jQuery查找dom的几种方法效率详解

console.time('Scott');
console.log('seven');
console.timeEnd('Scott');
seven
Scott: 0.256ms

1.$(‘.parent .child');
2.$(‘.parent').find(‘.child');
3.$(‘.child','.parent');

jQuery = function( selector, context ) {
 // The jQuery object is actually just the init constructor 'enhanced'
 // Need init if jQuery is called (just allow error to be thrown if not included)
 return new jQuery.fn.init( selector, context );
}

init = jQuery.fn.init = function( selector, context ) {
 var match, elem;

 // HANDLE: $(""), $(null), $(undefined), $(false)
 if ( !selector ) {
 return this;
 }

 // Handle HTML strings
 if ( typeof selector === "string" ) {
 if ( selector.charAt(0) === "<" && selector.charAt( selector.length - 1 ) === ">" && selector.length >= 3 ) {
 // Assume that strings that start and end with <> are HTML and skip the regex check
 match = [ null, selector, null ];

 } else {
 match = rquickExpr.exec( selector );
 }

 // Match html or make sure no context is specified for #id
 if ( match && (match[1] || !context) ) {

 // HANDLE: $(html) -> $(array)
 if ( match[1] ) {
 context = context instanceof jQuery ? context[0] : context;

 // scripts is true for back-compat
 // Intentionally let the error be thrown if parseHTML is not present
 jQuery.merge( this, jQuery.parseHTML(
 match[1],
 context && context.nodeType ? context.ownerDocument || context : document,
 true
 ) );

 // HANDLE: $(html, props)
 if ( rsingleTag.test( match[1] ) && jQuery.isPlainObject( context ) ) {
 for ( match in context ) {
 // Properties of context are called as methods if possible
 if ( jQuery.isFunction( this[ match ] ) ) {
 this[ match ]( context[ match ] );

 // ...and otherwise set as attributes
 } else {
 this.attr( match, context[ match ] );
 }
 }
 }

 return this;

 // HANDLE: $(#id)
 } else {
 elem = document.getElementById( match[2] );

 // Check parentNode to catch when Blackberry 4.6 returns
 // nodes that are no longer in the document #6963
 if ( elem && elem.parentNode ) {
 // 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: $(expr, $(...))
 } else if ( !context || context.jquery ) {
 return ( context || rootjQuery ).find( selector );

 // HANDLE: $(expr, context)
 // (which is just equivalent to: $(context).find(expr)
 } else {
 return this.constructor( context ).find( selector );
 }

 // HANDLE: $(DOMElement)
 } else if ( selector.nodeType ) {
 this.context = this[0] = selector;
 this.length = 1;
 return this;

 // HANDLE: $(function)
 // Shortcut for document ready
 } else if ( jQuery.isFunction( selector ) ) {
 return typeof rootjQuery.ready !== "undefined" ?
 rootjQuery.ready( selector ) :
 // Execute immediately if ready is not present
 selector( jQuery );
 }

 if ( selector.selector !== undefined ) {
 this.selector = selector.selector;
 this.context = selector.context;
 }

 return jQuery.makeArray( selector, this );
 }

1.$(‘.parent .child');
3.$(‘.child','.parent');

find: function( selector ) {
 var i,
 ret = [],
 self = this,
 len = self.length;

 if ( typeof selector !== "string" ) {
 return this.pushStack( jQuery( selector ).filter(function() {
 for ( i = 0; i < len; i++ ) {
  if ( jQuery.contains( self[ i ], this ) ) {
  return true;
  }
 }
 }) );
 }

 for ( i = 0; i < len; i++ ) {
 jQuery.find( selector, self[ i ], ret );
 }

 // Needed because $( selector, context ) becomes $( context ).find( selector )
 ret = this.pushStack( len > 1 ? jQuery.unique( ret ) : ret );
 ret.selector = this.selector ? this.selector + " " + selector : selector;
 return ret;
 }