как сравнить значение в инпуте с массивом json?

document.querySelector(".btn").addEventListener("click", function(){
  
  if (!countries.includes(document.querySelector('#autocomplete-input').value))
  {
   alert('Error');
  }
});


function Get(url) {
  var HttpReq = new XMLHttpRequest(); 
  HttpReq.open("GET", url, false);
  HttpReq.send(null);
  return HttpReq.responseText;
}

var countries = JSON.parse(  Get("https://gist.githubusercontent.com/Miserlou/c5cd8364bf9b2420bb29/raw/2bf258763cdddd704f8ffd3ea9a3e81d25e2c6f6/cities.json")
);

countries.map(function(element, index) {
  countries[index] = element.city;
});

// variables
var input = document.querySelector("#autocomplete-input");
var results, countries_to_show = [];
var autocomplete_results = document.getElementById("autocomplete-results");

// functions
function autocomplete(val) {
  var countries_returned = [];

  for (i = 0; i < countries.length; i++) {
    if (val === countries[i].toLowerCase().slice(0, val.length)) {
      countries_returned.push(countries[i]);
    }
  }

  return countries_returned;
}

// events
input.onkeyup = function(e) {
  input_val = this.value.toLowerCase();

  if (input_val.length > 0) {
    autocomplete_results.innerHTML = "";
    countries_to_show = autocomplete(input_val);

    for (i = 0; i < countries_to_show.length; i++) {
      autocomplete_results.innerHTML +=
        "<li id=" +
        countries_to_show[i] +
        ' class="list-item">' +
        countries_to_show[i] +
        "</li>";
    }
    autocomplete_results.style.display = "block";
  } else {
    countries_to_show = [];
    autocomplete_results.innerHTML = "";
  }
};

// Get the element, add a click listener...
document
  .getElementById("autocomplete-results")
  .addEventListener("click", function(e) {
    // e.target is the clicked element!
    // If it was a list item
    if (e.target && e.target.nodeName == "LI") {
      // List item found!  Output the value!
      console.log(e.target.innerHTML);
      input.value = e.target.innerHTML;
      autocomplete_results.innerHTML = null; //empty the value
    }
  });

/*CSS RESET*/
html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed,
figure, figcaption, footer, header, hgroup,
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
  margin: 0;
  padding: 0;
  border: 0;
  font: inherit;
  font-size: 100%;
  vertical-align: baseline;
}

html {
  line-height: 1;
}

ol, ul {
  list-style: none;
}
/*STYLES*/
body {
  background: #123;
  height: 100%;
  width: 100%;
  font-family:"Lucida Grande", "Lucida Sans Unicode", Tahoma, Sans-Serif;
}
#autocomplete-container {
  width: 300px;
  margin: 50px auto;
}
#autocomplete-input {
  background: white;
  -moz-appearance: none;
  -webkit-appearance: none;
  width: 100%;
  padding: 20px 15px;
  border: pink;
  outline: none;
  color: black;
  font-size: 20px;
}
#autocomplete-input:-webkit-placeholder {
  color: grey;
}
#autocomplete-input::-moz-placeholder {
  color: grey;
}
#autocomplete-results {
  display: none;
  width: 100%;
  margin-top: -2px;
  color: #eee;
}
#autocomplete-results li {
  width: 100%;
  padding: 7px 15px;
  border-right: 2px solid #eee;
  border-bottom: 2px solid #eee;
  border-left: 2px solid #eee;
}
#autocomplete-results li:hover {
  background:rgba(105, 111, 118, 0.8);;
}
.btn{
  display:block;
  width:200px;
  margin: 0 auto;
}
.Input2{
  display:block;
  width:200px;
  margin: 20px auto;
}

autocomplete-input<div id="autocomplete-container">
  <input type="text" autofocus="true" name="autofocus sample" placeholder="Search by Country" id="autocomplete-input"></input>
  <ul id="autocomplete-results">
  </ul>
</div>
<button class="btn">Go</button>
  <input type="text" class="Input2"></input>