1. when a method has no arguments, the assignment can be done directly with the onclick = method name

  window.onload = function() {
        $('btnTest').onclick = test;  
  }
  function test() {
        alert(val);
  }

2. when the method has parameters, the error occurs when using onclick = method name (parameter), so we need to add function() before the method name.

  window.onload = function() {
        $('btnTest').onclick = function() { test(1) };  
  }
  function test(val) {
        alert(val);
  }