window.onload = someFunctionName与window.onload = someFunctionName（）

Question

So the question is what is the difference between

window.onload = someFunctionName vs. window.onload = someFunctionName() and should I use this parentheses?

Answer 1

Here is an excellent specification: https://developer.mozilla.org/en-US/docs/Web/API/GlobalEventHandlers.onload

There are several ways to set it: 1. Javascript, which requires setting it to a function. 2. Setting it via an HTML tag as a string value.

Examples of both here and are valid ways. I would recommend Javascript directly passing a function reference: http://www.w3schools.com/jsref/event_onload.asp

Answer 2

window.onload = someFunctionName(); will call the someFunctionName function and assign its return value to the onload property.

window.onload = someFunctionName; assigns the someFunctionName function to the property onload . The browser will call that function when the load event occurs. Functions are objects. When you refer to them by their name without calling them, you're referring to the object.

Example:

window.onload = test();

function test() {
    alert("outer");
    return function(){
    alert("inner");
    }
}

this will alert twice as test() is called it will alert outer and return innerfunction to load and onload it will alert inner.

while

window.onload = test;

function test() {
    alert("outer");
    return function(){
    alert("inner");
    }
}

above code will only alert outer because innerfunction returned but never get called.

Here is fiddle

Answer 3

window.onload = someFunction; - assigns someFunction to onload event

window.onload = someFunction(); - assigns a RESULT of someFunction to onload event.

Answer 4

You need to be aware that in Javascript a function can return another function after execution . For this reason, because you are not showing any code, both examples might be correct.

But there are differences between the two:

• window.onload = someFunctionName assigns the someFunctionName itself as the onload handler;

• window.onload = someFunctionName() does not assign the function someFunctionName itself but the returning result of the execution of the someFunctionName function. Assuming someFunctionName itself returns a function then it might be correct, otherwise you will get unexpected results.

Javascript functions are objects. When you use parentheses you are invoking the function. When you use the function without parentheses you are referring to the function object (ie the function itself).

So it depends on what you are doing...