使用promises执行类似while循环的惯用方法是什么？所以：
做某事 如果条件仍然存在再做一次 重复 然后做点别的事。dosomething.then(possilblydomoresomethings).then(finish)
我是这样做的，我想知道是否有更好/更多的自我方式？
var q = require('q');
var index = 1;
var useless = function(){
var currentIndex = index;
console.log(currentIndex)
var deferred = q.defer();
setTimeout(function(){
if(currentIndex > 10)
deferred.resolve(false);
else deferred.resolve(true);
},500);
return deferred.promise;
}
var control = function(cont){
var deferred = q.defer();
if(cont){
index = index + 1;
useless().then(control).then(function(){
deferred.resolve();
});
}
else deferred.resolve();
return deferred.promise;
}
var chain = useless().then(control).then(function(){console.log('done')});
输出： 1 2 3 4 五 6 7 8 9 10 11 完成
答案 0 :(得分：57)
这是一个可重复使用的功能，我认为非常清楚。
var Q = require("q");
// `condition` is a function that returns a boolean
// `body` is a function that returns a promise
// returns a promise for the completion of the loop
function promiseWhile(condition, body) {
var done = Q.defer();
function loop() {
// When the result of calling `condition` is no longer true, we are
// done.
if (!condition()) return done.resolve();
// Use `when`, in case `body` does not return a promise.
// When it completes loop again otherwise, if it fails, reject the
// done promise
Q.when(body(), loop, done.reject);
}
// Start running the loop in the next tick so that this function is
// completely async. It would be unexpected if `body` was called
// synchronously the first time.
Q.nextTick(loop);
// The promise
return done.promise;
}
// Usage
var index = 1;
promiseWhile(function () { return index <= 11; }, function () {
console.log(index);
index++;
return Q.delay(500); // arbitrary async
}).then(function () {
console.log("done");
}).done();
答案 1 :(得分：24)
这是我发现表达基本模式的最简单方法：定义一个调用promise的函数，检查其结果，然后再次调用自身或终止。
const doSomething = value =>
new Promise(resolve =>
setTimeout(() => resolve(value >= 5 ? 'ok': 'no'), 1000))
const loop = value =>
doSomething(value).then(result => {
console.log(value)
if (result === 'ok') {
console.log('yay')
} else {
return loop(value + 1)
}
})
loop(1).then(() => console.log('all done!'))
如果您使用的是解析或拒绝的承诺，则应定义then和catch，而不是使用if子句。
如果你有一系列的承诺，你只需要改变loop每次换班或弹出下一张。
编辑：这是一个使用async/await的版本，因为它是2018年：
const loop = async value => {
let result = null
while (result != 'ok') {
console.log(value)
result = await doSomething(value)
value = value + 1
}
console.log('yay')
}
正如您所看到的，它使用了正常的while循环而没有递归。
答案 2 :(得分：18)
我使用一个对象来包装值。这样你就可以拥有一个done属性来让循环知道你已经完成了。
// fn should return an object like
// {
// done: false,
// value: foo
// }
function loop(promise, fn) {
return promise.then(fn).then(function (wrapper) {
return !wrapper.done ? loop(Q(wrapper.value), fn) : wrapper.value;
});
}
loop(Q.resolve(1), function (i) {
console.log(i);
return {
done: i > 10,
value: i++
};
}).done(function () {
console.log('done');
});
答案 3 :(得分：12)
这是蓝鸟而不是q但是因为你没有特别提到q ..在bluebird api doc中，作者提到返回一个承诺生成函数比使用deferreds更惯用。
var Promise = require('bluebird');
var i = 0;
var counter = Promise.method(function(){
return i++;
})
function getAll(max, results){
var results = results || [];
return counter().then(function(result){
results.push(result);
return (result < max) ? getAll(max, results) : results
})
}
getAll(10).then(function(data){
console.log(data);
})
答案 4 :(得分：5)
由于我无法对Stuart K的答案发表评论，我将在这里添加一些内容。根据Stuart K的回答，您可以将其归结为一个非常简单的概念：重用未实现的承诺。他拥有的基本上是：
Stuart的答案是提供更通用的解决方案，但基础知识非常棒（一旦你意识到它是如何工作的）。
答案 5 :(得分：4)
现在使用q-flow可以更轻松地调用此模式。一个例子，对于上述问题：
var q = require('q');
require('q-flow');
var index = 1;
q.until(function() {
return q.delay(500).then(function() {
console.log(index++);
return index > 10;
});
}).done(function() {
return console.log('done');
});
答案 6 :(得分：3)
以下是Promise原型的扩展，用于模仿for循环的行为。它支持初始化，条件，循环体和增量部分的promise或立即值。它还完全支持异常，并且没有内存泄漏。下面给出了一个如何使用它的例子。
var Promise = require('promise');
// Promise.loop([properties: object]): Promise()
//
// Execute a loop based on promises. Object 'properties' is an optional
// argument with the following fields:
//
// initialization: function(): Promise() | any, optional
//
// Function executed as part of the initialization of the loop. If
// it returns a promise, the loop will not begin to execute until
// it is resolved.
//
// Any exception occurring in this function will finish the loop
// with a rejected promise. Similarly, if this function returns a
// promise, and this promise is reject, the loop finishes right
// away with a rejected promise.
//
// condition: function(): Promise(result: bool) | bool, optional
//
// Condition evaluated in the beginning of each iteration of the
// loop. The function should return a boolean value, or a promise
// object that resolves with a boolean data value.
//
// Any exception occurring during the evaluation of the condition
// will finish the loop with a rejected promise. Similarly, it this
// function returns a promise, and this promise is rejected, the
// loop finishes right away with a rejected promise.
//
// If no condition function is provided, an infinite loop is
// executed.
//
// body: function(): Promise() | any, optional
//
// Function acting as the body of the loop. If it returns a
// promise, the loop will not proceed until this promise is
// resolved.
//
// Any exception occurring in this function will finish the loop
// with a rejected promise. Similarly, if this function returns a
// promise, and this promise is reject, the loop finishes right
// away with a rejected promise.
//
// increment: function(): Promise() | any, optional
//
// Function executed at the end of each iteration of the loop. If
// it returns a promise, the condition of the loop will not be
// evaluated again until this promise is resolved.
//
// Any exception occurring in this function will finish the loop
// with a rejected promise. Similarly, if this function returns a
// promise, and this promise is reject, the loop finishes right
// away with a rejected promise.
//
Promise.loop = function(properties)
{
// Default values
properties = properties || {};
properties.initialization = properties.initialization || function() { };
properties.condition = properties.condition || function() { return true; };
properties.body = properties.body || function() { };
properties.increment = properties.increment || function() { };
// Start
return new Promise(function(resolve, reject)
{
var runInitialization = function()
{
Promise.resolve().then(function()
{
return properties.initialization();
})
.then(function()
{
process.nextTick(runCondition);
})
.catch(function(error)
{
reject(error);
});
}
var runCondition = function()
{
Promise.resolve().then(function()
{
return properties.condition();
})
.then(function(result)
{
if (result)
process.nextTick(runBody);
else
resolve();
})
.catch(function(error)
{
reject(error);
});
}
var runBody = function()
{
Promise.resolve().then(function()
{
return properties.body();
})
.then(function()
{
process.nextTick(runIncrement);
})
.catch(function(error)
{
reject(error);
});
}
var runIncrement = function()
{
Promise.resolve().then(function()
{
return properties.increment();
})
.then(function()
{
process.nextTick(runCondition);
})
.catch(function(error)
{
reject(error);
});
}
// Start running initialization
process.nextTick(runInitialization);
});
}
// Promise.delay(time: double): Promise()
//
// Returns a promise that resolves after the given delay in seconds.
//
Promise.delay = function(time)
{
return new Promise(function(resolve)
{
setTimeout(resolve, time * 1000);
});
}
// Example
var i;
Promise.loop({
initialization: function()
{
i = 2;
},
condition: function()
{
return i < 6;
},
body: function()
{
// Print "i"
console.log(i);
// Exception when 5 is reached
if (i == 5)
throw Error('Value of "i" reached 5');
// Wait 1 second
return Promise.delay(1);
},
increment: function()
{
i++;
}
})
.then(function()
{
console.log('LOOP FINISHED');
})
.catch(function(error)
{
console.log('EXPECTED ERROR:', error.message);
});
答案 7 :(得分：1)
var Q = require('q')
var vetor = ['a','b','c']
function imprimeValor(elements,initValue,defer){
console.log( elements[initValue++] )
defer.resolve(initValue)
return defer.promise
}
function Qloop(initValue, elements,defer){
Q.when( imprimeValor(elements, initValue, Q.defer()), function(initValue){
if(initValue===elements.length){
defer.resolve()
}else{
defer.resolve( Qloop(initValue,elements, Q.defer()) )
}
}, function(err){
defer.reject(err)
})
return defer.promise
}
Qloop(0, vetor,Q.defer())
答案 8 :(得分：1)
我现在正在使用它：
function each(arr, work) {
function loop(arr, i) {
return new Promise(function(resolve, reject) {
if (i >= arr.length) {resolve();}
else try {
Promise.resolve(work(arr[i], i)).then(function() {
resolve(loop(arr, i+1))
}).catch(reject);
} catch(e) {reject(e);}
});
}
return loop(arr, 0);
}
这接受数组arr和函数work并返回Promise。为数组中的每个元素调用一次提供的函数，并传递当前元素及其在数组中的索引。它可能是同步或异步，在这种情况下它必须返回一个Promise。
你可以像这样使用它：
var items = ['Hello', 'cool', 'world'];
each(items, function(item, idx) {
// this could simply be sync, but can also be async
// in which case it must return a Promise
return new Promise(function(resolve){
// use setTimeout to make this async
setTimeout(function(){
console.info(item, idx);
resolve();
}, 1000);
});
})
.then(function(){
console.info('DONE');
})
.catch(function(error){
console.error('Failed', error);
})
将依次处理数组中的每个项目。处理完所有代码后，将运行.then()的代码，或者，如果发生某些错误，则会将代码提供给.catch()。在work函数内，您可以throw Error（在同步函数的情况下）或reject Promise（在异步函数的情况下）中止循环。
function each(arr, work) {
function loop(arr, i) {
return new Promise(function(resolve, reject) {
if (i >= arr.length) {resolve();}
else try {
Promise.resolve(work(arr[i], i)).then(function() {
resolve(loop(arr, i+1))
}).catch(reject);
} catch(e) {reject(e);}
});
}
return loop(arr, 0);
}
var items = ['Hello', 'cool', 'world'];
each(items, function(item, idx) {
// this could simply be sync, but can also be async
// in which case it must return a Promise
return new Promise(function(resolve){
// use setTimeout to make this async
setTimeout(function(){
console.info(item, idx);
resolve();
}, 1000);
});
})
.then(function(){
console.info('DONE');
})
.catch(function(error){
console.error('Failed', error);
})
答案 9 :(得分：1)
这是一个使用 ES6 承诺的通用解决方案：
/**
* Simulates a while loop where the condition is determined by the result of a Promise.
*
* @param {Function} condition
* @param {Function} action
* @returns {Promise}
*/
function promiseWhile (condition, action) {
return new Promise((resolve, reject) => {
const loop = function () {
if (!condition()) {
resolve();
} else {
Promise.resolve(action())
.then(loop)
.catch(reject);
}
}
loop();
})
}
/**
* Simulates a do-while loop where the condition is determined by the result of a Promise.
*
* @param {Function} condition
* @param {Function} action
* @returns {Promise}
*/
function promiseDoWhile (condition, action) {
return Promise.resolve(action())
.then(() => promiseWhile(condition, action));
}
export default promiseWhile;
export {promiseWhile, promiseDoWhile};
你可以这样使用它：
let myCounter = 0;
function myAsyncFunction () {
return new Promise(resolve => {
setTimeout(() => {
console.log(++myCounter);
resolve()
}, 1000)
});
}
promiseWhile(() => myCounter < 5, myAsyncFunction).then(() => console.log(`Timer completed: ${myCounter}`));
答案 10 :(得分：0)
使用ES6 Promise，我想出了这个。它链接承诺并返回承诺。它在技术上不是一个while循环，但确实显示了如何同步迭代promises。
function chain_promises(list, fun) {
return list.reduce(
function (promise, element) {
return promise.then(function () {
// I only needed to kick off some side-effects. If you need to get
// a list back, you would append to it here. Or maybe use
// Array.map instead of Array.reduce.
fun(element);
});
},
// An initial promise just starts things off.
Promise.resolve(true)
);
}
// To test it...
function test_function (element) {
return new Promise(function (pass, _fail) {
console.log('Processing ' + element);
pass(true);
});
}
chain_promises([1, 2, 3, 4, 5], test_function).then(function () {
console.log('Done.');
});
答案 11 :(得分：0)
我想我也不妨把我的帽子戴在戒指上，使用ES6 Promises ......
function until_success(executor){
var before_retry = undefined;
var outer_executor = function(succeed, reject){
var rejection_handler = function(err){
if(before_retry){
try {
var pre_retry_result = before_retry(err);
if(pre_retry_result)
return succeed(pre_retry_result);
} catch (pre_retry_error){
return reject(pre_retry_error);
}
}
return new Promise(executor).then(succeed, rejection_handler);
}
return new Promise(executor).then(succeed, rejection_handler);
}
var outer_promise = new Promise(outer_executor);
outer_promise.before_retry = function(func){
before_retry = func;
return outer_promise;
}
return outer_promise;
}
executor参数是the same，因为它传递给Promise构造函数，但会被重复调用，直到它触发成功回调。 before_retry函数允许对失败的尝试进行自定义错误处理。如果它返回一个真值，它将被视为一种成功形式，“循环”将结束，结果是真实的。如果未注册before_retry函数，或者它返回falsey值，则循环将运行另一次迭代。第三个选项是before_retry函数本身会抛出错误。如果发生这种情况，那么“循环”将结束，将该错误作为错误传递。
以下是一个例子：
var counter = 0;
function task(succ, reject){
setTimeout(function(){
if(++counter < 5)
reject(counter + " is too small!!");
else
succ(counter + " is just right");
}, 500); // simulated async task
}
until_success(task)
.before_retry(function(err){
console.log("failed attempt: " + err);
// Option 0: return falsey value and move on to next attempt
// return
// Option 1: uncomment to get early success..
//if(err === "3 is too small!!")
// return "3 is sort of ok";
// Option 2: uncomment to get complete failure..
//if(err === "3 is too small!!")
// throw "3rd time, very unlucky";
}).then(function(val){
console.log("finally, success: " + val);
}).catch(function(err){
console.log("it didn't end well: " + err);
})
选项0的输出：
failed attempt: 1 is too small!!
failed attempt: 2 is too small!!
failed attempt: 3 is too small!!
failed attempt: 4 is too small!!
finally, success: 5 is just right
选项1的输出：
failed attempt: 1 is too small!!
failed attempt: 2 is too small!!
failed attempt: 3 is too small!!
finally, success: 3 is sort of ok
选项2的输出：
failed attempt: 1 is too small!!
failed attempt: 2 is too small!!
failed attempt: 3 is too small!!
it didn't end well: 3rd time, very unlucky
答案 12 :(得分：0)
这里的很多答案以及您要实现的目标都不是很实际。但这应该可行。这是在aws lambda函数中实现的，对于Node.js 10，它将一直持续到函数超时。它还可能会消耗大量的内存。
exports.handler = async (event) => {
let res = null;
while (true) {
try{
res = await dopromise();
}catch(err){
res = err;
}
console.log(res);
}//infinite will time out
};
function dopromise(){
return new Promise((resolve, reject) => {
//do some logic
//if error reject
//reject('failed');
resolve('success');
});
}
在lambda上进行测试，并且可以正常运行5分钟以上。但是正如别人所说，这不是一件好事。
答案 13 :(得分：-1)
我写了一个模块，可以帮助你用promises做异步任务的链接循环，它是基于juandopazo提供的上述答案
/**
* Should loop over a task function which returns a "wrapper" object
* until wrapper.done is true. A seed value wrapper.seed is propagated to the
* next run of the loop.
*
* todo/maybe? Reject if wrapper is not an object with done and seed keys.
*
* @param {Promise|*} seed
* @param {Function} taskFn
*
* @returns {Promise.<*>}
*/
function seedLoop(seed, taskFn) {
const seedPromise = Promise.resolve(seed);
return seedPromise
.then(taskFn)
.then((wrapper) => {
if (wrapper.done) {
return wrapper.seed;
}
return seedLoop(wrapper.seed, taskFn);
});
}
// A super simple example of counting to ten, which doesn't even
// do anything asynchronous, but if it did, it should resolve to
// a promise that returns the { done, seed } wrapper object for the
// next call of the countToTen task function.
function countToTen(count) {
const done = count > 10;
const seed = done ? count : count + 1;
return {done, seed};
}
seedLoop(1, countToTen).then((result) => {
console.log(result); // 11, the first value which was over 10.
});