Question

This is the script that i tested when the page loads.

window.onload = initAll;

function initAll() {
document.getElementById("pTag").innerHTML = "Hello Java Script !";
}

The above script works fine till i put parenthesis like initAll() during the call. _( window.onload=initAll(); )_ . Nothing happens if use initAll() during function call. It is like the script wasn’t there.

Why is it so ?

Answer 1

window.onload expects the value you set for it to be a function (functions are like any other object in JavaScript).

If you put parens after initAll, you actually invoke the function and set window.onload to the function’s return value. Since the function does not return anything explicitly, this value is undefined. So in practice you can think of this code:

window.onload = initAll();

function initAll() {
    // do something with document
}

as equivalent to this code:

var result = initAll();
window.onload = result;

function initAll() {
    return null;
}

I ‘m sure you can immediately see why this would not work.