JavaScript: Find duplicate values in a array

Last update on May 30 2022 10:08:59 (UTC/GMT +8 hours)

JavaScript Array: Exercise-20 with Solution

Write a JavaScript program to find duplicate values in a JavaScript array.

Sample Solution:

HTML Code:

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Find duplicate values in a JavaScript array</title>
</head>
<body>
</body>
</html>

JavaScript Code:

function find_duplicate_in_array(arra1) {
        var object = {};
        var result = [];

        arra1.forEach(function (item) {
          if(!object[item])
              object[item] = 0;
            object[item] += 1;
        })

        for (var prop in object) {
           if(object[prop] >= 2) {
               result.push(prop);
           }
        }

        return result;

    }

    console.log(find_duplicate_in_array([1, 2, -2, 4, 5, 4, 7, 8, 7, 7, 71, 3, 6]));

Sample Output:

["4","7"]

Flowchart:

ES6 Version:

function find_duplicate_in_array(arra1) {
        const object = {};
        const result = [];

        arra1.forEach(item => {
          if(!object[item])
              object[item] = 0;
            object[item] += 1;
        })

        for (const prop in object) {
           if(object[prop] >= 2) {
               result.push(prop);
           }
        }

        return result;

    }

    console.log(find_duplicate_in_array([1, 2, -2, 4, 5, 4, 7, 8, 7, 7, 71, 3, 6]));

Live Demo:

See the Pen JavaScript - Find duplicate values in a array - array-ex- 20 by w3resource (@w3resource) on CodePen.

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Next: Write a JavaScript program to flatten a nested (any depth) array.

JavaScript: Tips of the Day

How to insert an item into an array at a specific index (JavaScript)?

What you want is the splice function on the native array object.

arr.splice(index, 0, item); will insert item into arr at the specified index (deleting 0 items first, that is, it's just an insert). In this example we will create an array and add an element to it into index 2:

var arr = []; arr[0] = "Jani"; arr[1] = "Hege"; arr[2] = "Stale"; arr[3] = "Kai Jim"; arr[4] = "Borge"; console.log(arr.join()); arr.splice(2, 0, "Lene"); console.log(arr.join());

Ref: https://bit.ly/2BXbp04