check an array for a duplicate javascript code example

Example 1: 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 2: javascript duplicate an array

let arr =["a","b","c"];
// ES6 way
const duplicate = [...arr];

// older method
const duplicate = Array.from(arr);