javascript onclick happening on page load

Question

I'm trying to make a simple Javascript function to open a window with a bigger image when an image is clicked on. What happens is this test alert pops up on the page load and then does nothing when i click on the image. Here is the code

function zoom()
{
alert("test!");
}
document.getElementById("us").onclick=zoom();

Answer 1

Try this

function zoom()
{
    alert("test!");
}
document.getElementById("us").onclick=zoom;

Answer 2

function zoom()
{
    alert("test!");
}
document.getElementById("us").onclick=function(){zoom(variable1)};

If you need to pass a variable.

Answer 3

for es6 onclick = () => zoom(i) works