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
Node 7.4现在支持带有和谐标志的async/await调用。
试试这个:
async function getExample(){
let response = await returnPromise();
let response2 = await returnPromise2();
console.log(response, response2)
}
getExample()
然后运行文件:
node——harmony-async-await getExample.js
尽可能的简单!
其他回答
对“可变上下文状态”不那么苛刻的解释
对于您提出的问题，使用局部作用域对象来收集承诺链中的中间结果是一种合理的方法。考虑下面的代码片段:
function getExample(){
//locally scoped
const results = {};
return promiseA(paramsA).then(function(resultA){
results.a = resultA;
return promiseB(paramsB);
}).then(function(resultB){
results.b = resultB;
return promiseC(paramsC);
}).then(function(resultC){
//Resolve with composite of all promises
return Promise.resolve(results.a + results.b + resultC);
}).catch(function(error){
return Promise.reject(error);
});
}
Global variables are bad, so this solution uses a locally scoped variable which causes no harm. It is only accessible within the function. Mutable state is ugly, but this does not mutate state in an ugly manner. The ugly mutable state traditionally refers to modifying the state of function arguments or global variables, but this approach simply modifies the state of a locally scoped variable that exists for the sole purpose of aggregating promise results...a variable that will die a simple death once the promise resolves. Intermediate promises are not prevented from accessing the state of the results object, but this does not introduce some scary scenario where one of the promises in the chain will go rogue and sabotage your results. The responsibility of setting the values in each step of the promise is confined to this function and the overall result will either be correct or incorrect...it will not be some bug that will crop up years later in production (unless you intend it to!) This does not introduce a race condition scenario that would arise from parallel invocation because a new instance of the results variable is created for every invocation of the getExample function.
示例在jsfiddle上可用
我不打算在我自己的代码中使用这种模式，因为我不太喜欢使用全局变量。然而，在紧要关头它还是有用的。
用户是一个承诺的猫鼬模型。
var globalVar = '';
User.findAsync({}).then(function(users){
globalVar = users;
}).then(function(){
console.log(globalVar);
});
Node 7.4现在支持带有和谐标志的async/await调用。
试试这个:
async function getExample(){
let response = await returnPromise();
let response2 = await returnPromise2();
console.log(response, response2)
}
getExample()
然后运行文件:
node——harmony-async-await getExample.js
尽可能的简单!
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);
});
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>
