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in javascript, when () is attached or not attached

About the cases where () is attached or not in Javascript

When you have onclick attribute in html and in event property on script side
When I wrote oclick, I would like to know why ();

Applicable source code 
<! DOCTYPE html>
<html>
  <head>
    <title>test</title>
  </head>
  <body>
    <p id = "ab">test</p>
    <script type = "text/javascript">
    function a () {
      console.log ("Hellow");
    }
    let p = document.getElementById ("ab");
    p.onclick = a;
    </script>
  </body>
</html>
<! DOCTYPE html>
<html>
  <head>
    <title>test</title>
  </head>
  <body>
    <p onclick = "a ();">test</p>
    <script type = "text/javascript">
    function a () {
      console.log ("Hellow");
    }
    </script>
  </body>
</html>

`` `Enter the language name here
javascript

If i add ();in the event properties, it will fire without permission.

Supplemental information (FW/tool version etc.)

chrome latest version

javascript
  • Answer # 1

    p.onclick = a ();
    In this process, the return value obtained by executing the functionais assigned top.onclick
    It isp.onclick = abecause you must assign the function you want to call when you click.

    test


    Sinceonclick = "// JavaScript is described"in HTML description,a ();

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