Question

I’m reading Bibeault’s jQuery in Action and I’m having troublel understanding the value parameter of the attr() method. The book says that the parameter can be a function whose parameters are index and previousValue. What is the purpose of these parameters? I don’t understand the text’s explanation.

Specifically I want to know:

1. Are these parameters mandatory?
2. What is an example of how these parameters are used?
3. Can I use other parameters within the function?

Answer 1

1) No parameters are mandatory in javascript. You use whatever amount you want. These parameters are available for you in your function.

2) examples:

Let’s say you have this html:

<a href="#" title="Google"></a>
<a href="#" title="Yahoo"></a>
<a href="#" title="Bing"></a>

Now, run this snippet:

$('a').attr('title', function(index, previousValue){
    return previousValue + ' - An external link';
});

This will add the string ” – An external link” to the end of every title.

Now, look at this:

$('a').attr('title', function(index, previousValue){
    return previousValue + ' - Link number ' + index;
});

This will result in the following html:

<a href="#" title="Google - Link number 0"></a>
<a href="#" title="Yahoo - Link number 1"></a>
<a href="#" title="Bing - Link number 2"></a>

As you can see, you can see, these parameters are very handy.

3) Not sure what you mean by using other parameters. Please clarify.

---

It seems that you are not familiar with Javascript’s scope lookup chain. You do not have to explicitly pass parameters to a function. If they’re defined in a scope above it, the function will have access to it:

var num1 = 23;
var num2 = 54;

$('a').attr('title', function(){
    return num1 + num2;
});