JavaScript Base64 エンコードガイド：btoa() と Buffer の使い方

// Encoding an API credential pair for an HTTP Basic Auth header
const serviceId  = 'deploy-bot'
const apiKey     = 'sk-prod-a7f2c91e4b3d8'

const credential = btoa(`${serviceId}:${apiKey}`)
// → 'ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg='

const headers = new Headers({
  Authorization: `Basic ${credential}`,
  'Content-Type': 'application/json',
})

console.log(headers.get('Authorization'))
// Basic ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg=

// Round-trip: encode, transmit, decode
const payload = 'service:payments region:eu-west-1 env:production'

const encoded = btoa(payload)
const decoded = atob(encoded)

console.log(encoded)
// c2VydmljZTpwYXltZW50cyByZWdpb246ZXUtd2VzdC0xIGVudjpwcm9kdWN0aW9u

console.log(decoded === payload) // true

btoa('Müller & Søren') // ❌ Uncaught DOMException: String contains an invalid character
btoa('résumé')         // ❌ 'é' is U+00E9 = 233 — within Latin-1, this one actually works
btoa('田中太郎')         // ❌ Throws — all CJK characters are above U+00FF

// Utility functions for Unicode-safe Base64
function toBase64(text: string): string {
  const bytes = new TextEncoder().encode(text)
  const chars = Array.from(bytes, byte => String.fromCharCode(byte))
  return btoa(chars.join(''))
}

function fromBase64(encoded: string): string {
  const binary = atob(encoded)
  const bytes  = Uint8Array.from(binary, ch => ch.charCodeAt(0))
  return new TextDecoder().decode(bytes)
}

// Works with any language or script
const orderNote = '確認済み：田中太郎 — 東京倉庫、数量：250'
const encoded   = toBase64(orderNote)
const decoded   = fromBase64(encoded)

console.log(encoded)
// 56K644GX44G/77ya55Sw5Lit5aSq6YOO4oCU5p2x5Lqs5YCJ5bqr44CB5pWZ6K+V77yaMjUw

console.log(decoded === orderNote) // true

// Encoding a server configuration object for storage in an env variable
const dbConfig = JSON.stringify({
  host:           'db-primary.internal',
  port:           5432,
  database:       'analytics_prod',
  maxConnections: 100,
  ssl:            { rejectUnauthorized: true },
})

const encoded = Buffer.from(dbConfig, 'utf8').toString('base64')
console.log(encoded)
// eyJob3N0IjoiZGItcHJpbWFyeS5pbnRlcm5hbCIsInBvcnQiOjU0MzIsImRhdGFiYXNlIjoiYW5h...

// Decoding back
const decoded = Buffer.from(encoded, 'base64').toString('utf8')
const config  = JSON.parse(decoded)

console.log(config.host)           // db-primary.internal
console.log(config.maxConnections) // 100

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

// Read a TLS certificate and encode it for embedding in a config file
const certPem     = readFileSync(join(process.cwd(), 'ssl', 'server.crt'))
const certBase64  = certPem.toString('base64')

// Store as a single-line string — suitable for env vars or JSON configs
writeFileSync('./dist/cert.b64', certBase64, 'utf8')

console.log(`Certificate encoded: ${certBase64.length} characters`)
// Certificate encoded: 2856 characters

// Restore the binary cert from the encoded value
const restored = Buffer.from(certBase64, 'base64')
console.log(restored.equals(certPem)) // true

import { readFile } from 'node:fs/promises'

async function encodeFileToBase64(filePath: string): Promise<string> {
  try {
    const buffer = await readFile(filePath)
    return buffer.toString('base64')
  } catch (err) {
    const code = (err as NodeJS.ErrnoException).code
    if (code === 'ENOENT') throw new Error(`File not found: ${filePath}`)
    if (code === 'EACCES') throw new Error(`Permission denied: ${filePath}`)
    throw err
  }
}

// Encode a PDF for an email attachment payload
const reportBase64 = await encodeFileToBase64('./reports/q1-financials.pdf')

const emailPayload = {
  to:          'finance-team@company.internal',
  subject:     'Q1 Financial Report',
  attachments: [{
    filename:    'q1-financials.pdf',
    content:     reportBase64,
    encoding:    'base64',
    contentType: 'application/pdf',
  }],
}

console.log(`Attachment: ${reportBase64.length} chars`)

function toBase64Url(text: string): string {
  // For ASCII-safe input (e.g., JSON with only ASCII chars)
  return btoa(text)
    .replace(/+/g, '-')
    .replace(///g, '_')
    .replace(/=/g, '')
}

function fromBase64Url(encoded: string): string {
  // Restore standard Base64 characters and padding before decoding
  const base64  = encoded.replace(/-/g, '+').replace(/_/g, '/')
  const padded  = base64 + '==='.slice(0, (4 - base64.length % 4) % 4)
  return atob(padded)
}

// JWT header — must be URL-safe Base64
const header  = JSON.stringify({ alg: 'HS256', typ: 'JWT' })
const encoded = toBase64Url(header)
console.log(encoded) // eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9

const decoded = fromBase64Url(encoded)
console.log(decoded) // {"alg":"HS256","typ":"JWT"}

// Node.js 18 added 'base64url' as a first-class Buffer encoding
const sessionPayload = JSON.stringify({
  userId:     'usr_9f2a1c3e8b4d',
  role:       'editor',
  workspaceId:'ws_3a7f91c2',
  exp:        Math.floor(Date.now() / 1000) + 3600,
})

const encoded = Buffer.from(sessionPayload, 'utf8').toString('base64url')
// No + or / or = characters in the output
// eyJ1c2VySWQiOiJ1c3JfOWYyYTFjM2U4YjRkIiwicm9sZSI6ImVkaXRvciIsIndvcmtzcGFjZUlkIjoid3NfM2E3ZjkxYzIiLCJleHAiOjE3MTcyMDM2MDB9

const decoded = Buffer.from(encoded, 'base64url').toString('utf8')
console.log(JSON.parse(decoded).role) // editor

// Modern approach: File.arrayBuffer() (Chrome 76+, Firefox 69+, Safari 14+)
async function encodeFile(file: File): Promise<string> {
  const buffer = await file.arrayBuffer()
  const bytes  = new Uint8Array(buffer)
  const chars  = Array.from(bytes, b => String.fromCharCode(b))
  return btoa(chars.join(''))
}

const uploadInput = document.getElementById('avatar') as HTMLInputElement

uploadInput.addEventListener('change', async (e) => {
  const file = (e.target as HTMLInputElement).files?.[0]
  if (!file) return

  try {
    const encoded = await encodeFile(file)
    const dataUri = `data:${file.type};base64,${encoded}`

    // Preview the image inline
    const img   = document.getElementById('preview') as HTMLImageElement
    img.src     = dataUri
    img.hidden  = false

    console.log(`Encoded ${file.name} (${file.size} bytes) → ${encoded.length} Base64 chars`)
  } catch (err) {
    console.error('Encoding failed:', err)
  }
})

// GitHub Contents API returns file content as Base64 with embedded newlines
async function fetchRepoFile(
  owner: string,
  repo:  string,
  path:  string,
  token: string,
): Promise<string> {
  const res = await fetch(
    `https://api.github.com/repos/${owner}/${repo}/contents/${path}`,
    {
      headers: {
        Authorization: `Bearer ${token}`,
        Accept: 'application/vnd.github.v3+json',
      },
    }
  )

  if (!res.ok) throw new Error(`GitHub API ${res.status}: ${res.statusText}`)

  const data = await res.json() as { content: string; encoding: string; size: number }

  if (data.encoding !== 'base64') {
    throw new Error(`Unexpected encoding from GitHub: ${data.encoding}`)
  }

  // GitHub wraps output at 60 chars — strip newlines before decoding
  const clean = data.content.replace(/\n/g, '')
  return atob(clean)
}

const openApiSpec = await fetchRepoFile(
  'acme-corp', 'platform-api', 'openapi.json', process.env.GITHUB_TOKEN!
)
const spec = JSON.parse(openApiSpec)
console.log(`API version: ${spec.info.version}`)

# ── macOS / Linux system base64 ───────────────────────────────────────
# Standard encoding
echo -n "deploy-bot:sk-prod-a7f2c91e4b3d8" | base64
# ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg=

# URL-safe variant (replace chars and strip padding)
echo -n "deploy-bot:sk-prod-a7f2c91e4b3d8" | base64 | tr '+/' '-_' | tr -d '='

# Encode a file inline (macOS: -b 0 removes line wrapping; Linux: --wrap=0)
base64 -b 0 ./config/production.json
# or on Linux:
base64 --wrap=0 ./config/production.json

# Decode
echo "ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg=" | base64 --decode

# ── Node.js one-liner — works on Windows too ───────────────────────────
node -e "process.stdout.write(Buffer.from(process.argv[1]).toString('base64'))" "my:secret"
# bXk6c2VjcmV0

# URL-safe from Node.js 18+
node -e "process.stdout.write(Buffer.from(process.argv[1]).toString('base64url'))" "my:secret"
# bXk6c2VjcmV0  (same here since there are no special chars)

# Decode in Node.js
node -e "console.log(Buffer.from(process.argv[1], 'base64').toString())" "ZGVwbG95LWJvdA=="

npm install js-base64
# or
pnpm add js-base64

import { toBase64, fromBase64, toBase64Url, fromBase64Url, isValid } from 'js-base64'

// Standard encoding — Unicode-safe, works in browser and Node.js
const telemetryEvent = JSON.stringify({
  eventId:   'evt_7c3a9f1b2d',
  type:      'checkout_completed',
  currency:  'EUR',
  amount:    14900,
  userId:    'usr_4e2b8d6a5c',
  timestamp: 1717200000,
})

const encoded    = toBase64(telemetryEvent)
const urlEncoded = toBase64Url(telemetryEvent) // No +, /, or = characters

const decoded = fromBase64(encoded)
console.log(JSON.parse(decoded).type) // checkout_completed

// Binary data — pass a Uint8Array as second argument
const pngMagicBytes = new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a])
const binaryEncoded = toBase64(pngMagicBytes, true) // true = binary mode

// Validation before decoding
const suspicious = 'not!valid@base64#'
console.log(isValid(suspicious)) // false

import { createReadStream, createWriteStream } from 'node:fs'
import { pipeline } from 'node:stream/promises'

// Stream a large binary file to a Base64-encoded output file
async function streamEncodeToBase64(
  inputPath:  string,
  outputPath: string,
): Promise<void> {
  const readStream  = createReadStream(inputPath, { highWaterMark: 3 * 1024 * 256 }) // 768 KB chunks (multiple of 3)
  const writeStream = createWriteStream(outputPath, { encoding: 'utf8' })

  let buffer = Buffer.alloc(0)

  await pipeline(
    readStream,
    async function* (source) {
      for await (const chunk of source) {
        buffer = Buffer.concat([buffer, chunk as Buffer])

        // Encode in complete 3-byte groups to avoid mid-stream padding
        const remainder = buffer.length % 3
        const safe      = buffer.subarray(0, buffer.length - remainder)
        buffer          = buffer.subarray(buffer.length - remainder)

        if (safe.length > 0) yield safe.toString('base64')
      }
      // Flush remaining bytes (may add 1 or 2 '=' padding chars)
      if (buffer.length > 0) yield buffer.toString('base64')
    },
    writeStream,
  )
}

// Usage: encode a 200 MB video attachment
await streamEncodeToBase64(
  './uploads/product-demo.mp4',
  './dist/product-demo.b64',
)
console.log('Stream encoding complete')