Can I override the onChange trigger of a selectize input?

I have a selectize input and want to change its behaviour. I normally use selectize inputs from within some framework (shiny), so I hope the example that I give here is understandable and precise.

I want to change the logic, when an onChange - event is triggered. I only want this to happen when there is one element selected, not when none is selected. In case the user deletes the element and leaves the form untouched, I want to restore the value that was active before.

Have a look at the following code:

$(document).ready(function() {
  var selectize = $('#select-beast').selectize({
    delimiter: ',',
    persist: false,
    create: function(input) {
      return {
        value: input,
        text: input
      }
    }
  });

  selectize[0].selectize.on('change', function() {
    var value = selectize[0].selectize.getValue();
    console.log(value);
  });
});

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Selectize Example</title>
  <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.13.3/css/selectize.default.min.css">
</head>
<body>
  <select id="select-beast" multiple placeholder="Select a beast...">
    <option value="lion">Lion</option>
    <option value="tiger">Tiger</option>
    <option value="bear">Bear</option>
    <option value="elephant">Elephant</option>
    <option value="rhino">Rhino</option>
  </select>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
  <script src="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.13.3/js/standalone/selectize.min.js"></script>
</body>
</html>

So in my case the log message should only ever print out something like Array["bear"] but never an empty array. I know that in this case one could do a if before the console.log but I am explicitly asking for changing the onChange condition because in my real problem I have no direct control on what my framework is doing. Is there a way of achieving my goal?

no change-event with empty selection
restoring the value that was there before, once the user gives focus to another element

I would claim that it is pretty natural to want something like that, so maybe there is even a plugin capable of achieving my goals?

Solution

I don't quite understand what you want.
But you can override the change event and not call your function if value.length === 0 (this will work with <select multiple> and without multiple). It's a bit dirty:

const selectize = $('#select-beast').selectize({
  delimiter: ',',
  persist: false,
  create: function(input) {
    return {
      value: input,
      text: input
    }
  }
});

/* We save the event */
const originalOnChange = selectize[0].selectize._events.change; 

/* Your function. (By the way, value is in the first argument) */
selectize[0].selectize.on('change', function() {
  var value = selectize[0].selectize.getValue();
  console.log(value);
});

/* We turn off the event */
selectize[0].selectize.off('change');

selectize[0].selectize._events.change = [];
selectize[0].selectize._events.change[0] = function(value) {
  /* If is no value - we do nothing */
  if (!value.length) {
    return;
  }
  /* Else call previously saved functions */
  if (Array.isArray(originalOnChange)) {
    originalOnChange[0] && originalOnChange[0].call(this);
    originalOnChange[1] && originalOnChange[1]();
  }
}

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.13.3/css/selectize.default.min.css">
<select id="select-beast" multiple placeholder="Select a beast...">
  <option value="lion">Lion</option>
  <option value="tiger">Tiger</option>
  <option value="bear">Bear</option>
  <option value="elephant">Elephant</option>
  <option value="rhino">Rhino</option>
</select>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.13.3/js/standalone/selectize.min.js"></script>

If you want to set a value after blur when the select is empty, you need to store that value somewhere. I added this to a modified change function:

const selectize = $('#select-beast').selectize({
  delimiter: ',',
  persist: false,
  create: function(input) {
    return {
      value: input,
      text: input
    }
  }
});

/* We save the event */
const originalOnChange = selectize[0].selectize._events.change; 

/* Your function. (By the way, value is in the first argument) */
selectize[0].selectize.on('change', function() {
  var value = selectize[0].selectize.getValue();
  console.log(value);
});

/* blur event */
selectize[0].selectize.on('blur', function() {
 const value = selectize[0].selectize.getValue();
 if(!value.length){
    selectize[0].selectize.setValue(selectize.data('value'))
 }
});

/* We turn off the event */
selectize[0].selectize.off('change');

selectize[0].selectize._events.change = [];
selectize[0].selectize._events.change[0] = function(value) {
  /* If is no value - we do nothing */
  if (!value.length) {
    return;
  }
  
  /* Keep the value for `blur` */
  selectize.data('value', value)
  
  /* Else call previously saved functions */
  if (Array.isArray(originalOnChange)) {
    originalOnChange[0] && originalOnChange[0].call(this);
    originalOnChange[1] && originalOnChange[1]();
  }
}

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.13.3/css/selectize.default.min.css">
<select id="select-beast" multiple placeholder="Select a beast...">
  <option value="lion">Lion</option>
  <option value="tiger">Tiger</option>
  <option value="bear">Bear</option>
  <option value="elephant">Elephant</option>
  <option value="rhino">Rhino</option>
</select>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/selectize.js/0.13.3/js/standalone/selectize.min.js"></script>