How to update object file with certain value, keeping other values intact, front to back using JSON, ? node.js

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Javascript

Node.js

I am sending a post request to node server that contains a JSON object with a value. Hoping to update a certain value, of an Object in a JSON file (no DB just experimental). I just cannot figure out how, I hope the code will be self-explanatory I put all the examples there

Server File Obj: (datafile Already on the server) { "Show": {"Cars": {"Name": "Audi A4","MaxSpeed": 260,"Color": "Blue"}}}

Server Post Obj: (req.body) {"Show": {"Cars": {"Name": "Audi TT"}}}

Problem is my current Code Returns: {"Show": {"Cars": {"Name": "Audi TT"}}} And removes "MaxSpeed": 260,"Color": "Blue" from the obj. Thanks for your effort in advance.

const dataFile = JSON.parse(fs.readFileSync(`${__dirname}/../Data/Data.json`)
    );
router.post('/app', (req, res, next) => {
// Update coming from Front-End

    let post = req.body
    let mutatedObj = Object.assign(dataFile, post)

fs.writeFile(`${__dirname}/../Data/Data.json`, JSON.stringify(mutatedObj), err => { res.send('we got your request !!') })
}

What I have tried:

Server Post Obj: (req.body) {"Show": {"Cars": {"Name": "Audi TT"}}}