写文件异步node.js的，明白为什么不工作

我有这个功能，是工作的罚款：

        const playlistPath = path.join(config.PLAYLIST_PATH, this.playlist.id + '.json')
        let playlistString = JSON.stringify(this.playlist)

        mkdirp(config.PLAYLIST_PATH, function (_) {
            fs.writeFile(playlistPath, playlistString, function (err) {
                if (err) return console.log('error saving album to playlist %s: %o', playlistPath, err)
                console.log('The album has been added to the playlist');
            })
        })

但是，如果我想删除了var playlistString并直接在不工作使用JSON.stringify，该文件中写入一个未定义的。

        const playlistPath = path.join(config.PLAYLIST_PATH, this.playlist.id + '.json')

        mkdirp(config.PLAYLIST_PATH, function (_) {
            fs.writeFile(playlistPath, JSON.stringify(this.playlist), function (err) {
                if (err) return console.log('error saving album to playlist %s: %o', playlistPath, err)
                console.log('The album has been added to the playlist');
            })
        })

为什么呢？

问题是由this的作用域造成的。在当你写this.playlist第二个代码块，它指的是调用函数。在这种情况下，它是谁拥有function (err)...回调this。

为了解决这个问题分配this给一个变量，然后使用该变量引用到你想要的内容。

const playlistPath = path.join(config.PLAYLIST_PATH, this.playlist.id + '.json');

// Hold the value of "this" in a variable "ref"
const ref = this;

mkdirp(config.PLAYLIST_PATH, function (_) {
    fs.writeFile(playlistPath, JSON.stringify(ref.playlist), function (err) {
        if (err) return console.log('error saving album to playlist %s: %o', playlistPath, err)
        console.log('The album has been added to the playlist');
    })
})

写文件异步node.js的，明白为什么不工作

我有这个功能，是工作的罚款：

        const playlistPath = path.join(config.PLAYLIST_PATH, this.playlist.id + '.json')
        let playlistString = JSON.stringify(this.playlist)

        mkdirp(config.PLAYLIST_PATH, function (_) {
            fs.writeFile(playlistPath, playlistString, function (err) {
                if (err) return console.log('error saving album to playlist %s: %o', playlistPath, err)
                console.log('The album has been added to the playlist');
            })
        })

但是，如果我想删除了var playlistString并直接在不工作使用JSON.stringify，该文件中写入一个未定义的。

        const playlistPath = path.join(config.PLAYLIST_PATH, this.playlist.id + '.json')

        mkdirp(config.PLAYLIST_PATH, function (_) {
            fs.writeFile(playlistPath, JSON.stringify(this.playlist), function (err) {
                if (err) return console.log('error saving album to playlist %s: %o', playlistPath, err)
                console.log('The album has been added to the playlist');
            })
        })

为什么呢？

问题是由this的作用域造成的。在当你写this.playlist第二个代码块，它指的是调用函数。在这种情况下，它是谁拥有function (err)...回调this。

为了解决这个问题分配this给一个变量，然后使用该变量引用到你想要的内容。

const playlistPath = path.join(config.PLAYLIST_PATH, this.playlist.id + '.json');

// Hold the value of "this" in a variable "ref"
const ref = this;

mkdirp(config.PLAYLIST_PATH, function (_) {
    fs.writeFile(playlistPath, JSON.stringify(ref.playlist), function (err) {
        if (err) return console.log('error saving album to playlist %s: %o', playlistPath, err)
        console.log('The album has been added to the playlist');
    })
})