how to find duplicate number in array javascript code example

Example 1: count duplicates array js

uniqueCount = ["a","b","c","d","d","e","a","b","c","f","g","h","h","h","e","a"];
var count = {};
uniqueCount.forEach(function(i) { count[i] = (count[i]||0) + 1;});
console.log(count);

Example 2: how to get duplicate values from array in javascript

var array = [1, 2, 2, 3, 3, 4, 5, 6, 2, 3, 7, 8, 5, 22, 1, 2, 511, 12, 50, 22];

console.log([...new Set(
  array.filter((value, index, self) => self.indexOf(value) !== index))]
);

Example 3: javascript find duplicate in array

// JavaScript - finds if there is duplicate in an array. 
// Returns True or False.

const isThereADuplicate = function(arrayOfNumbers) {
    // Create an empty associative array or hash. 
    // This is preferred,
    let counts = {};
    // // but this also works. Comment in below and comment out above if you want to try.
    // let counts = [];

    for(var i = 0; i <= arrayOfNumbers.length; i++) {
        // As the arrayOfNumbers is being iterated through,
        // the counts hash is being populated.
        // Each value in the array becomes a key in the hash. 
        // The value assignment of 1, is there to complete the hash structure.
        // Once the key exists, meaning there is a duplicate, return true.
        // If there are no duplicates, the if block completes and returns false.
        if(counts[arrayOfNumbers[i]] === undefined) {
            counts[arrayOfNumbers[i]] = 1;
        } else {
            return true;
        }
    }
    return false;
}

Example 4: how to get duplicate values from array in javascript

var input = [1, 2, 3, 1, 3, 1];

var duplicates = input.reduce(function(acc, el, i, arr) {
  if (arr.indexOf(el) !== i && acc.indexOf(el) < 0) acc.push(el); return acc;
}, []);

document.write(duplicates); // = 1,3 (actual array == [1, 3])