Write file async node.js , understand why is not working

Question

I have this function and is working fine:

        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');
            })
        })

But if I want to delete the var playlistString and use the JSON.stringify directly in is not working, the file is written with an undefined.

        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');
            })
        })

why ?

molamk · Accepted Answer · 2019-02-08 20:34:37Z

2

The problem is caused by the scoping of this. In the second code block when you write this.playlist, it refers to the calling function. In this case, it's your callback function (err)... who holds this.

To solve the problem assign this to a variable, then use that variable to refer to the context you want.

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');
    })
})

answered Feb 8, 2019 at 20:34

molamk

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2 Comments

Merlyn007 Over a year ago

Yes is that, thanks for the answer, anyway ill use the first solution to avoid use this out of the scope.

molamk Over a year ago

yes, that's what I would do too. Much cleaner to leave this as clean as possible

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