Question

以下代码可以正常使用：

var fs         = require('fs');
var async      = require('async');
var addErrParm = function (err, done) {return function(exists) {
    done(err, exists);
}}
function testAsync() {var response = '';
    function checkIfTheFileExists(done) {
        fs.exists('file.txt', addErrParm(null, done));
    }
    function readTheFile(exists, done) {
        if (!exists) {
            done('notFound');
        } else {
            fs.readFile('file.txt', 'utf8', done);
        }
    }
    function showTheFile(err) {
        if (err) {
            response = err;
        } else {
            response = 'file was read';
        }
        console.log(response);
    }
    async.waterfall([ //  <-- waterfall
        checkIfTheFileExists,
        readTheFile
    ], showTheFile);
}
testAsync() // "file was read"

以下似乎不起作用。唯一的区别是第一次使用async.waterfall而不是async.series。

var fs         = require('fs');
var async      = require('async');
var addErrParm = function (err, done) {return function(exists) {
    done(err, exists);
}}
function testAsync() {var response = '';
    function checkIfTheFileExists(done) {
        fs.exists('file.txt', addErrParm(null, done));
    }
    function readTheFile(exists, done) {
        if (!exists) {
            done('notFound');
        } else {
            fs.readFile('file.txt', 'utf8', done);
        }
    }
    function showTheFile(err) {
        if (err) {
            response = err;
        } else {
            response = 'file was read';
        }
        console.log(response);
    }
    async.series([ //   <-- this is the only line of code that is different.
        checkIfTheFileExists,
        readTheFile
    ], showTheFile);
}
testAsync() //     <-- nothing was shown on the console, not even a blank line.

async.series版本未记录对控制台的任何响应。什么导致async.series版本没有响应？

我还尝试了一个使用async.parallelLimit的版本，将限制设置为1.我希望它由于限制而在系列中运行任务，但在控制台上又没有收到任何内容。这是async.parallelLimit版本：

var fs         = require('fs');
var async      = require('async');
var addErrParm = function (err, done) {return function(exists) {
    done(err, exists);
}}
function testAsync() {var response = '';
    function checkIfTheFileExists(done) {
        fs.exists('file.txt', addErrParm(null, done));
    }
    function readTheFile(exists, done) {
        if (!exists) {
            done('notFound');
        } else {
            fs.readFile('file.txt', 'utf8', done);
        }
    }
    function showTheFile(err) {
        if (err) {
            response = err;
        } else {
            response = 'file was read';
        }
        console.log(response);
    }
    async.parallelLimit([  //    <--- this line is different
        checkIfTheFileExists,
        readTheFile
    ], 1, showTheFile); //   <--- and this line is different.  I added the "1".
}
testAsync() //           <--- produces nothing on the console.

该文件确实存在于所有三个测试用例中。

Answer 1

async.series版本未记录对控制台的任何响应。什么导致async.series版本没有响应？

你误解了系列的语义。它像瀑布一样依次排列，但与瀑布不同，每个工人的功能完全独立。因此，checkIfTheFileExists的结果不会传递给readTheFile。系列没有这样做。

为什么瀑布和series / parallelLimit（1）之间的async.js不一致？

1 个答案: