A script for for the article "Adding KB search to your website with AJAX" at https://support.zendesk.com/hc/en-us/articles/206287378

search_kb.js

document.getElementById('search_btn').addEventListener('click', search_kb, false);

function search_kb(event) {
  event.preventDefault();
  var search_string = encodeURIComponent(document.getElementById('query').value);
  var url = 'https://your_subdomain.zendesk.com/api/v2/help_center/articles/search.json?query=' + search_string;
  var my_request = new XMLHttpRequest();
  my_request.open('get', url, true);
  my_request.onload = show_results;
  my_request.onerror = show_error;
  my_request.send();
}

function show_error() {
  alert('Request error');
}

function show_results() {
  clearPreviousResults();
  var form = document.getElementById('searchForm');
  var resultDiv = document.createElement('div');
  resultDiv.id = 'search_results';
  document.body.insertBefore(resultDiv, form.nextSibling);

  var data = JSON.parse(this.responseText);
  if (data.results.length) {      // if results exist
    var resultList = document.createElement('ul');
    resultDiv.appendChild(resultList);

    for (var i = 0; i < data.results.length; ++i) {
      var article = data.results[i];
      var resultItem = document.createElement('li');
      resultList.appendChild(resultItem);
      resultItem.innerHTML = '<a href="' + article.html_url + '" target="_blank">' + article.title + '</a>';
    }
  } else {		// no results
    var result = document.createElement('p');
    result.innerHTML = 'No results';
    resultDiv.appendChild(result);
  }
}

function clearPreviousResults(){
  var results = this.document.getElementById('search_results');
  if (results) {
    results.remove();
  }
}