To create a multi-select autocomplete combobox in JavaScript, you can use a combination of HTML, CSS, and JavaScript.

Here’s an example of how to create a multi-select autocomplete combobox:

HTML:

<input type="text" id="myComboBox" placeholder="Type to search...">
<ul id="myComboBoxList"></ul>

CSS:

#myComboBoxList {
  list-style: none;
  padding: 0;
  margin: 0;
  border: 1px solid #ccc;
  border-top: none;
  position: absolute;
  width: 100%;
  max-height: 150px;
  overflow-y: scroll;
}

#myComboBoxList li {
  padding: 5px;
  cursor: pointer;
}

#myComboBoxList li:hover {
  background-color: #f2f2f2;
}

JavaScript:

var myComboBox = document.getElementById("myComboBox");
var myComboBoxList = document.getElementById("myComboBoxList");
var selectedItems = [];

myComboBox.addEventListener("input", function() {
  var inputValue = this.value;
  myComboBoxList.innerHTML = "";

  // Perform search for inputValue and get the matching results
  // ...

  // Display the matching results in the dropdown list
  for (var i = 0; i < matchingResults.length; i++) {
    var listItem = document.createElement("li");
    listItem.innerText = matchingResults[i].text;
    listItem.addEventListener("click", function() {
      // Add selected item to selectedItems array
      selectedItems.push(this.innerText);

      // Clear the input box and hide the dropdown list
      myComboBox.value = "";
      myComboBoxList.innerHTML = "";

      // Display selected items in the input box
      updateSelectedItems();
    });
    myComboBoxList.appendChild(listItem);
  }
});

function updateSelectedItems() {
  myComboBox.value = selectedItems.join(", ");
}

In the above example, the myComboBox variable is used to reference the input box, and the myComboBoxList variable is used to reference the dropdown list. The selectedItems array is used to store the selected items.

The input event is added to the input box to perform a search based on the user’s input and display the matching results in the dropdown list. When a user clicks on an item in the dropdown list, the selected item is added to the selectedItems array and the input box is cleared.

The updateSelectedItems() function is called to update the input box with the selected items. This function joins the selected items array into a string and sets the value of the input box to the selected items string.

Note that this is a simplified example and you will need to modify it to suit your specific requirements, such as performing the search using AJAX, using a different data structure for the selected items, or modifying the styling of the dropdown list.