If you use fs.readFile and want to do an action after the file has been read, you must do the action (write and read a file in your case) inside the callback function. Also, you can use fs.readFileSync if you can read synchronously.

First off, we need to explain a few things:

fs.readFile() is non-blocking and asynchronous. That means that when you call it, it starts the operation and then returns immediately and starts the execute the code that comes right after it. Then, some time later, it calls its callback.

So, your code is:

1. Calling fs.readFile()
2. Then, immediately setting a timer
3. Then, it's an indeterminate race between the fs.readFile() callback and the timer to see who finishes first. If the timer finishes first, then it will call its callback and you will attempt to access data BEFORE it has been filled in (because the fs.readFile() callback has not yet been called).

You cannot write reliable code this way as you are guessing on the timing of indeterminate, asynchronous operations. Instead, you have to use the asynchronous result from within the callback because that's the only place that you know the timing for when it finished and thus when it's valid. So, one way to solve your problem is by chaining the asynchronous operations so you put the second one INSIDE the callback of the first:

function timeSeries(obj, callback) {
    //read file named as passed object's _id
    fs.readFile("./api/assignment_data/" + obj._id + ".json", "utf-8", function read(err, datas) {
        if (err) {
            console.log(err);
            // tell caller about our error
            callback(err)
            return;
        } else {
            let data = [{original_value: []}];
        
            const filedata = JSON.parse(datas);
            for (let line of filedata) {
                data[0].original_value.push(line.original_value);
            }

            fs.writeFile("./api/timeseries.json", JSON.stringify(data), { encoding: 'utf8' }, (err) => {
                if (err) {
                    console.log(err);
                    callback(err);
                    return;
                } else {
                    // give our data to the caller
                    callback(data);
                }
            });
        }
    })
}

Then, to call this function, you pass it a callback and in the callback you can either see the error or get the data.

In modern nodejs, it's a bit easier to use async/await and the promise-based interfaces in the fs module:

const fsp = require('fs').promises;

async function timeSeries(obj) {
    //read file named as passed object's _id
    try {
        let datas = await fsp.readFile("./api/assignment_data/" + obj._id + ".json", "utf-8");
        const filedata = JSON.parse(datas);
        let data = [{original_value: []}];

        for (let line of filedata) {
            data[0].original_value.push(line.original_value);
        }

        await fsp.writeFile("./api/timeseries.json", JSON.stringify(data), { encoding: 'utf8' });
        return data;
    } catch(e) {
        console.log(e);
        // handle error here or throw back to the caller
        throw e;
    }
}

For this version, the caller can use await and try/catch to get errors:

try {
    let data = await timeSeries(obj);
    // do something with data here
} catch(e) {
    // handle error here
}

Based on what code you have written , I could just modify it using simple async-await - hope this helps

import fs from 'fs'
async function timeSeries(obj) {
    const data = [{
        original_value: []
    }]
    const assData = fs.readFileSync('./api/assignment_data/metrics.json', 'utf8')
    const filedata = JSON.parse(assData)
    filedata.map(line => data[0].original_value.push(line.original_value))
    // no need for timeOut 
    fs.writeFileSync('./api/timeseries.json', JSON.stringify(data));
    //return data if u need
    return data
}

router.route("/api/debug/:num").get(async (req, res) => {
        try {
            const metricData = fs.readFileSync('./api/assignment_data/metrics.json', 'utf8')

            const objdata = JSON.parse(data)

            const timeSeriesData = await timeSeries(objdata[req.params.num])
            // returning TimeSeriesData
            res.status(200).json(timeSeriesData)
        })
}
catch (error) {
    res.status(500).send(error.message)
}