Rails%203%20In%20Action

238 CHAPTER 9 File uploading
Open app/assets/javascripts/tickets.js.coffee, and we’ll build up your function line
by line so you can understand what you’re doing. Let’s begin by putting this line first:
$(->
It seems like a random amalgamation of characters, but this line is really helpful. It
calls the jQuery $ 9 function and passes it a function as an argument. This line runs the
function only when the page has fully loaded. 10 You need this because the JavaScript
otherwise would be executed before the link you’re going to be referencing is loaded.
Let’s add a second line to this:
$(->
$('a#add_another_file').click(->
This line uses jQuery’s $ function to select an element on the page called a, which has
an id attribute of add_another_file that will soon be your Add Another File link. This
would happen only after the page is ready. After this, you call the click function on it
and pass it a function that runs when you click on this link. Let’s now add a third line:
$(->
$('a#add_another_file').click(->
url = "/files/new?number=" + $('#files input').length
The double-space indent here indicates to CoffeeScript that this code belongs inside
the function passed to click. 11 Here, you define a variable called url, which will be
the URL you use to request a new file field on your page. At the end of this URL you
specify the number parameter with some additional jQuery code. This code selects all
the input elements inside the element on the page with the id attribute of files
and stores them in an array. To find out how many elements are in that array, you
call length on it. The URL for the first time you click this link would now look something
like /files/new?number=1, indicating that you already have one file field on
your page.
Let’s make the fourth line now:
$(->
$('a#add_another_file').click(->
url = "/files/new?number=" + $('#files input').length
$.get(url,
This line is pretty simple; you call the jQuery function $, and then call the get 12 function
on it, which starts an asynchronous request to the specified URL that is the first
argument here, using the variable you set up on the previous line. Another line:
$(->
$('a#add_another_file').click(->
url = "/files/new?number=" + $('#files input').length
9 Aliased from the jQuery function: http://api.jquery.com/jquery/.
10 For the meaning of “loaded,” see this: http://api.jquery.com/ready
11 http://api.jquery.org/click.
12 http://api.jquery.com/jQuery.get.