The syntax that I don't understand is this:
$("#profilePhotoFileUpload")[0]
I have been seeing this syntax pretty frequently and I've ignored it for a while because I never had to use it. But now, in order to understand the code from this post How do I upload an image using the Parse.com javascriptSDK?, I cannot ignore it any longer.
I know that [0] this syntax is used to refer to an array usually. But it seems a little weird that a reference to an id would generate an array of some sort.
By appending [0] to the jQuery object will return the first DOM element.
As you're using id selector here, there will be only one element in the array so using [0] makes sense. If you are selecting multiple elements you can also use any number which is between 0 and number of elements you can get corresponding DOM element.
From jQuery Docs
A jQuery object contains a collection of Document Object Model (DOM) elements that have been created from an HTML string or selected from a document. Since jQuery methods often use CSS selectors to match elements from a document, the set of elements in a jQuery object is often called a set of "matched elements" or "selected elements".
The jQuery object itself behaves much like an array; it has a
lengthproperty and the elements in the object can be accessed by their numeric indices[0]to[length-1]. Note that a jQuery object is not actually a Javascript Array object, so it does not have all the methods of a true Array object such asjoin().
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