我将代码重构为承诺，并构建了一个奇妙的长而平坦的承诺链，由多个.then()回调组成。最后我想返回一些复合值，并需要访问多个中间承诺结果。然而，从序列中间的分辨率值不在最后回调的范围内，我如何访问它们?
function getExample() {
return promiseA(…).then(function(resultA) {
// Some processing
return promiseB(…);
}).then(function(resultB) {
// More processing
return // How do I gain access to resultA here?
});
}
当前回答
我认为你可以使用RSVP哈希。
内容如下:
const mainPromise = () => {
const promise1 = new Promise((resolve, reject) => {
setTimeout(() => {
console.log('first promise is completed');
resolve({data: '123'});
}, 2000);
});
const promise2 = new Promise((resolve, reject) => {
setTimeout(() => {
console.log('second promise is completed');
resolve({data: '456'});
}, 2000);
});
return new RSVP.hash({
prom1: promise1,
prom2: promise2
});
};
mainPromise()
.then(data => {
console.log(data.prom1);
console.log(data.prom2);
});
其他回答
另一个答案，使用babel节点版本<6
使用async - await
NPM install -g babel@5.6.14
example.js:
async function getExample(){
let response = await returnPromise();
let response2 = await returnPromise2();
console.log(response, response2)
}
getExample()
然后，运行babel-node example.js，瞧!
另一个答案，使用顺序执行器nsynjs:
function getExample(){
var response1 = returnPromise1().data;
// promise1 is resolved at this point, '.data' has the result from resolve(result)
var response2 = returnPromise2().data;
// promise2 is resolved at this point, '.data' has the result from resolve(result)
console.log(response, response2);
}
nynjs.run(getExample,{},function(){
console.log('all done');
})
更新:增加了工作示例
function synchronousCode() { var urls=[ "https://ajax.googleapis.com/ajax/libs/jquery/1.7.0/jquery.min.js", "https://ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js", "https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js" ]; for(var i=0; i<urls.length; i++) { var len=window.fetch(urls[i]).data.text().data.length; // ^ ^ // | +- 2-nd promise result // | assigned to 'data' // | // +-- 1-st promise result assigned to 'data' // console.log('URL #'+i+' : '+urls[i]+", length: "+len); } } nsynjs.run(synchronousCode,{},function(){ console.log('all done'); }) <script src="https://rawgit.com/amaksr/nsynjs/master/nsynjs.js"></script>
这几天，我也遇到了一些像你一样的问题。最后，我用这个问题找到了一个很好的解决方案，它简单易懂。我希望这能帮助到你。
根据how-to-chain-javascript-promises
好的，让我们看看代码:
const firstPromise = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('first promise is completed');
resolve({data: '123'});
}, 2000);
});
};
const secondPromise = (someStuff) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('second promise is completed');
resolve({newData: `${someStuff.data} some more data`});
}, 2000);
});
};
const thirdPromise = (someStuff) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log('third promise is completed');
resolve({result: someStuff});
}, 2000);
});
};
firstPromise()
.then(secondPromise)
.then(thirdPromise)
.then(data => {
console.log(data);
});
function getExample() {
var retA, retB;
return promiseA(…).then(function(resultA) {
retA = resultA;
// Some processing
return promiseB(…);
}).then(function(resultB) {
// More processing
//retA is value of promiseA
return // How do I gain access to resultA here?
});
}
简单的方法:D
打破枷锁
When you need to access the intermediate values in your chain, you should split your chain apart in those single pieces that you need. Instead of attaching one callback and somehow trying to use its parameter multiple times, attach multiple callbacks to the same promise - wherever you need the result value. Don't forget, a promise just represents (proxies) a future value! Next to deriving one promise from the other in a linear chain, use the promise combinators that are given to you by your library to build the result value.
这将导致一个非常直接的控制流程，清晰的功能组合，因此易于模块化。
function getExample() {
var a = promiseA(…);
var b = a.then(function(resultA) {
// some processing
return promiseB(…);
});
return Promise.all([a, b]).then(function([resultA, resultB]) {
// more processing
return // something using both resultA and resultB
});
}
而不是在Promise之后的回调中进行参数解构。在ES5中，then调用将被许多承诺库(Q, Bluebird, when，…)提供的一个漂亮的助手方法所取代:.spread(function(resultA, resultB) {....
Bluebird还提供了一个专用的连接功能来取代承诺。All +扩展组合，更简单(更有效)的结构:
…
return Promise.join(a, b, function(resultA, resultB) { … });
