Ch15 Views.js File Point 12

[zibrnp 0]

Hello,

I'm going over the "view.js" file on page 585 and in point 12 we have:

"bidAjax.onreadystatechange = handleBidAjaxResponse;"
why "()" is missing in the function call?
Why it is not like this:
"bidAjax.onreadystatechange = handleBidAjaxResponse();"

Thanks

[HartleySan 826]

Because it's not a function call, it's a function reference.

Basically, you assign a function reference to the event handler so that when the event occurs (i.e., not now), the system knows which function to call.

 

If you assigned a function call to the event handler, then the function would be immediately executed, which would likely lead to an unintended result.

 

That make sense?

[zibrnp 0]

Thanks for the answer…
Yes it does, It's described on page 245 of the book in "Functions as Variables Values" paragraph.

Basically in this line "bidAjax.onreadystatechange = handleBidAjaxResponse;" we are assigning value of function "handleBidAjaxResponse" to the property "onreadystatechange" of the object bidAjax.

 

Am I right?

Thanks

[HartleySan 826]

Yes, that's right.

 

Furthermore, properties that have functions assigned to them are often called "methods", but that's purely a semantical differentiation from properties.

As a result though, assigning a function (not a function call) to onreadystatechange essentially makes onreadystatechange a method of the bidAjax object, and that method is called whenever the readystatechange event occurs for the bidAjax object.