Node JS Promise.all and forEach

问题:

I have an array like structure that exposes async methods. The async method calls return array structures that in turn expose more async methods. I am creating another JSON object to store values obtained from this structure and so I need to be careful about keeping track of references in callbacks.

I have coded a brute force solution, but I would like to learn a more idiomatic or clean solution.

1. The pattern should be repeatable for n levels of nesting.
2. I need to use promise.all or some similar technique to determine when to resolve the enclosing routine.
3. Not every element will necessarily involve making an async call. So in a nested promise.all I can't simply make assignments to my JSON array elements based on index. Nevertheless, I do need to use something like promise.all in the nested forEach to ensure that all property assignments have been made prior to resolving the enclosing routine.
4. I am using the bluebird promise lib but this is not a requirement

Here is some partial code -

var jsonItems = [];  items.forEach(function(item){    var jsonItem = {};   jsonItem.name = item.name;   item.getThings().then(function(things){   // or Promise.all(allItemGetThingCalls, function(things){      things.forEach(function(thing, index){        jsonItems[index].thingName = thing.name;       if(thing.type === 'file'){          thing.getFile().then(function(file){ //or promise.all?            jsonItems[index].filesize = file.getSize(); 

回答1:

It's pretty straightforward with some simple rules:

• Whenever you create a promise in a then, return it - any promise you don't return will not be waited for outside.
• Whenever you create multiple promises, .all them - that way it waits for all the promises and no error from any of them are silenced.
• Whenever you nest thens, you can typically return in the middle - then chains are usually at most 1 level deep.
• Whenever you perform IO, it should be with a promise - either it should be in a promise or it should use a promise to signal its completion.

And some tips:

• Mapping is better done with .map than with for/push - if you're mapping values with a function, map lets you concisely express the notion of applying actions one by one and aggregating the results.
• Concurrency is better than sequential execution if it's free - it's better to execute things concurrently and wait for them Promise.all than to execute things one after the other - each waiting before the next.

Ok, so let's get started:

var items = [1, 2, 3, 4, 5]; var fn = function asyncMultiplyBy2(v){ // sample async action     return new Promise(resolve => setTimeout(() => resolve(v * 2), 100)); }; // map over forEach since it returns  var actions = items.map(fn); // run the function over all items  // we now have a promises array and we want to wait for it  var results = Promise.all(actions); // pass array of promises  results.then(data => // or just .then(console.log)     console.log(data) // [2, 4, 6, 8, 10] );  // we can nest this of course, as I said, `then` chains:  var res2 = Promise.all([1, 2, 3, 4, 5].map(fn)).then(     data => Promise.all(data.map(fn)) ).then(function(data){     // the next `then` is executed after the promise has returned from the previous     // `then` fulfilled, in this case it's an aggregate promise because of      // the `.all`      return Promise.all(data.map(fn)); }).then(function(data){     // just for good measure     return Promise.all(data.map(fn)); });  // now to get the results:  res2.then(function(data){     console.log(data); // [16, 32, 48, 64, 80] }); 

回答2:

Here's a simple example using reduce. It runs serially, maintains insertion order, and does not require Bluebird.

/**  *   * @param items An array of items.  * @param fn A function that accepts an item from the array and returns a promise.  * @returns {Promise}  */ function forEachPromise(items, fn) {     return items.reduce(function (promise, item) {         return promise.then(function () {             return fn(item);         });     }, Promise.resolve()); } 

And use it like this:

var items = ['a', 'b', 'c'];  function logItem(item) {     return new Promise((resolve, reject) => {         process.nextTick(() => {             console.log(item);             resolve();         })     }); }  forEachPromise(items, logItem).then(() => {     console.log('done'); }); 

We have found it useful to send an optional context into loop. The context is optional and shared by all iterations.

function forEachPromise(items, fn, context) {     return items.reduce(function (promise, item) {         return promise.then(function () {             return fn(item, context);         });     }, Promise.resolve()); } 

Your promise function would look like this:

function logItem(item, context) {     return new Promise((resolve, reject) => {         process.nextTick(() => {             console.log(item);             context.itemCount++;             resolve();         })     }); } 

回答3:

I had through the same situation. I solved using two Promise.All().

I think was really good solution, so I published it on npm: https://www.npmjs.com/package/promise-foreach

I think your code will be something like this

var promiseForeach = require('promise-foreach') var jsonItems = []; promiseForeach.each(jsonItems,     [function (jsonItems){         return new Promise(function(resolve, reject){             if(jsonItems.type === 'file'){                 jsonItems.getFile().then(function(file){ //or promise.all?                     resolve(file.getSize())                 })             }         })     }],     function (result, current) {         return {             type: current.type,             size: jsonItems.result[0]         }     },     function (err, newList) {         if (err) {             console.error(err)             return;         }         console.log('new jsonItems : ', newList)     }) 

文章来源: Node JS Promise.all and forEach