/*
	Implementation Summary

1. Enforce API surface and semantic compatibility with 1.9.x branch
	2. Improve the module's maintainability by reducing the storage
		paths to a single mechanism.
	3. Use the same single mechanism to support "private" and "user" data.
	4. _Never_ expose "private" data to user code (TODO: Drop _data, _removeData)
	5. Avoid exposing implementation details on user objects (eg. expando properties)
	6. Provide a clear path for implementation upgrade to WeakMap in 2014
*/
var data_user, data_priv,
	rbrace = /(?:\{[\s\S]*\}|\[[\s\S]*\])$/,
	rmultiDash = /([A-Z])/g;

function Data() {
	this.cache = {};
	this.expando = jQuery.expando + Math.random();
}

Data.uid = 1;

Data.prototype = {
	key: function( owner ) {
		var descriptor = {},
			// Check if the owner object already has a cache key
			unlock = owner[ this.expando ];

// If not, create one
		if ( !unlock ) {
			unlock = Data.uid++;

// Secure it in a non-enumerable, non-writable property
			try {
				descriptor[ this.expando ] = { value: unlock };
				Object.defineProperties( owner, descriptor );

// Support: Android<4
			// Fallback to a less secure definition
			} catch ( e ) {
				descriptor[ this.expando ] = unlock;
				jQuery.extend( owner, descriptor );
			}
		}

// Ensure the cache object
		if ( !this.cache[ unlock ] ) {
			this.cache[ unlock ] = {};
		}

return unlock;
	},
	set: function( owner, data, value ) {
		var prop,
			// There may be an unlock assigned to this node,
			// if there is no entry for this "owner", create one inline
			// and set the unlock as though an owner entry had always existed
			unlock = this.key( owner ),
			cache = this.cache[ unlock ];

// Handle: [ owner, key, value ] args
		if ( typeof data === "string" ) {
			cache[ data ] = value;

// Handle: [ owner, { properties } ] args
		} else {
			// Support an expectation from the old data system where plain
			// objects used to initialize would be set to the cache by
			// reference, instead of having properties and values copied.
			// Note, this will kill the connection between
			// "this.cache[ unlock ]" and "cache"
			if ( jQuery.isEmptyObject( cache ) ) {
				this.cache[ unlock ] = data;
			// Otherwise, copy the properties one-by-one to the cache object
			} else {
				for ( prop in data ) {
					cache[ prop ] = data[ prop ];
				}
			}
		}

return this;
	},
	get: function( owner, key ) {
		// Either a valid cache is found, or will be created.
		// New caches will be created and the unlock returned,
		// allowing direct access to the newly created
		// empty data object. A valid owner object must be provided.
		var cache = this.cache[ this.key( owner ) ];

return key === undefined ?
			cache : cache[ key ];
	},
	access: function( owner, key, value ) {
		// In cases where either:
		//
		//   1. No key was specified
		//   2. A string key was specified, but no value provided
		//
		// Take the "read" path and allow the get method to determine
		// which value to return, respectively either:
		//
		//   1. The entire cache object
		//   2. The data stored at the key
		//
		if ( key === undefined ||
				((key && typeof key === "string") && value === undefined) ) {
			return this.get( owner, key );
		}

// [*]When the key is not a string, or both a key and value
		// are specified, set or extend (existing objects) with either:
		//
		//   1. An object of properties
		//   2. A key and value
		//
		this.set( owner, key, value );

// Since the "set" path can have two possible entry points
		// return the expected data based on which path was taken[*]
		return value !== undefined ? value : key;
	},
	remove: function( owner, key ) {
		var i, name,
				unlock = this.key( owner ),
				cache = this.cache[ unlock ];

if ( key === undefined ) {
			this.cache[ unlock ] = {};
		} else {
			// Support array or space separated string of keys
			if ( jQuery.isArray( key ) ) {
				// If "name" is an array of keys...
				// When data is initially created, via ("key", "val") signature,
				// keys will be converted to camelCase.
				// Since there is no way to tell _how_ a key was added, remove
				// both plain key and camelCase key. #12786
				// This will only penalize the array argument path.
				name = key.concat( key.map( jQuery.camelCase ) );
			} else {
				// Try the string as a key before any manipulation
				if ( key in cache ) {
					name = [ key ];
				} else {
					// If a key with the spaces exists, use it.
					// Otherwise, create an array by matching non-whitespace
					name = jQuery.camelCase( key );
					name = name in cache ?
						[ name ] : ( name.match( core_rnotwhite ) || [] );
				}
			}

i = name.length;
			while ( i-- ) {
				delete cache[ name[i] ];
			}
		}
	},
	hasData: function( owner ) {
		return !jQuery.isEmptyObject(
			this.cache[ this.key( owner ) ]
		);
	},
	discard: function( owner ) {
		delete this.cache[ this.key( owner ) ];
	}
};

// This will be used by remove()/cleanData() in manipulation to sever
// remaining references to node objects. One day we'll replace the dual
// arrays with a WeakMap and this won't be an issue.
// (Splices the data objects out of the internal cache arrays)
function data_discard( owner ) {
	data_user.discard( owner );
	data_priv.discard( owner );
}

// These may be used throughout the jQuery core codebase
data_user = new Data();
data_priv = new Data();

jQuery.extend({
	// This is no longer relevant to jQuery core, but must remain
	// supported for the sake of jQuery 1.9.x API surface compatibility.
	acceptData: function() {
		return true;
	},

hasData: function( elem ) {
		return data_user.hasData( elem ) || data_priv.hasData( elem );
	},

data: function( elem, name, data ) {
		return data_user.access( elem, name, data );
	},

removeData: function( elem, name ) {
		return data_user.remove( elem, name );
	},

// TODO: Replace all calls to _data and _removeData with direct
	// calls to
	//
	// data_priv.access( elem, name, data );
	//
	// data_priv.remove( elem, name );
	//
	_data: function( elem, name, data ) {
		return data_priv.access( elem, name, data );
	},

_removeData: function( elem, name ) {
		return data_priv.remove( elem, name );
	}
});

jQuery.fn.extend({
	data: function( key, value ) {
		var attrs, name,
			elem = this[0],
			i = 0,
			data = null;

// Gets all values
		if ( key === undefined ) {
			if ( this.length ) {
				data = data_user.get( elem );

if ( elem.nodeType === 1 && !data_priv.get( elem, "hasDataAttrs" ) ) {
					attrs = elem.attributes;
					for ( ; i < attrs.length; i++ ) {
						name = attrs[i].name;

if ( name.indexOf( "data-" ) === 0 ) {
							name = jQuery.camelCase( name.substring(5) );
							dataAttr( elem, name, data[ name ] );
						}
					}
					data_priv.set( elem, "hasDataAttrs", true );
				}
			}

return data;
		}

// Sets multiple values
		if ( typeof key === "object" ) {
			return this.each(function() {
				data_user.set( this, key );
			});
		}

return jQuery.access( this, function( value ) {
			var data,
					camelKey = jQuery.camelCase( key );

// The calling jQuery object (element matches) is not empty
			// (and therefore has an element appears at this[0]) and the
			// `value` parameter was not undefined. An empty jQuery object
			// will result in `undefined` for elem = this[0] which will
			// throw an exception if an attempt to read a data cache is made.
			if ( elem && value === undefined ) {
				// Attempt to get data from the cache
				// with the key as-is
				data = data_user.get( elem, key );
				if ( data !== undefined ) {
					return data;
				}

// Attempt to get data from the cache
				// with the key camelized
				data = data_user.get( elem, camelKey );
				if ( data !== undefined ) {
					return data;
				}

// Attempt to "discover" the data in
				// HTML5 custom data-* attrs
				data = dataAttr( elem, camelKey, undefined );
				if ( data !== undefined ) {
					return data;
				}

// We tried really hard, but the data doesn't exist.
				return undefined;
			}

// Set the data...
			this.each(function() {
				// First, attempt to store a copy or reference of any
				// data that might've been store with a camelCased key.
				var data = data_user.get( this, camelKey );

// For HTML5 data-* attribute interop, we have to
				// store property names with dashes in a camelCase form.
				// This might not apply to all properties...*
				data_user.set( this, camelKey, value );

// *... In the case of properties that might ACTUALLY
				// have dashes, we need to also store a copy of that
				// unchanged property.
				if ( /-/.test( key ) && data !== undefined ) {
					data_user.set( this, key, value );
				}
			});
		}, null, value, arguments.length > 1, null, true );
	},

removeData: function( key ) {
		return this.each(function() {
			data_user.remove( this, key );
		});
	}
});

function dataAttr( elem, key, data ) {
	var name;

// If nothing was found internally, try to fetch any
	// data from the HTML5 data-* attribute
	if ( data === undefined && elem.nodeType === 1 ) {

name = "data-" + key.replace( rmultiDash, "-$1" ).toLowerCase();
		data = elem.getAttribute( name );

if ( typeof data === "string" ) {
			try {
				data = data === "true" ? true :
				data === "false" ? false :
				data === "null" ? null :
				// Only convert to a number if it doesn't change the string
				+data + "" === data ? +data :
				rbrace.test( data ) ?
					JSON.parse( data ) : data;
			} catch( e ) {}

// Make sure we set the data so it isn't changed later
			data_user.set( elem, key, data );
		} else {
			data = undefined;
		}
	}
	return data;
}

src/data.js

Maintainability

Lines of code

Difficulty

Estimated Errors

Function weight

By Complexity

By SLOC