Difference between setTimeout with and without quotes and parentheses

Question

I am learning JavaScript and I have learned recently about JavaScript timing events. When I learned about setTimeout at W3Schools, I noticed a strange figure wh

Answer 1

With the parentheses:

setTimeout("alertMsg()", 3000); // It work, here it treat as a function

Without the quotes and the parentheses:

setTimeout(alertMsg, 3000); // It also work, here it treat as a function

And the third is only using quotes:

setTimeout("alertMsg", 3000); // It not work, here it treat as a string

function alertMsg1() {
        alert("message 1");
    }
    function alertMsg2() {
        alert("message 2");
    }
    function alertMsg3() {
        alert("message 3");
    }
    function alertMsg4() {
        alert("message 4");
    }

    // this work after 2 second
    setTimeout(alertMsg1, 2000);

    // This work immediately
    setTimeout(alertMsg2(), 4000);

    // this fail
    setTimeout('alertMsg3', 6000);

    // this work after 8second
    setTimeout('alertMsg4()', 8000);

In the above example first alertMsg2() function call immediately (we give the time out 4S but it don't bother) after that alertMsg1() (A time wait of 2 Second) then alertMsg4() (A time wait of 8 Second) but the alertMsg3() is not working because we place it within the quotes without parties so it is treated as a string.