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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br")])]),t("p",[s._v("上面两种写法，"),t("code",[s._v("getFoo")]),s._v("和"),t("code",[s._v("getBar")]),s._v("都是同时触发，这样就会缩短程序的执行时间。")]),s._v(" "),t("p",[s._v("第三点，"),t("code",[s._v("await")]),s._v("命令只能用在"),t("code",[s._v("async")]),s._v("函数之中，如果用在普通函数，就会报错。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("async")]),s._v(" 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br")])]),t("p",[s._v("上面代码会报错，因为"),t("code",[s._v("await")]),s._v("用在普通函数之中了。但是，如果将"),t("code",[s._v("forEach")]),s._v("方法的参数改成"),t("code",[s._v("async")]),s._v("函数，也有问题。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("dbFuc")]),t("span",{pre:!0,attrs:{class:"token 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与其他异步处理方法的比较")]),s._v(" "),t("p",[s._v("我们通过一个例子，来看 async 函数与 Promise、Generator 函数的比较。")]),s._v(" "),t("p",[s._v("假定某个 DOM 元素上面，部署了一系列的动画，前一个动画结束，才能开始后一个。如果当中有一个动画出错，就不再往下执行，返回上一个成功执行的动画的返回值。")]),s._v(" "),t("p",[s._v("首先是 Promise 的写法。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("chainAnimationsPromise")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("elem"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" animations")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n\n  "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 变量ret用来保存上一个动画的返回值")]),s._v("\n  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写法更清晰，用户定义的操作全部都出现在"),t("code",[s._v("spawn")]),s._v("函数的内部。这个写法的问题在于，必须有一个任务运行器，自动执行 Generator 函数，上面代码的"),t("code",[s._v("spawn")]),s._v("函数就是自动执行器，它返回一个 Promise 对象，而且必须保证"),t("code",[s._v("yield")]),s._v("语句后面的表达式，必须返回一个 Promise。")]),s._v(" "),t("p",[s._v("最后是 async 函数的写法。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("async")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("chainAnimationsAsync")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("elem"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" animations")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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函数的实现最简洁，最符合语义，几乎没有语义不相关的代码。它将 Generator 写法中的自动执行器，改在语言层面提供，不暴露给用户，因此代码量最少。如果使用 Generator 写法，自动执行器需要用户自己提供。")]),s._v(" "),t("h2",{attrs:{id:"实例-按顺序完成异步操作"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#实例-按顺序完成异步操作"}},[s._v("#")]),s._v(" 实例：按顺序完成异步操作")]),s._v(" "),t("p",[s._v("实际开发中，经常遇到一组异步操作，需要按照顺序完成。比如，依次远程读取一组 URL，然后按照读取的顺序输出结果。")]),s._v(" "),t("p",[s._v("Promise 的写法如下。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("logInOrder")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("urls")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token 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