如何在 Node.js 中跳过“async.forEachOf”循环迭代

Question

async.waterfall 嵌套在 async.forEachOfLimit 循环中，如下面的代码所示。

问题：当代码在async.waterfall 中执行一个步骤时，如何跳过async.forEachLimit 的迭代？换句话说，突破async.waterfall 并回到async.forEachLimit。我已经在代码中注释了应该进行此检查的位置

当前代码报错Callback was already called.

另外，如果我想突破async.waterfall 时使用return callback() 代替cb()，则不会发生错误，但不会被跳过。

var async = require('async')
var users = ['a','b','c']

async.forEachOfLimit(users, 1, function(user, index, cb) {

    console.log(index + ': ' + user)

    async.waterfall([
        function(callback) {
            callback(null);
        },
        function(callback) {
            // Skip async.forEAchOfLimit iteration when index == 1
            if(index == 1)
                cb()

            callback(null);
        }
    ], function (err, result) {
        console.log(index + ": done")
        cb()
    });

}, function() {
    console.log('ALL done')
})

错误

0: a
0: done
1: b
2: c
1: done

/Users/x/test/node_modules/async/lib/async.js:43
            if (fn === null) throw new Error("Callback was already called.");
                                   ^
Error: Callback was already called.

期望的输出

0: a
0: done
1: b
2: c
2: done
ALL done

---

使用return callback()

var async = require('async')
var users = ['a','b','c']

async.forEachOfLimit(users, 1, function(user, index, cb) {

    console.log(index + ': ' + user)

    async.waterfall([
        function(callback) {
            callback(null);
        },
        function(callback) {
            // Skip async.forEAchOfLimit iteration when index == 1
            if(index == 1)
                return callback()

            callback(null);
        }
    ], function (err, result) {
        console.log(index + ": done")
        cb()
    });

}, function() {
    console.log('ALL done')
})

输出

不会爆发...

0: a
0: done
1: b
1: done
2: c
2: done
ALL done

Answer 1

在您的第一个解决方案中，当索引匹配 1 时，cb 被调用两次，这就是为什么您不断收到 Callback was already called 错误的原因。尽管您调用 forEachOfLimit 回调 cb，但您的代码不会停止执行并调用回调。在回调函数cb 中再执行一次。

var async = require('async')
var users = ['a','b','c']

async.forEachOfLimit(users, 1, function(user, index, cb) {

    console.log(index + ': ' + user)

    async.waterfall([
        function(callback) {
            callback(null);
        },
        function(callback) {
            // Skip async.forEAchOfLimit iteration when index == 1
            if(index == 1)
                cb() // First callback call

            callback(null);
        }
    ], function (err, result) {
        console.log(index + ": done")
        cb() // Second callback call
    });

}, function() {
    console.log('ALL done')
})

在第二种解决方案中，如果索引匹配 1，它会调用不带参数的回调，并跳过带空参数的回调。仍然没有跳出瀑布。

要使用瀑布解决您的问题，您有两种选择。

1. 使用错误参数调用瀑布的方法回调，它会跳出瀑布并在瀑布的回调中处理此错误。

   var async = require('async')
   var users = ['a','b','c']
   
   async.forEachOfLimit(users, 1, function(user, index, cb) {
   
       console.log(index + ': ' + user)
   
       async.waterfall([
           function(callback) {
               callback(null);
           },
           function(callback) {
               // Skip async.forEAchOfLimit iteration when index == 1
               if(index == 1)
                   return callback(new Error('Index equals 1'));
   
               callback(null);
           }
       ], function (err, result) {
           console.log(index + ": done")
   
           if(err.message == 'Index equals 1') {
               err = null; // If you want to continue executing forEachOfLimit no error must be passed to cb
           }
   
           cb(err, result);
       });
   
   }, function() {
       console.log('ALL done')
   });
   

2. 在每个瀑布式方法的开头跳过其余代码并立即调用回调（这是您在第二次尝试中所做的）

   var async = require('async')
   var users = ['a','b','c']
   
   async.forEachOfLimit(users, 1, function(user, index, cb) {
   
       console.log(index + ': ' + user)
   
       async.waterfall([
           function(callback) {
               callback(null);
           },
           function(callback) {
               // Skip execution of the rest of waterfall method immediately
               if(index == 1)
                   return callback()
   
               // Some additional code here
   
               callback(null);
           }
       ], function (err, result) {
           console.log(index + ": done")
           cb()
       });
   
   }, function() {
       console.log('ALL done')
   })