How exactly is JS triggering the button's onclick function even before its been clicked during page load?

P.S:I come from PYTHON background

var btn = document.getElementById("btn")

// this works fine (alert box appears when clicked)

btn.onclick = function(){
    alert('clicked')
}

// this shows alert box when the page loads when clearly onclick is mentioned

function foo(){
    alert('clickedd')
}
btn.onclick = foo()

This is for the simple HTML button inside a form table.