how to find duplicates in an array javascript code example

Example 1: 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 2: 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 3: how to check for duplicate syntax in javascript

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

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

var names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']

var uniq = names
  .map((name) => {
    return {
      count: 1,
      name: name
    }
  })
  .reduce((a, b) => {
    a[b.name] = (a[b.name] || 0) + b.count
    return a
  }, {})

var duplicates = Object.keys(uniq).filter((a) => uniq[a] > 1)

console.log(duplicates) // [ 'Nancy' ]

Example 5: how to get duplicate values from array in javascript

let a = [1, 2, 3, 4, 2, 2, 4, 1, 5, 6]
let b = [...new Set(a.sort().filter((o, i) => o !== undefined && a[i + 1] !== undefined && o === a[i + 1]))]

// b is now [1, 2, 4]

Example 6: 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;
}