CSV to JSON JavaScript — Converter + Code Examples

function csvToJson(csv) {
  const lines = csv.trim().split('\n')
  const headers = lines[0].split(',').map(h => h.trim())

  const rows = lines.slice(1)
    .filter(line => line.trim() !== '')
    .map(line => {
      const values = line.split(',')
      return Object.fromEntries(
        headers.map((header, i) => [header, values[i]?.trim()])
      )
    })

  return JSON.stringify(rows, null, 2)
}

const csv = `server,port,region,status
api-gateway,8080,eu-north-1,healthy
auth-service,8443,eu-west-1,degraded
payments-api,9090,eu-central-1,healthy`

console.log(csvToJson(csv))
// [
//   { "server": "api-gateway", "port": "8080", "region": "eu-north-1", "status": "healthy" },
//   { "server": "auth-service", "port": "8443", "region": "eu-west-1", "status": "degraded" },
//   { "server": "payments-api", "port": "9090", "region": "eu-central-1", "status": "healthy" }
// ]

function coerceValue(val) {
  if (val === undefined || val === '') return null
  if (val === 'true') return true
  if (val === 'false') return false
  if (val === 'null') return null
  const num = Number(val)
  if (!isNaN(num) && val.trim() !== '') return num
  return val
}

function csvToJson(csv, { coerce = false } = {}) {
  const lines = csv.trim().split('\n')
  const headers = lines[0].split(',').map(h => h.trim())

  const rows = lines.slice(1)
    .filter(line => line.trim() !== '')
    .map(line => {
      const values = line.split(',')
      return Object.fromEntries(
        headers.map((header, i) => [
          header,
          coerce ? coerceValue(values[i]?.trim()) : values[i]?.trim()
        ])
      )
    })

  return JSON.stringify(rows, null, 2)
}

const csv = `endpoint,port,max_connections,debug
/api/v2/orders,8443,500,true
/api/v2/health,8080,100,false`

console.log(csvToJson(csv, { coerce: true }))
// [
//   { "endpoint": "/api/v2/orders", "port": 8443, "max_connections": 500, "debug": true },
//   { "endpoint": "/api/v2/health", "port": 8080, "max_connections": 100, "debug": false }
// ]

// Bygg en Map från CSV rubrik→värde-par
const headers = ['server', 'port', 'region']
const values = ['payments-api', '9090', 'eu-central-1']
const rowMap = new Map(headers.map((h, i) => [h, values[i]]))

// Map serialiseras INTE direkt
console.log(JSON.stringify(rowMap))
// "{}"  — tomt objekt, data förlorad!

// Konvertera till vanligt objekt först
const rowObj = Object.fromEntries(rowMap)
console.log(JSON.stringify(rowObj, null, 2))
// {
//   "server": "payments-api",
//   "port": "9090",
//   "region": "eu-central-1"
// }

class EmployeeRecord {
  constructor(csvRow) {
    this._rowIndex = csvRow._rowIndex  // internt, inte för JSON
    this.employeeId = csvRow.employee_id
    this.name = csvRow.name
    this.hiredAt = new Date(csvRow.hired_date)
    this.salary = Number(csvRow.salary)
  }

  toJSON() {
    // Exponera bara fält vi vill ha i JSON-utdata
    return {
      employee_id: this.employeeId,
      name: this.name,
      hired_at: this.hiredAt,  // Date.toJSON() → ISO-sträng automatiskt
      salary: this.salary,
    }
  }
}

const csvRow = {
  _rowIndex: 42,
  employee_id: 'EMP-2847',
  name: 'Erik Johansson',
  hired_date: '2024-03-15',
  salary: '128000',
}

const record = new EmployeeRecord(csvRow)
console.log(JSON.stringify(record, null, 2))
// {
//   "employee_id": "EMP-2847",
//   "name": "Erik Johansson",
//   "hired_at": "2024-03-15T00:00:00.000Z",
//   "salary": 128000
// }
// Obs: _rowIndex är exkluderat, lön är ett tal, datum är i ISO-format

const jsonString = '{"employee_id":"EMP-2847","name":"Erik Johansson","hired_at":"2024-03-15T00:00:00.000Z","salary":128000}'

const isoDatePattern = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}/

const parsed = JSON.parse(jsonString, (key, value) => {
  if (typeof value === 'string' && isoDatePattern.test(value)) {
    return new Date(value)
  }
  return value
})

console.log(parsed.hired_at instanceof Date)  // true
console.log(parsed.hired_at.getFullYear())    // 2024

const csv = `endpoint,method,avg_latency_ms,error_rate
/api/v2/orders,POST,342,0.02
/api/v2/health,GET,12,0.00
/api/v2/payments,POST,890,0.15
/api/v2/users,GET,45,0.01`

// Steg 1: konvertera CSV till JSON-sträng
const jsonString = csvToJson(csv, { coerce: true })

// Steg 2: parsa tillbaka till JavaScript-array
const endpoints = JSON.parse(jsonString)

// Verifiera att det är en array
console.log(Array.isArray(endpoints))  // true

// Filtrera slutpunkter med hög latens
const slow = endpoints.filter(ep => ep.avg_latency_ms > 200)
console.log(slow.map(ep => ep.endpoint))
// ["/api/v2/orders", "/api/v2/payments"]

// Destrukturera den första raden
const [first, ...rest] = endpoints
console.log(first.endpoint)  // "/api/v2/orders"
console.log(rest.length)     // 3

function safeParse(jsonString) {
  try {
    return { data: JSON.parse(jsonString), error: null }
  } catch (err) {
    return { data: null, error: err.message }
  }
}

// Giltig utdata
const result = safeParse(csvToJson(csv))
if (result.error) {
  console.error('CSV-konvertering producerade ogiltig JSON:', result.error)
} else {
  console.log(`Parsade ${result.data.length} rader`)
}

// Av misstag skicka rå CSV till JSON.parse — detta misslyckas
const bad = safeParse('name,email\nErik,erik@nexuslabs.se')
console.log(bad.error)  // "Unexpected token 'a', "name,email"... is not valid JSON"

const jsonString = csvToJson(`employeeId,firstName,hiredDate
EMP-2847,Erik,2024-03-15
EMP-3012,Anna,2023-11-01`, { coerce: false })

const camelToSnake = str => str.replace(/[A-Z]/g, c => '_' + c.toLowerCase())

const employees = JSON.parse(jsonString, function(key, value) {
  // Rotvärde — validera form
  if (key === '') {
    if (!Array.isArray(value)) throw new Error('Förväntade JSON-array från CSV')
    return value
  }
  // Byt namn på camelCase-rubriknycklar till snake_case
  if (typeof value === 'object' && value !== null && !Array.isArray(value)) {
    return Object.fromEntries(
      Object.entries(value).map(([k, v]) => [camelToSnake(k), v])
    )
  }
  return value
})

console.log(employees[0])
// { employee_id: 'EMP-2847', first_name: 'Erik', hired_date: '2024-03-15' }

import { readFileSync, writeFileSync } from 'node:fs'

function csvToJsonFromFile(inputPath, outputPath) {
  let csvText
  try {
    csvText = readFileSync(inputPath, 'utf8')
  } catch (err) {
    throw new Error(`Misslyckades med att läsa ${inputPath}: ${err.message}`)
  }

  const lines = csvText.trim().split('\n')
  if (lines.length < 2) {
    throw new Error(`${inputPath} har inga datarader (bara ${lines.length} rad)`)
  }

  const headers = lines[0].split(',').map(h => h.trim())
  const rows = lines.slice(1)
    .filter(line => line.trim() !== '')
    .map(line => {
      const values = line.split(',')
      return Object.fromEntries(headers.map((h, i) => [h, values[i]?.trim()]))
    })

  const jsonOutput = JSON.stringify(rows, null, 2)
  writeFileSync(outputPath, jsonOutput, 'utf8')
  console.log(`Konverterade ${rows.length} rader → ${outputPath}`)
  return rows
}

// Användning
const data = csvToJsonFromFile('inventory.csv', 'inventory.json')
console.log(data[0])
// { sku: "WDG-2847", warehouse: "eu-north-1", quantity: "150", ... }

async function fetchCsvAsJson(url) {
  const response = await fetch(url)
  if (!response.ok) {
    throw new Error(`HTTP ${response.status}: ${response.statusText}`)
  }

  const contentType = response.headers.get('content-type') || ''
  if (!contentType.includes('text/csv') && !contentType.includes('text/plain')) {
    console.warn(`Oväntat content-type: ${contentType}`)
  }

  const csvText = await response.text()
  const lines = csvText.trim().split('\n')
  const headers = lines[0].split(',').map(h => h.trim())
  const rows = lines.slice(1)
    .filter(line => line.trim() !== '')
    .map(line => {
      const values = line.split(',')
      return Object.fromEntries(headers.map((h, i) => [h, values[i]?.trim()]))
    })

  return rows
}

// Exempel: hämta valutakurs-CSV från en dataleverantör
try {
  const rates = await fetchCsvAsJson('https://data.ecb.internal/rates/daily.csv')
  console.log(JSON.stringify(rates.slice(0, 3), null, 2))
  // Skicka som JSON till nedströmstjänst
  await fetch('https://api.internal/v2/rates', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ data: rates }),
  })
} catch (err) {
  console.error('Kurssynkronisering misslyckades:', err.message)
}

# Pipe CSV till ett Node.js inline-skript
cat servers.csv | node -e "
  const lines = require('fs').readFileSync('/dev/stdin','utf8').trim().split('\n');
  const h = lines[0].split(',');
  const rows = lines.slice(1).map(l => Object.fromEntries(h.map((k,i) => [k.trim(), l.split(',')[i]?.trim()])));
  console.log(JSON.stringify(rows, null, 2));
"

# Miller är en schweizisk armékniv för strukturerad data
# Installera: brew install miller (macOS) eller apt install miller (Debian/Ubuntu)
mlr --icsv --ojson cat inventory.csv

# Filtrera rader under konvertering
mlr --icsv --ojson filter '$quantity > 100' inventory.csv

# Välj specifika kolumner
mlr --icsv --ojson cut -f sku,warehouse,quantity inventory.csv

# Installera globalt
npm install -g csvtojson

# Konvertera fil
csvtojson servers.csv > servers.json

# Pipe från stdin
cat exports/q1-metrics.csv | csvtojson > q1-metrics.json

npm install papaparse

import Papa from 'papaparse'

const csv = `product,description,price,in_stock
"Widget, Large","A premium widget with ""extra"" features",29.99,true
Bolt Assembly,Standard M8 bolt kit,4.50,true
"Gasket Set","Includes gasket, seal, and O-ring",12.75,false`

const { data, errors, meta } = Papa.parse(csv, {
  header: true,
  dynamicTyping: true,     // konverterar automatiskt tal och booleaner
  skipEmptyLines: true,
  transformHeader: h => h.trim().toLowerCase().replace(/\s+/g, '_'),
})

if (errors.length > 0) {
  console.error('Parsningsfel:', errors)
}

console.log(JSON.stringify(data, null, 2))
// [
//   {
//     "product": "Widget, Large",
//     "description": "A premium widget with \"extra\" features",
//     "price": 29.99,
//     "in_stock": true
//   },
//   ...
// ]
console.log(`Parsade ${data.length} rader, avgränsare: "${meta.delimiter}"`)

npm install cli-highlight

import { highlight } from 'cli-highlight'

// Efter att ha konverterat CSV till JSON-array
const jsonOutput = JSON.stringify(rows, null, 2)

// Skriv ut med syntaxmarkering
console.log(highlight(jsonOutput, { language: 'json' }))
// Nycklar, strängar, tal och booleaner får var sina distinkta färger

import { createReadStream, createWriteStream } from 'node:fs'
import { parse } from 'csv-parse'
import { Transform } from 'node:stream'
import { pipeline } from 'node:stream/promises'

// Transformera varje CSV-radobjekt till en JSON-rad
const toNdjson = new Transform({
  objectMode: true,
  transform(record, encoding, callback) {
    callback(null, JSON.stringify(record) + '\n')
  },
})

await pipeline(
  createReadStream('telemetry-2026-03.csv'),
  parse({
    columns: true,       // använd första raden som rubriker
    skip_empty_lines: true,
    trim: true,
    cast: true,          // konvertera automatiskt tal och booleaner
  }),
  toNdjson,
  createWriteStream('telemetry-2026-03.ndjson')
)

console.log('Strömmande konvertering klar')
// Varje rad i utdatafilen är ett JSON-objekt:
// {"timestamp":"2026-03-15T08:22:00Z","service":"gateway","latency_ms":42,"status":200}
// {"timestamp":"2026-03-15T08:22:01Z","service":"auth","latency_ms":156,"status":401}
// ...

import Papa from 'papaparse'
import { createReadStream } from 'node:fs'

let rowCount = 0
const errors = []

const fileStream = createReadStream('warehouse-inventory.csv')

Papa.parse(fileStream, {
  header: true,
  dynamicTyping: true,
  step(result) {
    // Bearbeta en rad åt gången — konstant minne
    rowCount++
    if (result.data.quantity === 0) {
      errors.push(`Rad ${rowCount}: ${result.data.sku} är slut i lager`)
    }
  },
  complete() {
    console.log(`Bearbetade ${rowCount} rader`)
    if (errors.length > 0) {
      console.log(`Problem hittades: ${errors.length}`)
      errors.forEach(e => console.log(`  ${e}`))
    }
  },
  error(err) {
    console.error('Parsning misslyckades:', err.message)
  },
})