check duplicates 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: js find duplicates in array

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

const count = names =>
  names.reduce((a, b) => ({ ...a,
    [b]: (a[b] || 0) + 1
  }), {}) // don't forget to initialize the accumulator

const duplicates = dict =>
  Object.keys(dict).filter((a) => dict[a] > 1)

console.log(count(names)) // { Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, Carl: 1 }
console.log(duplicates(count(names))) // [ 'Nancy' ]

Example 4: javascript check if array has duplicates

function hasDuplicates(array) {
    return (new Set(array)).size !== array.length;
}

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;
}

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