fastest way to detect if duplicate entry exists in javascript array?

Question

var arr = [\'test0\',\'test2\',\'test0\'];

Like the above,there are two identical entries with value \"test0\",how to check it most efficiently?

Answer 1

There are lots of answers here but not all of them "feel" nice... So I'll throw my hat in.

If you are using lodash:

function containsDuplicates(array) {
  return _.uniq(array).length !== array.length; 
}

If you can use ES6 Sets, it simply becomes:

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

With vanilla javascript:

function containsDuplicates(array) {
  return array
    .sort()
    .some(function (item, i, items) {
      return item === items[i + 1]
    })
}

However, sometimes you may want to check if the items are duplicated on a certain field.

This is how I'd handle that:

containsDuplicates([{country: 'AU'}, {country: 'UK'}, {country: 'AU'}], 'country')

function containsDuplicates(array, attribute) {
  return array
    .map(function (item) { return item[attribute] })
    .sort()
    .some(function (item, i, items) {
      return item === items[i + 1]
    })
}