes); // {0: 3, 3: 13, 4: 15, 5: 22}

If you want the last index of all values, not just duplicates, you can omit the count > 1 check:

var arr = [0, 0, 0, 0, 1, 2, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 5, 5, 5, 5, 5, 5, 5];
var counts = {};
var indexes = {};
arr.forEach(function(x, idx) {
  counts[x] = (counts[x] || 0) + 1;
  indexes[x] = idx; // update value index
});

// logs the last index of all values
console.log(indexes); // {0: 3, 1: 4, 2: 5, 3: 13, 4: 15, 5: 22}