Checking for duplicate strings in JavaScript array

javascript arrays compare
Bengall picture Bengall · Mar 11, 2018 · Viewed 103.4k times · Source

I have JS array with strings, for example:

var strArray = [ "q", "w", "w", "e", "i", "u", "r"];

I need to compare for duplicate strings inside array, and if duplicate string exists, there should be alert box pointing to that string.

I was trying to compare it with for loop, but I don't know how to write code so that array checks it`s own strings for duplicates, without already pre-determined string to compare.

Answer

Mike Ezzati picture Mike Ezzati · Mar 11, 2018

The findDuplicates function (below) compares index of all items in array with index of first occurrence of same item. If indexes are not same returns it as duplicate.

let strArray = [ "q", "w", "w", "w", "e", "i", "u", "r"];
let findDuplicates = arr => arr.filter((item, index) => arr.indexOf(item) != index)

console.log(findDuplicates(strArray)) // All duplicates
console.log([...new Set(findDuplicates(strArray))]) // Unique duplicates

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