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 it`s own strings for duplicates, without already pre-determined string to compare.

Answer 1

findDuplicates function 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

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

Answer 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.

Answer 4

Using some function on arrays: If any item in the array has an index number from the beginning is not equals to index number from the end, then this item exists in the array more than once.

// vanilla js
function hasDuplicates(arr) {
    return arr.some( function(item) {
        return arr.indexOf(item) !== arr.lastIndexOf(item);
    });
}

Answer 5

The following code uses a unique-filter (checks if every occurrence of an item is the first occurence) to compare the number of unique items in an array with the total number of items: if both are equal, the array only contains unique elements, otherwise there are some duplicates.

var firstUnique = (value, index, array) => array.indexOf(value) === index;
var numUnique = strArray.filter(firstUnique).length;
var allUnique = strArray.length === numUnique; 

Answer 6

Use object keys for good performance when you work with a big array (in that case, loop for each element and loop again to check duplicate will be very slowly).

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

var counting = {};
strArray.forEach(function (str) {
    counting[str] = (counting[str] || 0) + 1;
});

if (Object.keys(counting).length !== strArray.length) {
    console.log("Has duplicates");

    var str;
    for (str in counting) {
        if (counting.hasOwnProperty(str)) {
            if (counting[str] > 1) {
                console.log(str + " appears " + counting[str] + " times");
            }
        }
    }
}

Answer 7

Simple Javascript (if you don't know ES6)

function hasDuplicates(arr) {
    var counts = [];

    for (var i = 0; i <= arr.length; i++) {
        if (counts[arr[i]] === undefined) {
            counts[arr[i]] = 1;
        } else {
            return true;
        }
    }
    return false;
}

// [...]

var arr = [1, 1, 2, 3, 4];

if (hasDuplicates(arr)) {
  alert('Error: you have duplicates values !')
}