代码很简单直接上源码

function winnow( elements, qualifier, not ) {
	//如果qualifier是函数
	if ( jQuery.isFunction( qualifier ) ) {
		//not为true时,返回qualifier返回false的elements
		//not为false时,返回qualifier返回true的elements
		return jQuery.grep( elements, function( elem, i ) {
			/* jshint -W018 */
			return !!qualifier.call( elem, i, elem ) !== not;
		});

	}
	//如果qualifier是DOM元素
	if ( qualifier.nodeType ) {
		//not为true时,返回elements中和qualifier不是同一元素的elements
		//not为false时,返回elements中和qualifier是同一元素的elements
		return jQuery.grep( elements, function( elem ) {
			return ( elem === qualifier ) !== not;
		});

	}
	//如果qualifier是字符串(将qualifier当成selector)
	if ( typeof qualifier === "string" ) {
		// 如果selector不是多级selector(多级selector:div #id)或多selector(selector, selector)
		// 直接调用jQuery.filter( qualifier, elements, not )返回结果
		// risSimple = /^.[^:#\[\.,]*$/
		if ( risSimple.test( qualifier ) ) {
			return jQuery.filter( qualifier, elements, not );
		}
		// 如果selector是多级selector(多级selector:div #id)或多selector(selector, selector)
		// 解释: 因为多级selector或selector无法直接加上:not组成伪类选择器
		//       所以先过滤,再遍历根据not参数过滤
		qualifier = jQuery.filter( qualifier, elements );
	}

	return jQuery.grep( elements, function( elem ) {
		return ( indexOf.call( qualifier, elem ) >= 0 ) !== not;
	});
}
/**
 * filter方法是对自身进行筛选
 * find方法是对子集进行查找或匹配
 */
jQuery.filter = function( expr, elems, not ) {
	var elem = elems[ 0 ];
	
	//如果有not参数,修正selector为not伪类表达式
	if ( not ) {
		expr = ":not(" + expr + ")";
	}

	return elems.length === 1 && elem.nodeType === 1 ?
		//只有一个元素看是否和表达式匹配
		jQuery.find.matchesSelector( elem, expr ) ? [ elem ] : [] :
		jQuery.find.matches( expr, jQuery.grep( elems, function( elem ) {
			return elem.nodeType === 1;
		}));
};

jQuery.fn.extend({
	find: function( selector ) {
		var i,
			len = this.length,
			ret = [],
			self = this;
		// example:	
		// var e = document.getElementById('a');
		// var $li = $('div').find(e);
		if ( typeof selector !== "string" ) {
			return this.pushStack( jQuery( selector ).filter(function() {
				for ( i = 0; i < len; i++ ) {
					// this指向jQuery( selector )迭代的当前元素
					// 如果调用.find(selector)的JQuery对象中的的某个元素包含selector,返回true
					if ( jQuery.contains( self[ i ], this ) ) {
						return true;
					}
				}
			}) );
		}
		
		//$('ul').find('li');
		for ( i = 0; i < len; i++ ) {
			jQuery.find( selector, self[ i ], ret );
		}

		// Needed because $( selector, context ) becomes $( context ).find( selector )
		// 删除重复元素
		ret = this.pushStack( len > 1 ? jQuery.unique( ret ) : ret );
		// 拼接调用者和selector,example,$(div).find('ul'),ret.selector为div ul
		ret.selector = this.selector ? this.selector + " " + selector : selector;
		return ret;
	},
	// 保留匹配selector的元素
	filter: function( selector ) {
		return this.pushStack( winnow(this, selector || [], false) );
	},
	// 删除匹配selector的元素
	not: function( selector ) {
		return this.pushStack( winnow(this, selector || [], true) );
	},
	// 如果调用的jquery元素有一个符合selector返回true
	is: function( selector ) {
		return !!winnow(
			this,

			// If this is a positional/relative selector, check membership in the returned set
			// so $("p:first").is("p:last") won't return true for a doc with two "p".
			typeof selector === "string" && rneedsContext.test( selector ) ?
				jQuery( selector ) :
				selector || [],
			false
		).length;
	}