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

The following code works fine:

以下代码工作正常:

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"
va