JSFiddle - React, Tailwind, and code Playground

Replace

by hajime_nagahata

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
全角スペースを「□」に置き換え<br>
<input type="text" id="test">

JavaScript

var prev_text = "";
$('#test').on('keyup', function(){

  var start = this.selectionStart;
  var end = this.selectionEnd;
  
  if($(this).val().match(/　/)) {
    $(this).val($(this).val().replace(/　/, '□'));
    prev_text = $(this).val();
    this.setSelectionRange(start, end);
  }
})