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before"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'foo'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n after"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'bar'")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])])]),s("h4",{attrs:{id:"_2-4-10-常量注释"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_2-4-10-常量注释"}},[t._v("#")]),t._v(" 2.4.10 常量注释")]),t._v(" "),s("h5",{attrs:{id:"【强制】-常量必须使用-const-标记，并包含说明和类型信息。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【强制】-常量必须使用-const-标记，并包含说明和类型信息。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【强制】")]),t._v(" 常量必须使用 "),s("code",[t._v("@const")]),t._v(" 标记，并包含说明和类型信息。")]),t._v(" "),s("p",[t._v("示例：")]),t._v(" "),s("div",{staticClass:"language-javascript extra-class"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n * 常量说明\n *\n * @const\n * @type {string}\n */")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token constant"}},[t._v("REQUEST_URL")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'myurl.do'")]),s("span",{pre:!0,attrs:{class:"token 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"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n host"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'1.2.3.5'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v("\n port"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v(":")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("8081")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])])]),s("h4",{attrs:{id:"_2-4-12-细节注释"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_2-4-12-细节注释"}},[t._v("#")]),t._v(" 2.4.12 细节注释")]),t._v(" "),s("p",[t._v("对于内部实现、不容易理解的逻辑说明、摘要信息等，我们可能需要编写细节注释。")]),t._v(" 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punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("i"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 错误的遍历方式")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("for")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("i "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("in")]),t._v(" arr"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n console"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token 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"),s("strong",[t._v("【建议】")]),t._v(" 清空数组使用 "),s("code",[t._v(".length = 0")]),t._v("。")]),t._v(" "),s("h3",{attrs:{id:"_3-8-函数"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_3-8-函数"}},[t._v("#")]),t._v(" 3.8 函数")]),t._v(" "),s("h4",{attrs:{id:"_3-8-1-函数长度"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_3-8-1-函数长度"}},[t._v("#")]),t._v(" 3.8.1 函数长度")]),t._v(" "),s("h5",{attrs:{id:"【建议】-一个函数的长度控制在-50-行以内。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-一个函数的长度控制在-50-行以内。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 一个函数的长度控制在 "),s("code",[t._v("50")]),t._v(" 行以内。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("将过多的逻辑单元混在一个大函数中，易导致难以维护。一个清晰易懂的函数应该完成单一的逻辑单元。复杂的操作应进一步抽取，通过函数的调用来体现流程。")]),t._v(" "),s("p",[t._v("特定算法等不可分割的逻辑允许例外。")]),t._v(" "),s("p",[t._v("示例：")]),t._v(" "),s("div",{staticClass:"language-javascript extra-class"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token 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punctuation"}},[t._v("}")]),t._v("\n")])])]),s("h4",{attrs:{id:"_3-8-2-参数设计"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_3-8-2-参数设计"}},[t._v("#")]),t._v(" 3.8.2 参数设计")]),t._v(" "),s("h5",{attrs:{id:"【建议】-一个函数的参数控制在-6-个以内。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-一个函数的参数控制在-6-个以内。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 一个函数的参数控制在 "),s("code",[t._v("6")]),t._v(" 个以内。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("除去不定长参数以外，函数具备不同逻辑意义的参数建议控制在 6 个以内，过多参数会导致维护难度增大。")]),t._v(" "),s("h5",{attrs:{id:"【建议】-通过-options-参数传递非数据输入型参数。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-通过-options-参数传递非数据输入型参数。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 通过 "),s("code",[t._v("options")]),t._v(" 参数传递非数据输入型参数。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("有些函数的参数并不是作为算法的输入，而是对算法的某些分支条件判断之用，此类参数建议通过一个 options 参数传递。")]),t._v(" "),s("p",[t._v("如下函数：")]),t._v(" "),s("div",{staticClass:"language-javascript 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function"}},[t._v("removeChild")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("element"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("removeEventListeners"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n element"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("clearEventListeners")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("可以转换为下面的签名：")]),t._v(" "),s("div",{staticClass:"language-javascript extra-class"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("/**\n * 移除某个元素\n *\n * @param {Node} element 需要移除的元素\n * @param {Object} options 相关的逻辑配置\n * @param {boolean} options.removeEventListeners 是否同时将所有注册在元素上的事件移除\n */")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("removeElement")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("element"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" options")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n element"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("parent"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("removeChild")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("element"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("options"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("removeEventListeners"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n element"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("clearEventListeners")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("这种模式有几个显著的优势：")]),t._v(" "),s("ul",[s("li",[t._v("boolean 型的配置项具备名称，从调用的代码上更易理解其表达的逻辑意义。")]),t._v(" "),s("li",[t._v("当配置项有增长时，无需无休止地增加参数个数，不会出现 removeElement(element, true, false, false, 3) 这样难以理解的调用代码。")]),t._v(" "),s("li",[t._v("当部分配置参数可选时，多个参数的形式非常难处理重载逻辑，而使用一个 options 对象只需判断属性是否存在，实现得以简化。")])]),t._v(" "),s("h4",{attrs:{id:"_3-8-3-闭包"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_3-8-3-闭包"}},[t._v("#")]),t._v(" 3.8.3 闭包")]),t._v(" "),s("h5",{attrs:{id:"【建议】-在适当的时候将闭包内大对象置为-null。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-在适当的时候将闭包内大对象置为-null。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 在适当的时候将闭包内大对象置为 "),s("code",[t._v("null")]),t._v("。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("在 JavaScript 中，无需特别的关键词就可以使用闭包，一个函数可以任意访问在其定义的作用域外的变量。需要注意的是，函数的作用域是静态的，即在定义时决定，与调用的时机和方式没有任何关系。")]),t._v(" "),s("p",[t._v("闭包会阻止一些变量的垃圾回收，对于较老旧的JavaScript引擎，可能导致外部所有变量均无法回收。")]),t._v(" "),s("p",[t._v("首先一个较为明确的结论是，以下内容会影响到闭包内变量的回收：")]),t._v(" "),s("ul",[s("li",[t._v("嵌套的函数中是否有使用该变量。")]),t._v(" "),s("li",[t._v("嵌套的函数中是否有 "),s("strong",[t._v("直接调用eval")]),t._v("。")]),t._v(" "),s("li",[t._v("是否使用了 with 表达式。")])]),t._v(" "),s("p",[t._v("Chakra、V8 和 SpiderMonkey 将受以上因素的影响，表现出不尽相同又较为相似的回收策略，而JScript.dll和Carakan则完全没有这方面的优化，会完整保留整个 LexicalEnvironment 中的所有变量绑定，造成一定的内存消耗。")]),t._v(" "),s("p",[t._v("由于对闭包内变量有回收优化策略的 Chakra、V8 和 SpiderMonkey 引擎的行为较为相似，因此可以总结如下，当返回一个函数 fn 时：")]),t._v(" "),s("ol",[s("li",[t._v("如果 fn 的 [[Scope]] 是ObjectEnvironment（with 表达式生成 ObjectEnvironment，函数和 catch 表达式生成 DeclarativeEnvironment），则：\n"),s("ol",[s("li",[t._v("如果是 V8 引擎，则退出全过程。")]),t._v(" "),s("li",[t._v("如果是 SpiderMonkey，则处理该 ObjectEnvironment 的外层 LexicalEnvironment。")])])]),t._v(" "),s("li",[t._v("获取当前 LexicalEnvironment 下的所有类型为 Function 的对象，对于每一个 Function 对象，分析其 FunctionBody：\n"),s("ol",[s("li",[t._v("如果 FunctionBody 中含有 "),s("strong",[t._v("直接调用eval")]),t._v("，则退出全过程。")]),t._v(" "),s("li",[t._v("否则得到所有的 Identifier。")]),t._v(" "),s("li",[t._v("对于每一个 Identifier，设其为 name，根据查找变量引用的规则，从 LexicalEnvironment 中找出名称为 name 的绑定 binding。")]),t._v(" "),s("li",[t._v("对 binding 添加 notSwap 属性，其值为 true。")])])]),t._v(" "),s("li",[t._v("检查当前 LexicalEnvironment 中的每一个变量绑定，如果该绑定有 notSwap 属性且值为 true，则：\n"),s("ol",[s("li",[t._v("如果是V8引擎，删除该绑定。")]),t._v(" "),s("li",[t._v("如果是SpiderMonkey，将该绑定的值设为 undefined，将删除 notSwap 属性。")])])])]),t._v(" "),s("p",[t._v("对于Chakra引擎，暂无法得知是按 V8 的模式还是按 SpiderMonkey 的模式进行。")]),t._v(" "),s("p",[t._v("如果有 "),s("strong",[t._v("非常庞大")]),t._v(" 的对象，且预计会在 "),s("strong",[t._v("老旧的引擎")]),t._v(" 中执行，则使用闭包时，注意将闭包不需要的对象置为空引用。")]),t._v(" "),s("h5",{attrs:{id:"【建议】-使用-iife-避免-lift-效应。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-使用-iife-避免-lift-效应。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 使用 "),s("code",[t._v("IIFE")]),t._v(" 避免 "),s("code",[t._v("Lift 效应")]),t._v("。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("在引用函数外部变量时，函数执行时外部变量的值由运行时决定而非定义时，最典型的场景如下：")]),t._v(" "),s("div",{staticClass:"language-javascript extra-class"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" tasks "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("for")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("5")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" i"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("++")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n tasks"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("tasks"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("length"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n console"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Current cursor is at '")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+")]),t._v(" i"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" len "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" tasks"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("length"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("while")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("len"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("--")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n tasks"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("len"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n")])])]),s("p",[t._v("以上代码对 tasks 中的函数的执行均会输出 "),s("code",[t._v("Current cursor is at 5")]),t._v("，往往不符合预期。")]),t._v(" "),s("p",[t._v("此现象称为 "),s("strong",[t._v("Lift 效应")]),t._v(" 。解决的方式是通过额外加上一层闭包函数，将需要的外部变量作为参数传递来解除变量的绑定关系：")]),t._v(" "),s("div",{staticClass:"language-javascript extra-class"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" tasks "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("for")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("var")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" i "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token number"}},[t._v("5")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v(" i"),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("++")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 注意有一层额外的闭包")]),t._v("\n tasks"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),t._v("tasks"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),t._v("length"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[t._v("i")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token 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punctuation"}},[t._v(".")]),t._v("selected "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token boolean"}},[t._v("true")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("this")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("flushView")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(";")]),t._v("\n")])])]),s("p",[t._v("对于此类场景，需要使用额外的对象来维护，使用由自身控制，不与外部产生任何交互的 selectedNodeIndex 对象来维护节点的选中状态，不对 datasource 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"),s("h2",{attrs:{id:"_4-浏览器环境"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_4-浏览器环境"}},[t._v("#")]),t._v(" 4 浏览器环境")]),t._v(" "),s("h3",{attrs:{id:"_4-1-dom"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_4-1-dom"}},[t._v("#")]),t._v(" 4.1 DOM")]),t._v(" "),s("h4",{attrs:{id:"_4-1-1-元素获取"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_4-1-1-元素获取"}},[t._v("#")]),t._v(" 4.1.1 元素获取")]),t._v(" "),s("h5",{attrs:{id:"【建议】-对于单个元素，尽可能使用-document-getelementbyid-获取，避免使用document-all。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-对于单个元素，尽可能使用-document-getelementbyid-获取，避免使用document-all。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 对于单个元素，尽可能使用 "),s("code",[t._v("document.getElementById")]),t._v(" 获取，避免使用"),s("code",[t._v("document.all")]),t._v("。")]),t._v(" "),s("h5",{attrs:{id:"【建议】-对于多个元素的集合，尽可能使用-context-getelementsbytagname-获取。其中-context-可以为-document-或其他元素。指定-tagname-参数为-可以获得所有子元素。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-对于多个元素的集合，尽可能使用-context-getelementsbytagname-获取。其中-context-可以为-document-或其他元素。指定-tagname-参数为-可以获得所有子元素。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 对于多个元素的集合，尽可能使用 "),s("code",[t._v("context.getElementsByTagName")]),t._v(" 获取。其中 "),s("code",[t._v("context")]),t._v(" 可以为 "),s("code",[t._v("document")]),t._v(" 或其他元素。指定 "),s("code",[t._v("tagName")]),t._v(" 参数为 "),s("code",[t._v("*")]),t._v(" 可以获得所有子元素。")]),t._v(" "),s("h5",{attrs:{id:"【建议】-遍历元素集合时，尽量缓存集合长度。如需多次操作同一集合，则应将集合转为数组。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-遍历元素集合时，尽量缓存集合长度。如需多次操作同一集合，则应将集合转为数组。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 遍历元素集合时，尽量缓存集合长度。如需多次操作同一集合，则应将集合转为数组。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("原生获取元素集合的结果并不直接引用 DOM 元素，而是对索引进行读取，所以 DOM 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"),s("h5",{attrs:{id:"【建议】-获取元素实际样式信息时，应使用-getcomputedstyle-或-currentstyle。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-获取元素实际样式信息时，应使用-getcomputedstyle-或-currentstyle。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 获取元素实际样式信息时，应使用 "),s("code",[t._v("getComputedStyle")]),t._v(" 或 "),s("code",[t._v("currentStyle")]),t._v("。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("通过 style 只能获得内联定义或通过 JavaScript 直接设置的样式。通过 CSS class 设置的元素样式无法直接通过 style 获取。")]),t._v(" "),s("h4",{attrs:{id:"_4-1-3-样式设置"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_4-1-3-样式设置"}},[t._v("#")]),t._v(" 4.1.3 样式设置")]),t._v(" "),s("h5",{attrs:{id:"【建议】-尽可能通过为元素添加预定义的-classname-来改变元素样式，避免直接操作-style-设置。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-尽可能通过为元素添加预定义的-classname-来改变元素样式，避免直接操作-style-设置。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 尽可能通过为元素添加预定义的 className 来改变元素样式，避免直接操作 style 设置。")]),t._v(" "),s("h5",{attrs:{id:"【强制】-通过-style-对象设置元素样式时，对于带单位非-0-值的属性，不允许省略单位。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【强制】-通过-style-对象设置元素样式时，对于带单位非-0-值的属性，不允许省略单位。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【强制】")]),t._v(" 通过 style 对象设置元素样式时，对于带单位非 0 值的属性，不允许省略单位。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("除了 IE，标准浏览器会忽略不规范的属性值，导致兼容性问题。")]),t._v(" "),s("h4",{attrs:{id:"_4-1-4-dom-操作"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_4-1-4-dom-操作"}},[t._v("#")]),t._v(" 4.1.4 DOM 操作")]),t._v(" "),s("h5",{attrs:{id:"【建议】-操作-dom-时，尽量减少页面-reflow。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-操作-dom-时，尽量减少页面-reflow。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 操作 "),s("code",[t._v("DOM")]),t._v(" 时，尽量减少页面 "),s("code",[t._v("reflow")]),t._v("。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("页面 reflow 是非常耗时的行为，非常容易导致性能瓶颈。下面一些场景会触发浏览器的reflow：")]),t._v(" "),s("ul",[s("li",[t._v("DOM元素的添加、修改（内容）、删除。")]),t._v(" "),s("li",[t._v("应用新的样式或者修改任何影响元素布局的属性。")]),t._v(" "),s("li",[t._v("Resize浏览器窗口、滚动页面。")]),t._v(" "),s("li",[t._v("读取元素的某些属性（offsetLeft、offsetTop、offsetHeight、offsetWidth、scrollTop/Left/Width/Height、clientTop/Left/Width/Height、getComputedStyle()、currentStyle(in IE)) 。")])]),t._v(" "),s("h5",{attrs:{id:"【建议】-尽量减少-dom-操作。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-尽量减少-dom-操作。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 尽量减少 "),s("code",[t._v("DOM")]),t._v(" 操作。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("DOM 操作也是非常耗时的一种操作，减少 DOM 操作有助于提高性能。举一个简单的例子，构建一个列表。我们可以用两种方式：")]),t._v(" "),s("ol",[s("li",[t._v("在循环体中 createElement 并 append 到父元素中。")]),t._v(" "),s("li",[t._v("在循环体中拼接 HTML 字符串，循环结束后写父元素的 innerHTML。")])]),t._v(" "),s("p",[t._v("第一种方法看起来比较标准，但是每次循环都会对 DOM 进行操作，性能极低。在这里推荐使用第二种方法。")]),t._v(" "),s("h4",{attrs:{id:"_4-1-5-dom-事件"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_4-1-5-dom-事件"}},[t._v("#")]),t._v(" 4.1.5 DOM 事件")]),t._v(" "),s("h5",{attrs:{id:"【建议】-优先使用-addeventlistener-attachevent-绑定事件，避免直接在-html-属性中或-dom-的-expando-属性绑定事件处理。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-优先使用-addeventlistener-attachevent-绑定事件，避免直接在-html-属性中或-dom-的-expando-属性绑定事件处理。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 优先使用 "),s("code",[t._v("addEventListener / attachEvent")]),t._v(" 绑定事件，避免直接在 HTML 属性中或 DOM 的 "),s("code",[t._v("expando")]),t._v(" 属性绑定事件处理。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("expando 属性绑定事件容易导致互相覆盖。")]),t._v(" "),s("h5",{attrs:{id:"【建议】-使用-addeventlistener-时第三个参数使用-false。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-使用-addeventlistener-时第三个参数使用-false。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 使用 "),s("code",[t._v("addEventListener")]),t._v(" 时第三个参数使用 "),s("code",[t._v("false")]),t._v("。")]),t._v(" "),s("p",[t._v("解释：")]),t._v(" "),s("p",[t._v("标准浏览器中的 addEventListener 可以通过第三个参数指定两种时间触发模型：冒泡和捕获。而 IE 的 attachEvent 仅支持冒泡的事件触发。所以为了保持一致性，通常 addEventListener 的第三个参数都为 false。")]),t._v(" "),s("h5",{attrs:{id:"【建议】-在没有事件自动管理的框架支持下，应持有监听器函数的引用，在适当时候（元素释放、页面卸载等）移除添加的监听器。"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#【建议】-在没有事件自动管理的框架支持下，应持有监听器函数的引用，在适当时候（元素释放、页面卸载等）移除添加的监听器。"}},[t._v("#")]),t._v(" "),s("strong",[t._v("【建议】")]),t._v(" 在没有事件自动管理的框架支持下，应持有监听器函数的引用，在适当时候（元素释放、页面卸载等）移除添加的监听器。")])])}),[],!1,null,null,null);a.default=e.exports}}]);
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