test4.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>test4</title>
  </head>

  <body>
    <script src="./js/observer.js"></script>
    <script src="./js/watcher.js"></script>
    <script src="./js/compile.js"></script>
    <script>
      /* eslint-disable no-new */
      /* eslint-disable no-undef */
      function mvue(options) {
        this.$options = options
        this.$data = options.data
        this.$el = document.querySelector(options.el)
        this.init()
      }
      mvue.prototype.init = function() {
        observer(this.$data)
        new Compile(this)
      }
    </script>
    <div id="app">{{ word }}</div>
    <script>
      const vm = new mvue({
        el: '#app',
        data: {
          word: 'hello world, hello everyone!'
        }
      })
    </script>
  </body>
</html>