ES6中的async/await讓Promise變得更加簡(jiǎn)便，通常await處理的鏈?zhǔn)絇romise會(huì)包裹在函數(shù)中，返回結(jié)果仍然是一個(gè)Promise對(duì)象。

但是當(dāng)await直接處理鏈?zhǔn)絇romise時(shí)，編譯器是會(huì)返回一個(gè)Promise對(duì)象等待下次await，還是繼續(xù)執(zhí)行Promise對(duì)象直到返回結(jié)果不再是Promise？

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測(cè)試環(huán)境

NodeJS v8.7.0

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測(cè)試代碼

1 const util = require('util'); 2 3 const prom2 = util.promisify((a, b, cb) => { 4 cb(null, a + b); 5 }); 6 7 function promiseCal(a, b) { 8 return prom2(a, b) 9 .then((val) => { 10 console.log(`value in first then: ${val}`); 11 return val * 2; 12 }) 13 .then((val) => { 14 console.log(`Value in second then: ${val}`); 15 return val * 3 16 }); 17 } 18 19 async function test() { 20 // await getting value from chained promise 21 let val1 = await promiseCal(2, 4); 22 23 console.log(`Typeof val1: ${typeof val1}\nFinal returned value: ${val1}`); 24 25 // await getting value from another form 26 console.log(`[v2] Type of prom2: ${typeof prom2}`); 27 let val2 = await prom2(4, 6) 28 .then((val) => { 29 console.log(`[v2] Value in first then: ${val}`); 30 return val * 2; 31 }) 32 .then((val) => { 33 console.log(`[v2] Value in second then: ${val}`); 34 return val * 3 35 }); 36 console.log(`[v2] Typeof val2: ${typeof val2}\n[v2] Final returned value: ${val2}`); 37 } 38 39 test() 40 .catch(e => console.error(e.stack || e));

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運(yùn)行結(jié)果

value in first then: 6 Value in second then: 12 Typeof val1: number Final returned value: 36 [v2] Type of prom2: function [v2] Value in first then: 10 [v2] Value in second then: 20 [v2] Typeof val2: number [v2] Final returned value: 60

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結(jié)論

await操作符會(huì)沿著Promise鏈處理到返回結(jié)果不再是Promise位置，await語(yǔ)句返回的將是Primise鏈上最后一個(gè)then函數(shù)的返回值(或者拋出異常)。

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轉(zhuǎn)載于:https://www.cnblogs.com/harelion/p/9128674.html

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