JSFiddle - React, Tailwind, and code Playground

JavaScript

function unique(arr) {
  var unique = [],
      map = {};

  for (var i = 0; i < arr.length; i++) {
    var exists = map[arr[i]];
      console.log(exists);
    if (!exists)  {
        unique.push(arr[i]);
        map[arr[i]] = true;
    }
  }
  return unique;
}

document.write(unique([2,2,1,4,4,3,5,5]));