Checking for duplicate strings in JavaScript array

Solution 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

Solution 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

Solution 3:

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

strArray.forEach(function(str) {
  if (alreadySeen[str])
    alert(str);
  else
    alreadySeen[str] = true;
});

I added another duplicate in there from your original just to show it would find a non-consecutive duplicate.

Updated version with arrow function:

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

strArray.forEach(str => alreadySeen[str] ? alert(str) : alreadySeen[str] = true);