Transform nested .map methods with async/await model

Question

I'm managing a JSON file like this example:

[
    {
        "Item": "Item1",
        "ItemChild": [
            {
                "Category": "Category1",
                "Inputs": [
                    {
                        "InputName": "1",
                        "InputTitle": "Title1",
                    },
                ]
            },
            {
                "Category": "Category2",
                "Inputs": [
                    {
                        "InputName": "2",
                        "InputTitle": "Title2",
                    },
                ]
            }
        ]
    }
]

I'm parsing this file, getting all the Category, InputName and InputTitle keys values and appending them in a file using this logic:

myParsedJSON.map(({ ItemChild }) => {
    ItemChild.map(({ Category, Inputs }) => {
         fs.appendFileSync(myfile, Category);
         Inputs.map(({ InputName, InputTitle }) => {
             fs.appendFileSync(myfile, InputName + ' - ' + InputTitle);
         });
     });
});

I want use an asynchronous version of this logic with async/await model and async operations. What's the best way to do that? Thanks

Matt · Accepted Answer · 2020-10-28 11:54:45Z

2

Array functions don't play nice with async so convert them to for of loops

const fsp = require('fs').promises

for (const { ItemChild } of myParsedJSON) {
    for (const { Category, Inputs } of ItemChild) {
        await fsp.appendFile(myfile, Category);
        for (const { InputName, InputTitle } of Inputs) {
            await fsp.appendFile(myfile, `${InputName} - ${InputTitle}`);
        }
    }
}

answered Oct 28, 2020 at 11:54

Matt

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1 Comment

Flavio Del Grosso Over a year ago

Thanks!! @Matt, this solution is very easy and suitable to my case

Jared Smith · Accepted Answer · 2020-10-28 11:41:52Z

1

You would have to really contort this to use async/await, although you could easily convert it to something that you could await somewhere else:

const text = myParsedJSON.map(({ ItemChild }) => {
    return ItemChild
        .map(({ Category, Inputs }) => {
            return Category + '\n' + Inputs
                .map(({ InputName, InputTitle }) => {
                    return InputName + ' - ' + InputTitle;
                })
                .join('\n');
        })
        .join('\n');
});

const awaitableResult = new Promise((resolve, reject) => {
    fs.appendFile(myFile, text, (err) => {
        if (err) reject(err);
        resolve(true);
    });
});

You can now await the awaitableResult in any async function:

async someFunction() {
    await awaitableResult; // file IO done.
    // do whatever needs to happen next.
}

Note that we do all of the data processing before we start writing to the file: writing to disk is s-l-o-w, you want to do it as little as possible. If you really want to stay with the piecemeal approach, use fs.writeStream.

answered Oct 28, 2020 at 11:41

Jared Smith

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1 Comment

Flavio Del Grosso Over a year ago

Thanks @Jared Smith, your answer is very clear and usefull for me.

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