How can verify if exist duplicates of an element in an array?

Question

I am doing like this, this is my code to know if there are duplicates

const value = 'a'
const value2 = 'c'
const arr = [{
  name: 'a'
}, {
  name: 'b'
}]

const result = arr.filter((d) => d.name === value).length !== 1
console.log(result)
const result2 = arr.filter((d) => d.name === value2).length !== 1
console.log(result2)

there is another way to do this properly? with .some(), .includes or other functions, i think my function its hard

note: arr contains objects

Does this answer your question? [Checking for duplicate strings in JavaScript array](https://stackoverflow.com/questions/49215358/checking-for-duplicate-strings-in-javascript-array) — PM 77-1, Oct 03 '22 at 21:15
@letters I recommend showing a basic example of your array, that will give users the ability to test out and get a working example based on your actual data. — imvain2, Oct 03 '22 at 21:24
some() is what you are looking for `const hasDupe = arr.some(({name}) => name === value )` — epascarello, Oct 03 '22 at 21:27
@epascarello wouldn't that return true also when only 1 element fulfils the condition? I think OP needs at least 2 — GrafiCode, Oct 03 '22 at 21:39
You'll need to explain "at least 2". It changes the whole task you're doing from "checking if value exists in arr" to "checking arr for duplicates in general". And you didn't provide enough supporting code for us to be able to tell. — James, Oct 03 '22 at 22:04
Um, not sure where "2" came from when it would be one.... makes no sense. — epascarello, Oct 04 '22 at 00:36

Mister Jojo · Accepted Answer · 2022-10-04T00:34:02.627

1

you can use Array.reduce()

const
  value  = 'a'
, value2 = 'c'
, arr    = [{ name: 'a' }, { name: 'b' }]
, uniqElm = x => (1 === arr.reduce((r,{name}) => r + (name===x)?1:0,0))
  ;

console.log( value,  ', is present and unique ?', uniqElm(value)  )
console.log( value2, ', is present and unique ?', uniqElm(value2) )

edited Oct 04 '22 at 00:34

answered Oct 03 '22 at 22:52

Mister Jojo

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no, i need two appears in my array – letters Oct 04 '22 at 00:13
@letters oh, i'm having a little trouble understanding the real question, does updating my answer fit better? – Mister Jojo Oct 04 '22 at 00:28

score 0 · Answer 2 · answered Oct 03 '22 at 23:01

I think your logic could fail with the case I fix below:

const value = 'a'
const value2 = 'c'

const arr = [{
  name: 'a'
}, {
  name: 'b'
},
{
  name: 'a'
}
]

const result = arr.some((d) => d.name === value)
console.log(result)
const result2 = arr.some((d) => d.name === value2)
console.log(result2)

Semantically and better in performance I think it is better to use some. It will stop iterating with the first success match found.

How can verify if exist duplicates of an element in an array?

2 Answers2