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"),t("h3",{attrs:{id:"_4-4-promise-then-返回的新-promise-的结果状态由什么决定"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_4-4-promise-then-返回的新-promise-的结果状态由什么决定"}},[s._v("#")]),s._v(" 4.4 promise.then()返回的新 promise 的结果状态由什么决定?")]),s._v(" "),t("p",[s._v("(1) 简单表达: 由 then()指定的回调函数执行的结果决定")]),s._v(" "),t("p",[s._v("(2) 详细表达:")]),s._v(" "),t("p",[s._v("​ ① 如果抛出异常, 新 promise 变为 rejected, reason 为抛出的异常")]),s._v(" "),t("p",[s._v("​ ② 如果返回的是非 promise 的任意值, 新 promise 变为 resolved, value 为返回的值")]),s._v(" "),t("p",[s._v("​ ③ 如果返回的是另一个新 promise, 此 promise 的结果就会成为新 promise 的结果")]),s._v(" "),t("h3",{attrs:{id:"_4-5-promise-如何串连多个操作任务"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_4-5-promise-如何串连多个操作任务"}},[s._v("#")]),s._v(" 4.5 promise 如何串连多个操作任务?")]),s._v(" "),t("p",[s._v("(1) promise 的 then()返回一个新的 promise, 可以开成 then()的链式调用")]),s._v(" "),t("p",[s._v("(2) 通过 then 的链式调用串连多个同步/异步任务,这样就能用then()将多个同步或异步操作串联成一个同步队列")]),s._v(" 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class-name"}},[s._v("Promise")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("resolve"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" reject")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=>")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("setTimeout")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=>")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//有且只有这一个方式")]),s._v("\n    "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("then")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("value")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=>")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("222")]),t("span",{pre:!0,attrs:{class:"token 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async 函数内部，不能单独使用。")]),s._v(" "),t("li",[s._v("await 后面可以跟任何的JS 表达式。虽然说 await 可以等很多类型的东西，但是它最主要的意图是用来等待 Promise 对象的状态被 resolved。如果await的是 promise对象会造成异步函数停止执行并且等待 promise 的解决,如果等的是正常的表达式则立即执行")])])]),s._v(" "),t("li",[t("p",[s._v("async ==> 同步转异步")]),s._v(" "),t("ol",[t("li",[s._v("方法体内部的某个表达式使用await修饰，那么这个方法体所属方法必须要用async修饰所以使用awit方法会自动升级为异步方法")])])]),s._v(" "),t("li",[t("p",[s._v("mdn文档")]),s._v(" "),t("ul",[t("li",[t("a",{attrs:{href:"https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/async_function",target:"_blank",rel:"noopener noreferrer"}},[s._v("async"),t("OutboundLink")],1)]),s._v(" "),t("li",[t("a",{attrs:{href:"https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Operators/await",target:"_blank",rel:"noopener noreferrer"}},[s._v("await"),t("OutboundLink")],1)])])])]),s._v(" "),t("h3",{attrs:{id:"_5-1-async函数"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_5-1-async函数"}},[s._v("#")]),s._v(" 5.1 async函数")]),s._v(" "),t("ol",[t("li",[s._v("函数的返回值为 promise 对象")]),s._v(" "),t("li",[s._v("promise 对象的结果由 async 函数执行的返回值决定")])]),s._v(" "),t("h3",{attrs:{id:"_5-2-await表达式"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_5-2-await表达式"}},[s._v("#")]),s._v(" 5.2 await表达式")]),s._v(" "),t("ol",[t("li",[s._v("await 右侧的表达式一般为 promise 对象, 但也可以是其它的值")]),s._v(" "),t("li",[s._v("如果表达式是 promise 对象, await 返回的是 promise 成功的值")]),s._v(" "),t("li",[s._v("如果表达式是其它值, 直接将此值作为 await 的返回值")])]),s._v(" "),t("h3",{attrs:{id:"_5-3-注意"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_5-3-注意"}},[s._v("#")]),s._v(" 5.3 注意")]),s._v(" "),t("ol",[t("li",[s._v("await 必须写在 async 函数中, 但 async 函数中可以没有 await")]),s._v(" "),t("li",[s._v("如果 await 的 promise 失败了, 就会抛出异常, 需要通过 try...catch 捕获处理")])])])}),[],!1,null,null,null);t.default=e.exports}}]);