Q:

Javascript function being executed even if its not called

Hi so I'm fairly new to javascript and right now, I'm experimenting in defining functions and hiding/showing specific elements.
in this simple program that I made, I am trying to show a loading screen while a function is still executing. the problem is, the function that is inside the onlick event handler executes even without clicking the button
<body>
<button id="thisbutton" type="button">clickme</button>
<div>
    <p> The download will begin in <span id="countdowntimer">10 </span> Seconds</p>
</div>
<div id="loadss" class="LockOn"></div>

<script>

    document.getElementById("thisbutton").onclick(meow());
    document.onload = rawr();
        function rawr() {
        document.getElementById("loadss").style.visibility = "hidden";
        document.getElementById("loadss").style.display = "none";
        }

        function meow() {
            document.getElementById("loadss").style.visibility = "visible";
            document.getElementById("loadss").style.display = "block";
            time();
            document.getElementById("loadss").style.visibility = "hidden";
            document.getElementById("loadss").style.display = "none";
        };
        function time() {
            var timeleft = 10;
            var downloadTimer = setInterval(function () {
                timeleft--;
                document.getElementById("countdowntimer").textContent = timeleft;
                if (timeleft <= 0)
                    clearInterval(downloadTimer);
            }, 1000);
        }
</script>

A:

In Javascript, functions are first-class citizens.  This means you can treat them like most variables:
function test () { console.log('Hi!'); }

var x = test;

And you can execute them by using parentheses on any reference to them:
test(); // will execute function test
x(); // will _also_ execute function test

Hence, your code:
document.getElementById("thisbutton").onclick(meow());
document.onload = rawr();

is executing the functions meow and rawr.  You perhaps want to pass references to these functions:
document.getElementById("thisbutton").onclick(meow);
document.onload = rawr;

