JavaScript Base64 デコード完全ガイド — atob() と Buffer

// Decoding an HTTP Basic Auth credential pair received in a request header
// Authorization: Basic ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg=

function parseBasicAuth(header: string): { serviceId: string; apiKey: string } {
  const base64Part = header.replace(/^Basics+/i, '')
  const decoded    = atob(base64Part)
  const [serviceId, apiKey] = decoded.split(':')
  return { serviceId, apiKey }
}

const auth = parseBasicAuth('Basic ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg=')

console.log(auth.serviceId) // deploy-bot
console.log(auth.apiKey)    // sk-prod-a7f2c91e4b3d8

// Verify lossless recovery for ASCII-only content
const original = 'service:payments region:eu-west-1 env:production'

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

console.log(encoded)
// c2VydmljZTpwYXltZW50cyByZWdpb246ZXUtd2VzdC0xIGVudjpwcm9kdWN0aW9u

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

// '田中太郎' was UTF-8 encoded then Base64 encoded before transmission
const encoded = '55Sw5Lit5aSq6YOO'

// ❌ atob() returns the raw UTF-8 bytes as a Latin-1 string — garbled output
console.log(atob(encoded))
// "ç"°ä¸­å¤ªé ã"  ← byte values misread as Latin-1

// Unicode-safe Base64 decode utilities
function fromBase64(encoded: string): string {
  const binary = atob(encoded)
  const bytes  = Uint8Array.from(binary, ch => ch.charCodeAt(0))
  return new TextDecoder().decode(bytes)
}

function toBase64(text: string): string {
  const bytes = new TextEncoder().encode(text)
  const chars = Array.from(bytes, byte => String.fromCharCode(byte))
  return btoa(chars.join(''))
}

// 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(decoded === orderNote) // true
console.log(decoded)
// 確認済み：田中太郎 — 東京倉庫、数量：250

// Server config stored as Base64 in an env variable (avoids JSON escaping in shell)
// DB_CONFIG=eyJob3N0IjoiZGItcHJpbWFyeS5pbnRlcm5hbCIsInBvcnQiOjU0MzIsImRhdGFiYXNlIjoiYW5hbHl0aWNzX3Byb2QiLCJtYXhDb25uZWN0aW9ucyI6MTAwfQ==

const raw = Buffer.from(process.env.DB_CONFIG!, 'base64').toString('utf8')
const dbConfig = JSON.parse(raw)

console.log(dbConfig.host)           // db-primary.internal
console.log(dbConfig.port)           // 5432
console.log(dbConfig.maxConnections) // 100

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

// Read the Base64-encoded certificate and restore the original binary
const encoded = readFileSync(join(process.cwd(), 'dist', 'cert.b64'), 'utf8').trim()
const certBuf  = Buffer.from(encoded, 'base64')

writeFileSync('./ssl/server.crt', certBuf)

console.log(`Restored ${certBuf.length} bytes`)
// Restored 2142 bytes

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

async function decodeBase64File(
  encodedPath: string,
  outputPath:  string,
): Promise<number> {
  try {
    const encoded = await readFile(encodedPath, 'utf8')
    const binary  = Buffer.from(encoded.trim(), 'base64')
    await writeFile(outputPath, binary)
    return binary.length
  } catch (err) {
    const code = (err as NodeJS.ErrnoException).code
    if (code === 'ENOENT') throw new Error(`File not found: ${encodedPath}`)
    if (code === 'EACCES') throw new Error(`Permission denied: ${encodedPath}`)
    throw err
  }
}

// Restore a PDF stored as Base64
const bytes = await decodeBase64File('./uploads/invoice.b64', './out/invoice.pdf')
console.log(`Decoded ${bytes} bytes — PDF restored`)

function decodeBase64Url(input: string): string {
  const base64 = input.replace(/-/g, '+').replace(/_/g, '/')
  const padded = base64 + '==='.slice(0, (4 - base64.length % 4) % 4)
  const binary = atob(padded)
  const bytes  = Uint8Array.from(binary, ch => ch.charCodeAt(0))
  return new TextDecoder().decode(bytes)
}

// Inspect a JWT payload segment (the middle part between the two dots)
const jwtToken  = 'eyJ1c2VySWQiOiJ1c3JfOWYyYTFjM2U4YjRkIiwicm9sZSI6ImVkaXRvciIsIndvcmtzcGFjZUlkIjoid3NfM2E3ZjkxYzIiLCJleHAiOjE3MTcyMDM2MDB9'
const payload   = decodeBase64Url(jwtToken)
const claims    = JSON.parse(payload)

console.log(claims.userId)      // usr_9f2a1c3e8b4d
console.log(claims.role)        // editor
console.log(claims.workspaceId) // ws_3a7f91c2

// Node.js 18 added 'base64url' as a first-class Buffer encoding — no manual replace needed
function decodeJwtSegment(segment: string): Record<string, unknown> {
  const json = Buffer.from(segment, 'base64url').toString('utf8')
  return JSON.parse(json)
}

const token   = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VySWQiOiJ1c3JfOWYyYTFjM2U4YjRkIiwicm9sZSI6ImVkaXRvciIsIndvcmtzcGFjZUlkIjoid3NfM2E3ZjkxYzIiLCJleHAiOjE3MTcyMDM2MDB9.SIGNATURE'
const [headerB64, payloadB64] = token.split('.')

const header  = decodeJwtSegment(headerB64)
const payload = decodeJwtSegment(payloadB64)

console.log(header.alg)          // HS256
console.log(payload.role)        // editor
console.log(payload.workspaceId) // ws_3a7f91c2

// GitHub Contents API returns file content as Base64, wrapped at 60 chars per line
async function fetchDecodedFile(
  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 }
  if (data.encoding !== 'base64') throw new Error(`Unexpected encoding: ${data.encoding}`)

  // ⚠️ GitHub wraps at 60 chars — strip newlines before decoding
  const clean = data.content.replace(/\n/g, '')
  return Buffer.from(clean, 'base64').toString('utf8')
}

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

// Some APIs return binary content (images, PDFs) as Base64 JSON fields
async function downloadDecodedFile(endpoint: string, authToken: string): Promise<void> {
  const res = await fetch(endpoint, { headers: { Authorization: `Bearer ${authToken}` } })
  if (!res.ok) throw new Error(`Download failed: ${res.status}`)

  const { filename, content, mimeType } = await res.json() as {
    filename: string; content: string; mimeType: string
  }

  // Decode Base64 → binary bytes → Blob
  const binary = atob(content)
  const bytes  = Uint8Array.from(binary, ch => ch.charCodeAt(0))
  const blob   = new Blob([bytes], { type: mimeType })

  // Trigger browser download
  const url = URL.createObjectURL(blob)
  const a   = Object.assign(document.createElement('a'), { href: url, download: filename })
  a.click()
  URL.revokeObjectURL(url)
}

await downloadDecodedFile('/api/reports/latest', sessionStorage.getItem('auth_token')!)

# ── macOS / Linux system base64 ───────────────────────────────────────
# Standard decoding (macOS uses -D, Linux uses -d)
echo "ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg=" | base64 -d   # Linux
echo "ZGVwbG95LWJvdDpzay1wcm9kLWE3ZjJjOTFlNGIzZDg=" | base64 -D   # macOS

# Decode a .b64 file to its original binary
base64 -d ./dist/cert.b64 > ./ssl/server.crt       # Linux
base64 -D -i ./dist/cert.b64 -o ./ssl/server.crt   # macOS

# URL-safe Base64 — restore + and / before decoding
echo "eyJ1c2VySWQiOiJ1c3JfOWYyYTFjM2UifQ" | tr '-_' '+/' | base64 -d

# ── Node.js one-liner — works on Windows too ───────────────────────────
node -e "process.stdout.write(Buffer.from(process.argv[1], 'base64').toString())" "ZGVwbG95LWJvdA=="
# deploy-bot

# URL-safe (Node.js 18+)
node -e "process.stdout.write(Buffer.from(process.argv[1], 'base64url').toString())" "eyJhbGciOiJIUzI1NiJ9"
# {"alg":"HS256"}

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

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

// Standard decoding — Unicode-safe, works in browser and Node.js
const raw   = fromBase64('eyJldmVudElkIjoiZXZ0XzdjM2E5ZjFiMmQiLCJ0eXBlIjoiY2hlY2tvdXRfY29tcGxldGVkIiwiY3VycmVuY3kiOiJFVVIiLCJhbW91bnQiOjE0OTAwfQ==')
const event = JSON.parse(raw)
console.log(event.type)     // checkout_completed
console.log(event.currency) // EUR

// URL-safe decoding — no manual character replacement needed
const jwtPayload = fromBase64Url('eyJ1c2VySWQiOiJ1c3JfOWYyYTFjM2U4YjRkIiwicm9sZSI6ImVkaXRvciJ9')
const claims     = JSON.parse(jwtPayload)
console.log(claims.role) // editor

// Validate before decoding untrusted input
const untrusted = 'not!valid@base64#'
if (!isValid(untrusted)) {
  console.error('Rejected: invalid Base64 input')
}

// Binary output — second argument true returns Uint8Array
const pngBytes = fromBase64('iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==', true)
console.log(pngBytes instanceof Uint8Array) // true

npm install chalk
# chalk v5+ is ESM-only — use import, not require

import chalk from 'chalk'

// Decode and display any Base64 value with smart type detection
function inspectBase64(encoded: string, label = 'Decoded value'): void {
  let decoded: string
  try {
    decoded = Buffer.from(encoded.trim(), 'base64').toString('utf8')
  } catch {
    console.error(chalk.red('✗ Invalid Base64 input'))
    return
  }

  console.log(chalk.bold.cyan(`\n── ${label} ──`))

  // Attempt JSON pretty-print
  try {
    const parsed = JSON.parse(decoded)
    console.log(chalk.green('Type:'), chalk.yellow('JSON'))
    for (const [key, value] of Object.entries(parsed)) {
      const display = typeof value === 'object' ? JSON.stringify(value) : String(value)
      console.log(chalk.green(`  ${key}:`), chalk.white(display))
    }
    return
  } catch { /* not JSON */ }

  // Plain text fallback
  console.log(chalk.green('Type:'), chalk.yellow('text'))
  console.log(chalk.white(decoded))
}

// Inspect a Base64-encoded JWT payload
const tokenParts = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VySWQiOiJ1c3JfOWYyYTFjM2U4YjRkIiwicm9sZSI6ImVkaXRvciIsImV4cCI6MTcxNzIwMzYwMH0.SIGNATURE'.split('.')
inspectBase64(tokenParts[0], 'JWT Header')
inspectBase64(tokenParts[1], 'JWT Payload')
// ── JWT Header ──
// Type:   JSON
//   alg:  HS256
//   typ:  JWT
//
// ── JWT Payload ──
// Type:   JSON
//   userId: usr_9f2a1c3e8b4d
//   role:   editor
//   exp:    1717203600

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

async function streamDecodeBase64(inputPath: string, outputPath: string): Promise<void> {
  const readStream  = createReadStream(inputPath, { encoding: 'utf8', highWaterMark: 4 * 1024 * 192 })
  const writeStream = createWriteStream(outputPath)

  let buffer = ''

  await pipeline(
    readStream,
    async function* (source) {
      for await (const chunk of source) {
        buffer += (chunk as string).replace(/\s/g, '') // strip any whitespace/newlines

        // Decode only complete 4-char groups to avoid mid-stream padding issues
        const remainder = buffer.length % 4
        const safe      = buffer.slice(0, buffer.length - remainder)
        buffer          = buffer.slice(buffer.length - remainder)

        if (safe.length > 0) yield Buffer.from(safe, 'base64')
      }
      if (buffer.length > 0) yield Buffer.from(buffer, 'base64')
    },
    writeStream,
  )
}

// Decode a 200 MB video that was stored as Base64
await streamDecodeBase64('./uploads/product-demo.b64', './dist/product-demo.mp4')
console.log('Stream decode complete')