async.waterfall复制调用

Question

我有下面的异步代码。

  for fileName in sourceFiles
    console.log 'dealing with ', fileName
    async.waterfall [
      (callback) ->
        console.log "going to read ", fileName
        fs.readFile fileName, (err, content) ->
          if err then throw err
          callback null, fileName, content
          return
      (fileName, content, callback) ->
        console.log 'length of: ',  fileName, ' is: ', content.length

我预计输出会是这样的：

dealing with file1
going to read file1
length of file1 is 10
dealing with file2
going to read file2
length of file 2 is 20

相反，我得到的是：

dealing with file1
dealing with file2
going to read file2
going to read file2 <- note it is the same file repeated
length of file2 is 20
length of file2 is 20

我不明白为什么会这样。(这是一个coffeescript没有问题。它在JS中也是相同的输出)

Answer 1

async库不会复制调用，但是这里的作用域有问题。

当调用第一个函数时，循环中的fileName已经设置为file2，您应该尝试将其包装到另一个函数中，以便获得一个新的作用域，如下所示：

for fileName in sourceFiles
  (fileName) ->
    console.log 'dealing with ', fileName
    async.waterfall [
      (callback) ->
        console.log "going to read ", fileName
        fs.readFile fileName, (err, content) ->
          if err then throw err
          callback null, fileName, content
          return
      (fileName, content, callback) ->
        console.log 'length of: ',  fileName, ' is: ', content.length

Answer 2

看起来async.waterfall也是异步的，所以您的循环在瀑布回调之前终止，并且使用相同的fileName调用瀑布2次。

但是在这里，瀑布是没有用的，你可以简单地使用同步的readFileSync。

http://nodejs.org/api/fs.html#fs_fs_readfilesync_filename_options