how to check if an item is repeated in array js code example

Example 1: 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 2: javascript get duplicates in array

function getDuplicateArrayElements(arr){
    var sorted_arr = arr.slice().sort();
    var results = [];
    for (var i = 0; i < sorted_arr.length - 1; i++) {
        if (sorted_arr[i + 1] === sorted_arr[i]) {
            results.push(sorted_arr[i]);
        }
    }
    return results;
}

var colors = ["red","orange","blue","green","red","blue"];
var duplicateColors= getDuplicateArrayElements(colors);//["blue", "red"]