jQuery's initialization requires two steps: first the library creates the fn
alias for its prototype
object, then it
performs the necessary steps to handle CSS-like selector chain. Here's the code:
jQuery.fn = jQuery.prototype = { init: function( selector, context ) { var match, elem, ret, doc; // Handle $(""), $(null), or $(undefined) if ( !selector ) { return this; } // Handle $(DOMElement) if ( selector.nodeType ) { this.context = this[0] = selector; this.length = 1; return this; } // The body element only exists once, optimize finding it if ( selector === "body" && !context ) { this.context = document; this[0] = document.body; this.selector = "body"; this.length = 1; return this; } // Handle HTML strings if ( typeof selector === "string" ) { // Are we dealing with HTML string or an ID? match = quickExpr.exec( selector ); // Verify a match, and that no context was specified for #id if ( match && (match[1] || !context) ) { // HANDLE: $(html) -> $(array) if ( match[1] ) { doc = (context ? context.ownerDocument || context : document); // If a single string is passed in and it's a single tag // just do a createElement and skip the rest ret = rsingleTag.exec( selector ); if ( ret ) { if ( jQuery.isPlainObject( context ) ) { selector = [ document.createElement( ret[1] ) ]; jQuery.fn.attr.call( selector, context, true ); } else { selector = [ doc.createElement( ret[1] ) ]; } } else { ret = buildFragment( [ match[1] ], [ doc ] ); selector = (ret.cacheable ? ret.fragment.cloneNode(true) : ret.fragment).childNodes; } return jQuery.merge( this, selector ); // HANDLE: $("#id") } else { elem = document.getElementById( match[2] ); if ( elem ) { // Handle the case where IE and Opera return items // by name instead of ID if ( elem.id !== match[2] ) { return rootjQuery.find( selector ); } // Otherwise, we inject the element directly into the jQuery object this.length = 1; this[0] = elem; } this.context = document; this.selector = selector; return this; } // HANDLE: $("TAG") } else if ( !context && /^\w+$/.test( selector ) ) { this.selector = selector; this.context = document; selector = document.getElementsByTagName( selector ); return jQuery.merge( this, selector ); // HANDLE: $(expr, $(...)) } else if ( !context || context.jquery ) { return (context || rootjQuery).find( selector ); // HANDLE: $(expr, context) // (which is just equivalent to: $(context).find(expr) } else { return jQuery( context ).find( selector ); } // HANDLE: $(function) // Shortcut for document ready } else if ( jQuery.isFunction( selector ) ) { return rootjQuery.ready( selector ); } if (selector.selector !== undefined) { this.selector = selector.selector; this.context = selector.context; } return jQuery.makeArray( selector, this ); },
The private init()
method actually loads the core of the jQuery library, namely the selector chain syntax. Here are its progressive steps:
- a reference to a
null
orundefined
value is handled by binding the current selection tothis
, that is, to jQuery itself - the syntax
$(selector)[0]
allows you to use W3C DOM core methods on a selector chain - a reference to the
body
element is optimized for a better performance - by using its library constants, jQuery handles the following combinations:
- HTML strings
- a reference to an ID selector (
$('#id')
) - a reference to one or more generic simple selectors or descendant selectors
- a reference to a complex expression
- the shortcut
$(function)
which is an alias for$(document).ready()