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.

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

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);

You could take a Set and filter the values who are alreday seen.

var array = ["q", "w", "w", "e", "i", "u", "r"],_x000D_
    seen = array.filter((s => v => s.has(v) || !s.add(v))(new Set));_x000D_
_x000D_
console.log(seen);

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);
    });
}

This is the simplest solution I guess :

function diffArray(arr1, arr2) {
  return arr1
    .concat(arr2)
    .filter(item => !arr1.includes(item) || !arr2.includes(item));
}

   var elems = ['f', 'a','b','f', 'c','d','e','f','c'];

    elems.sort();

    elems.forEach(function (value, index, arr){

        let first_index = arr.indexOf(value);
        let last_index = arr.lastIndexOf(value);

         if(first_index !== last_index){

         console.log('Duplicate item in array ' + value);

         }else{

         console.log('unique items in array ' + value);

         }

    });

I think it can't be simpler than this.

const findDuplicates = arr => [...new Set(arr.filter(v => arr.indexOf(v) !== arr.lastIndexOf(v)))];

console.log(findDuplicates([ "q", "w", "w", "e", "i", "u", "r"]));

You could use reduce:

const arr = ["q", "w", "w", "e", "i", "u", "r"]
arr.reduce((acc, cur) => { 
  if(acc[cur]) {
    acc.duplicates.push(cur)
  } else {
    acc[cur] = true //anything could go here
  }
}, { duplicates: [] })

Result would look like this:

{ ...Non Duplicate Values, duplicates: ["w"] }

That way you can do whatever you want with the duplicate values!

You can do this using a Set. You have to create a Set and put all the values in your Array, in that Set. Then, you check whether they have the same length or not. If not, you know there are duplicate values, because a Set can only have unique values. It is explained in the link below:

https://medium.com/dailyjs/how-to-remove-array-duplicates-in-es6-5daa8789641c

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");
            }
        }
    }
}

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 !')
}

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 !')
}

Here's my solution if you are using typescript in a functional way:

const hasDuplicates = <T>(arr: T[]): boolean => {
  if (arr.length === 0) return false
  if (arr.lastIndexOf(arr[0]) !== 0) return true
  return hasDuplicates(arr.slice(1))
}

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;