haveDuplicates for chai

chai.haveDuplicates.js

/*

  expect([1,2,3]).to.not.haveDuplicates()
  expect([1,1,2,3]).to.haveDuplicates()

*/
chai.Assertion.addMethod('haveDuplicates', function() {
  const array = Array.from(this._obj)
  expect(array).to.be.an.instanceof(Array)

  const dups = []
  array.forEach((member, index) => {
    if (array.lastIndexOf(member) !== index){
      dups.push(member)
    }
  })

  this.assert(
    dups.length !== 0,
    "expected #{this} to have duplicates",
    "expected #{this} to not have duplicates",
    [],  // expected
    dups // actual
  );
})