问题
I'm sure this is a fairly simple task, but I'm not able to wrap my head around it at this time. I've got a nested set of forEach loops, and I need to have a callback for when all the loops are done running.
I'm open to using async.js
This is what I'm working with:
const scanFiles = function(accounts, cb) {
let dirs = ['pending', 'done', 'failed'];
let jobs = [];
accounts.forEach(function(account) {
dirs.forEach(function(dir) {
fs.readdir(account + '/' + dir, function(err, files) {
files.forEach(function(file) {
//do something
//add file to jobs array
jobs.push(file);
});
});
});
});
//return jobs array once all files have been added
cb(jobs);
}
回答1:
Using forEach's 2nd parameter, the index, you can carry out a check whether all loops are done each time you run the innermost loop.
Thus with only a few lines added to your code you get this:
const scanFiles = function(accounts, cb) {
let dirs = ['pending', 'done', 'failed'];
let jobs = [];
accounts.forEach(function(account, accIndex) {
dirs.forEach(function(dir, dirIndex) {
fs.readdir(account + '/' + dir, function(err, files) {
files.forEach(function(file, fileIndex) {
//do something
//add file to jobs array
jobs.push(file);
// Check whether each loop is on its last iteration
const filesDone = fileIndex >= files.length - 1;
const dirsDone = dirIndex >= dirs.length - 1;
const accsDone = accIndex >= accounts.length - 1;
// all three need to be true before we can run the callback
if (filesDone && dirsDone && accsDone) {
cb(jobs);
}
});
});
});
});
}
回答2:
Simpler solution
No need for loops and pushing to arrays
I noticed that all of the answers here use a lot of complicated code. You can make it much simpler:
let fs = require('mz/fs');
let path = require('path');
let d = ['pending', 'done', 'failed'];
let a = ['A', 'B', 'C']; // <-- example accounts
let paths = [].concat.apply([], d.map(d => (a.map(a => path.join(d,a)))));
Promise.all(paths.map(path => fs.readFile(path, 'utf-8'))).then(files => {
// you have all data here
}).catch(error => {
// handle errors here
});
Explanation
If you use the promise version of fs - currently you can use:
let fs = require('mz/fs');
with the mz module:
- https://www.npmjs.com/package/mz
and soon it will be native in Node, see:
- https://github.com/nodejs/node/pull/5020
then you will be able to do things like the code below. Using the data:
// directories:
let d = ['pending', 'done', 'failed'];
// accounts:
let a = ['A', 'B', 'C'];
You can easily create an array of paths:
let paths = [].concat.apply([], d.map(d => (a.map(a => path.join(d,a)))));
From which you can create an array of promises:
let promises = paths.map(path => fs.readFile(path, 'utf-8'));
You can even use Promise.all() to have all of your files read:
let data = Promise.all(promises);
Now you can use everything as:
data.then(files => {
// you have everything ready here
}).catch(error => {
// some error happened
});
Note: you need to require two modules for the above code to work:
let fs = require('mz/fs');
let path = require('path');
回答3:
You can use walk
walker.on("end", function () {
console.log("all done");
cb(jobs);
});
回答4:
Simple counter
One simple way could be just to keep a counter.
const scanFiles = function(accounts, cb) {
let dirs = ['pending', 'done', 'failed'];
let jobs = [];
// Variables to keep track of
const lastAccountIndex = accounts.length * dirs.length;
let indexCounter = 0;
accounts.forEach(function(account) {
dirs.forEach(function(dir) {
fs.readdir(account + '/' + dir, function(err, files) {
files.forEach(function(file) {
//do something
//add file to jobs array
jobs.push(file);
indexCounter++;
});
//return jobs array once all files have been added
if (lastAccountIndex === indexCounter) {
cb(jobs);
}
});
});
});
}
Promise
Alternatively, fs + promise could be very useful here.
const scanFiles = function(accounts) {
let dirs = ['pending', 'done', 'failed'];
let jobs = [];
const filePromises = [];
accounts.forEach(function(account) {
dirs.forEach(function(dir) {
filePromises.push(new Promise((resolve, reject) => {
fs.readdir(account + '/' + dir, function(err, files) {
files.forEach(function(file) {
resolve(file);
});
});
}));
});
});
return Promise.all(filePromises);
}
scanFiles(someAccounts)
.then((files) => {
files.forEach((file) => {
// At this point, iwll the files will be scanned
// So, do whatever you want with all the files here.
});
});
fs-promise
Or just use https://www.npmjs.com/package/fs-promise
回答5:
If you use asyc library https://caolan.github.io/async/docs.html, your code will be much faster. (forEach is blocking [JavaScript, Node.js: is Array.forEach asynchronous?).
const scanFiles = function (accounts, cb) {
let dirs = ['pending', 'done', 'failed'];
let jobs = [];
async.each(accounts, function (account, accountCallback) {
async.each(dirs, function (dir, dirCallback) {
fs.readdir(account + '/' + dir, function (err, files) {
if(err) console.log(err);
async.each(files, function (file, fileCallback) {
//do something
//add file to jobs array
jobs.push(file);
fileCallback();
}, dirCallback);
});
}, accountCallback);
}, function (err) {
//return jobs array once all files have been added
if (err) throw err;
cb(jobs)
});
};
回答6:
So the problem is that you were sending an empty result before fs.readdir was done because nodeJS is async. So the solution is to add the callback inside the fs.readdir function.
const scanFiles = function (accounts, cb) {
let dirs = ['pending', 'done', 'failed'];
let jobs = [];
accounts.forEach(function (account, i) {
dirs.forEach(function (dir, j) {
fs.readdir(account + '/' + dir, function (err, files) {
files.forEach(function (file, k) {
//do something
//add file to jobs array
jobs.push(file);
});
if (i === accounts.length - 1 && j === dirs.length - 1 && k === files.length - 1) {
//return jobs array once all files have been added
cb(jobs);
}
});
});
});
}
来源：https://stackoverflow.com/questions/43052015/javascript-callback-after-all-nested-foreach-loops-are-completed