Get all non-unique values (i.e.: duplicate/more than one occurrence) in an array

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I need to check a JavaScript array to see if there are any duplicate values. What’s the easiest way to do this? I just need to find what the duplicated values are – I don’t actually need their indexes or how many times they are duplicated.

I know I can loop through the array and check all the other values for a match, but it seems like there should be an easier way. Any ideas? Thanks!

Similar question:

You could sort the array and then run through it and then see if the next (or previous) index is the same as the current. Assuming your sort algorithm is good, this should be less than O(n2):

var arr = [9, 9, 111, 2, 3, 4, 4, 5, 7];
var sorted_arr = arr.slice().sort(); // You can define the comparing function here.
// JS by default uses a crappy string compare.
// (we use slice to clone the array so the
// original array won't be modified)
var results = [];
for (var i = 0; i < sorted_arr.length - 1; i++) {
if (sorted_arr[i + 1] == sorted_arr[i]) {
results.push(sorted_arr[i]);
}
}

console.log(results);

If you want to elimate the duplicates, try this great solution:

function eliminateDuplicates(arr) {
var i,
len = arr.length,
out = [],
obj = {};

for (i = 0; i < len; i++) {
obj[arr[i]] = 0;
}
for (i in obj) {
out.push(i);
}
return out;
}

This is my answer from the duplicate thread (!):

When writing this entry 2014 – all examples were for-loops or jQuery. Javascript has the perfect tools for this: sort, map and reduce.

Find duplicate items

var names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']

var uniq = names
.map((name) => {
return {
count: 1,
name: name
}
})
.reduce((a, b) => {
a[b.name] = (a[b.name] || 0) + b.count
return a
}, {})

var duplicates = Object.keys(uniq).filter((a) => uniq[a] > 1)

console.log(duplicates) // [ 'Nancy' ]

More functional syntax:

@Dmytro-Laptin pointed out some code code be removed. This is a more compact version of the same code. Using some ES6 tricks and higher order functions:

const names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']

const count = names =>
names.reduce((a, b) => ({ ...a,
[b]: (a[b] || 0) + 1
}), {}) // don't forget to initialize the accumulator

const duplicates = dict =>
Object.keys(dict).filter((a) => dict[a] > 1)

console.log(count(names)) // { Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, Carl: 1 }
console.log(duplicates(count(names))) // [ 'Nancy' ]
Reprint:https://stackoverflow.com/questions/840781/get-all-non-unique-values-i-e-duplicate-more-than-one-occurrence-in-an-array