Question

jQuery passes a version of itself to global scope with this statement:

window.jQuery = window.$ = jQuery;

as shown in this SO Post.

Looking at the jQuery source we can see the definition is in top scope for jQuery and defined like this:

// Define a local copy of jQuery
jQuery = function( selector, context ) {
    // The jQuery object is actually just the init constructor 'enhanced'
    return new jQuery.fn.init( selector, context, rootjQuery );
},

The selector seems pretty straight forward to use: Just pass in a string to select an element or elements from the DOM.

However, how do you use the context parameter correctly?

Answer 1

From the jQuery manual:

jQuery()

[…]

Selector Context

By default, selectors perform their searches within
the DOM starting at the document root. However, an alternate context
can be given for the search by using the optional second parameter to
the $() function. For example, to do a search within an event handler,
the search can be restricted like so:

$('div.foo').click(function() { $('span', this).addClass('bar'); });

When the search for the span selector is restricted to the context of
this, only spans within the clicked element will get the additional
class.

Internally, selector context is implemented with the .find() method,
so $('span', this) is equivalent to $(this).find('span').

Note that jQuery looks at the presence and type of its parameters and acts accordingly.