Answers for "how to set button onclick function in javascript"

set onclick javascript

<button id="myBtn">Try it</button>
<p id="demo"></p>

<script>
document.getElementById("myBtn").onclick=function(){displayDate()};

function displayDate()
{
    document.getElementById("demo").innerHTML=Date();
}
</script>

javascript change button onclick

function Bar() {
  document.getElementById("a").onclick = Foo();
}