javascript - Node 异步库 filterLimit 总是返回 undefined

Question

我正在尝试异步过滤文件路径列表以仅返回现有文件。我正在使用 async 库的 filterLimit连同 fs-extra。功能如下:

const asyncFilter = require('async').filterLimit;
const fsExtra = require('fs-extra');

async function filterFiles(fileList){
  const existing = await asyncFilter(fileList, 5, (file, callback) => {
    console.log(file); // looks good

    fsExtra.pathExists(file, (err, exists) => {
       const result = err ? false : exists;
       console.log(result); // looks good (boolean)

       callback(null, result);
    });
  });

  console.log(existing); // always undefined...
}

Answer 1

编辑 2:下面的两个建议都不起作用...这是一个工作版本，其中已弃用的 fs.exists() 被替换为 fs。访问()

const async = require('async');
const fs = require('fs');

async function filterFiles(fileList) {
    try {

        const result = await async.filterLimit(fileList, 5, function(filePath, callback) {
            fs.access(filePath, function(err) {
                callback(null, !err)
            });
        });

        console.log(result);
    } catch(err) {
        console.log(err);
    }
};

---

你不能简单地做:

不工作

    const asyncFilter = require('async').filterLimit;
    const fs = require('fs');

    async function filterFiles(fileList) {
        let result = [];
        await asyncFilter(fileList, 5, fs.exists, function(existing) {
            // array of existing files
            console.log(existing);
            // now assign the existing files to result array
            result = existing;
        });
        // should be the same as existing in callback function
        console.log(result);
    };

不需要 fs-extra。我认为您的问题是 async.filterLimit() 不返回任何内容，而是执行回调函数，因此仅通过将 async.filterLimit() 分配给变量并不意味着你在那里得到结果，你会得到 undefined 因为函数不返回任何东西!

编辑:根据@Jaromanda X 的评论从上面改编的片段:

    const asyncFilter = require('async').filterLimit;
    const fs = require('fs');

    async function filterFiles(fileList) {
        let result = await asyncFilter(fileList, 5, fs.exists);
        // should contain only existing files
        console.log(result);
    };