How to invoke javascript functions?
Javascript functions can be written in a javascript file or directly in an HTML document.
In both cases, when they are defined, they need to be called in order to be executed.



Automatically Callback Functions in JavaScript


 <html>
<body>

 <p id="demo"></p>
 
<script type="text/javascript">
function addText() {
 let a=document.getElementById("demo");
 a.innerText="The function executed";
}
 addText();
</script>
</body>
</html>  
  

result:







 <html>
<head>
<style>
button {width:100px;
        height:55px;
        color:red;}
</style>

</head>
<body>
  <button  onclick="message();">PRESS</button>
<script type="text/javascript">
function message()
{
alert("This is simple function called message()");
}
</script>
</body>
</html>  
  

result:







<script>

</script>
 
result: