How to use 'how to start json server' in JavaScript

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function startServer(server, port = 3000) {
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  server.use(bodyParser.json())
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  // The new /save-subscription endpoint
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  server.post('/save-subscription', async function(req, res){
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    const subscription = req.body
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    await saveToDatabase(subscription) //Method to save the subscription to Database
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    res.json({ message: 'success' })
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  })
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  server.get('/send-notification', function(req, res) {
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    const subscription = dummyDb.subscription //get subscription from your databse here.
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    if (subscription) {
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      const message = 'Hello World'
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      sendNotification(subscription, message)
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      res.json({ message: 'message sent' })
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    } else {
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      res.json( {error: 'nothing'})
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    }
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  })
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  server.start({
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    port: process.env.PORT || port,
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    endpoint: '/graphql',
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    playground: '/playground',
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    subscriptions: {
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      path: '/subscriptions',
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      onConnect,
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      onDisconnect,
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    }
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  }, (options) => {
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    console.log(`Server is running on http://localhost:${port}`)
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    const maxListenersExceededWarning = require('max-listeners-exceeded-warning');
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    maxListenersExceededWarning();
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    }
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  )
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}