如何在没有嵌套回调的情况下访问变量？

Question

我有一个使用Node FileSystem模块的简单嵌套回调，并且在尝试访问仅由于作用域链而变得可用的变量时遇到了困难。 我的目标是尽可能减少嵌套的回调。

var fs = require('fs');
var directory = '/Users/johndoe/desktop/temp';

fs.readdir(directory, function(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, function(err, data) {
      // I have access to filePath variable here.  This works just fine.
      console.log(filePath);
    });
  });
});

但这是我想写的：

var fs = require('fs');
var directory = '/Users/johndoe/desktop/temp';

fs.readdir(directory, processFiles);

function processFiles(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, processSingleFile);
  });
}

function processSingleFile(err, data) {
  // how do I get the filePath variable here?
  console.log(filePath);
}

在第二个示例中，如何在此处获取filePath变量？

Answer 1

您可以通过绑定将filePath作为第一个参数传递给processSingleFile 。

function processFiles(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, processSingleFile.bind(this, filePath));
  });
}

function processSingleFile(filePath, err, data) {
  console.log(filePath);
}

Answer 2

将processSingleFile更改为function returning function ，并将变量filePath作为变量传递给该function returning function 。 像波纹管

function processSingleFile(filePath) {
  return function(err, data){
    console.log(filePath);
    console.log(data);
  }
}

叫它像波纹管

fs.readFile(filePath, processSingleFile(filePath));

Answer 3

var fs = require('fs');
var directory = '/Users/johndoe/desktop/temp';

fs.readdir(directory, processFiles);

// declaring filePath outside to allow scope access
var filePath;

function processFiles(err, files) {
  files.forEach(function(file) {
    filePath = directory + "/" + file;
    fs.readFile(filePath, processSingleFile);
  });
}

function processSingleFile(err, data) {
  // You can access filePath here
  console.log(filePath);
}

Answer 4

function processFiles(err, files) {
  files.forEach(function(file) {
    var filePath = directory + "/" + file;
    fs.readFile(filePath, function(err,data){
        processSingleFile(data,filePath);
    });
  });
}

function processSingleFile(data, filePath) {
  // You can access filePath here
  console.log(filePath);
}