It is really simple creating an express API server.

All you have to do is basically requiring express, creating/importing the JSON data, setting up a route, setting up a listening port, and bam there is API access to your data.

Steps for creating an Express API server:

Step 1: Importing Express, assigning it to app variable

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const express = require('express')
const app = express()

Step 2: Creating JSON data

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let persons = [
    {
      "id": 1,
      "name": "Arto Hellas",
      "number": "040-123456"
    },
    {
      "id": 2,
      "name": "Ada Lovelace",
      "number": "39-44-5323523"
    },
    {
      "id": 3,
      "name": "Dan Abramov",
      "number": "12-43-234345"
    },
    {
      "id": 4,
      "name": "Mary Poppendieck",
      "number": "39-23-6423122"
    }
]

Step 3: Setting up a route

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app.get('/api/persons', (request, response) => {
  response.json(persons)
})

Step 4: Setting up a port:

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const PORT = 3001

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`)
})