CODEWITHSUNDEEP
javascript
Navneet Saini
Navneet Saini

Reputation: 944

JavaScript! window.onload = someFunction and window.onload = someFunction()

Is there a difference between:

  1. window.onload = someFunction;

  2. window.onload = someFunction();

The parentheses at the end. Do they make any difference?

We generally use the first one! What if we Have to pass some parameter to the function. How will we do it by using the first statement?

Upvotes: 3

Views: 15633

Answers (5)

phtrivier
phtrivier

Reputation: 13361

As explained otherwise, the first form 

window.onload = someFunction

Simply set the "onload" variable to be equals to the "someFunction" function ; when the page finishes loading, this function is called.

The other form :

window.onload = someFunction()

Sets the "onload" variable to be the result of calling someFunction. Unless "someFunction" itself returns a function, this is probably not what you want to do.

By default, the onload function is called with a single "event" argument. If you want to pass arguments, you might be able to do something like this : 

window.onload = function (event) {
  someFunction(someArg, someOtherArg)
}

Upvotes: 7

MMM
MMM

Reputation: 7310

Your second statement assigns the result of someFunction() to window.onload.

If you want to add a parameter, you can do the following:

window.onload = function () {
    someFunction(parameter);
};

Upvotes: 4

Quentin
Quentin

Reputation: 943515

window.onload = someFunction; assigns a function to onload.

window.onload = someFunction(); calls a function and assigns its return value to onload. (This is not desirable unless the return value of that function is another function).

What if we Have to pass some parameter to the function

Usually you define a new function which does nothing except call the original function with some arguments, then you assign the new function to the event handler.

Upvotes: 3

Jeremy Blalock
Jeremy Blalock

Reputation: 2619

Yes. If you put window.onload = someFunction() it is expected that the result of someFunction() is another function. This could be used for wrapping a function, or passing parameters. For instance: 

window.onload = myFunction(arg1, arg2)

myFunction(arg1, arg2) would be expected to return some function incorporating these two variables.

Upvotes: 0

Jakub Kotrs
Jakub Kotrs

Reputation: 6229

if you use

window.onload = someFunction;

a function with the name of someFunction is called on window.onload

if you use

window.onload = someFunction();

someFunction() runs and the result is assigned to window.onload

Upvotes: 1

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