Checking for duplicate strings in JavaScript array

Question

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 its own strings for duplicates, without already pre-determined string to compare.

Answer 1

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

Answer 2

Using ES6 features

• Because each value in the Set has to be unique, the value equality will be checked.

function checkIfDuplicateExists(arr) {     return new Set(arr).size !== arr.length }    var arr = ["a", "a", "b", "c"]; var arr1 = ["a", "b", "c"];  console.log(checkIfDuplicateExists(arr)); // true console.log(checkIfDuplicateExists(arr1)); // false