


jQuery source code analysis - detailed explanation of looping techniques in jQuery_jquery
There are many techniques worth learning from the source code of jQuery. This article collects various traversal techniques and scenarios that appear in jQuery. The specific analysis is as follows:
// 简单的for-in(事件) for ( type in events ) { } // 缓存length属性,避免每次都去查找length属性,稍微提升遍历速度 // 但是如果遍历HTMLCollection时,性能提升非常明显,因为每次访问HTMLCollection的属性,HTMLCollection都会内部匹配一次所有的节点 for ( var j = 0, l = handlers.length; j < l; j++ ) { } // 不比较下标,直接判断元素是否为true(强制类型转换) var elem; for ( var i = 0; elems[i]; i++ ) { elem = elems[i]; // ... } // 遍历动态数组(事件),不能缓存length属性,j++之前先执行j--,保证不会因为数组下标的错误导致某些数组元素遍历不到 for ( j = 0; j < eventType.length; j++ ) { eventType.splice( j--, 1 ); } for ( var i = 1; i < results.length; i++ ) { if ( results[i] === results[ i - 1 ] ) { results.splice( i--, 1 ); } } // 迭代过程中尽可能减少遍历次数(事件),如果你能知道从哪里开始遍历的话,这里是pos for ( j = pos || 0; j < eventType.length; j++ ) { } //倒序遍历(事件),减少了几个字符:循环条件判断,合并i自减和i取值,倒序遍历会有浏览器优化,稍微提升遍历速度 for ( var i = this.props.length, prop; i; ) { prop = this.props[ --i ]; event[ prop ] = originalEvent[ prop ]; } // 倒序遍历,中规中矩,倒序会有浏览器优化,稍微提升遍历速度 for ( j = tbody.length - 1; j >= 0 ; --j ) { if ( jQuery.nodeName( tbody[ j ], "tbody" ) && !tbody[ j ].childNodes.length ) { tbody[ j ].parentNode.removeChild( tbody[ j ] ); } } //不判断下标,直接判断元素(选择器) for ( i = 0; checkSet[i] != null; i++ ) { if ( checkSet[i] && (checkSet[i] === true || checkSet[i].nodeType === 1 && Sizzle.contains(context, checkSet[i])) ) { results.push( set[i] ); } } for ( ; array[i]; i++ ) { ret.push( array[i] ); } // 不判断下标,取出元素然后判断元素(选择器) for ( var i = 0; (item = curLoop[i]) != null; i++ ) { } // 遍历DOM子元素 for ( node = parent.firstChild; node; node = node.nextSibling ) { if ( node.nodeType === 1 ) { node.nodeIndex = ++count; } } // 动态遍历DOM子元素(DOM遍历),dir参数表示元素的方向属性,如parentNode、nextSibling、previousSibling、lastChild和firstChild for ( ; cur; cur = cur[dir] ) { if ( cur.nodeType === 1 && ++num === result ) { break; } } // while检查下标i var i = promiseMethods.length; while( i-- ) { obj[ promiseMethods[i] ] = deferred[ promiseMethods[i] ]; } // while检查元素 while( (type = types[ i++ ]) ) { } // while遍历动态数组(AJAX),总是获取第一个元素,检查是否与特殊值相等,如果相等就从数组头部移除,直到遇到不相等的元素或数组为空 while( dataTypes[ 0 ] === "*" ) { dataTypes.shift(); if ( ct === undefined ) { ct = s.mimeType || jqXHR.getResponseHeader( "content-type" ); } } // while遍历动态数组(异步队列),总是获取第一个元素,直到数组为空,或遇到值为undefined的元素 while( callbacks[ 0 ] ) { callbacks.shift().apply( context, args ); } // while反复调用RegExp.exec(AJAX),能够否反复调是exec比re.test、String.match更加强大的原因,每次调用都将lastIndex属性设置到紧接着匹配字符串的字符位置 while( ( match = rheaders.exec( responseHeadersString ) ) ) { responseHeaders[ match[1].toLowerCase() ] = match[ 2 ]; // 将响应头以key-value的方式存在responseHeaders中 }
I hope this article will be helpful to everyone’s jQuery WEB programming design.

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