Headers & Authentication in JavaScript

// Headers object
const headers = new Headers()
headers.append('Content-Type', 'application/json')
headers.set('Authorization', 'Bearer token123')
headers.get('Content-Type') // 'application/json'
headers.has('Authorization') // true
headers.delete('X-Custom')

// Headers in request
const response = await fetch(url, {
  headers: {
    'Content-Type': 'application/json',
    'Authorization': 'Bearer token123',
    'X-API-Key': 'secret-key',
    'Accept-Language': 'en-US',
    'User-Agent': 'MyApp/1.0'
  }
})

// Headers from response
const contentType = response.headers.get('content-type')
const lastModified = response.headers.get('last-modified')
const customHeader = response.headers.get('x-custom-header')

// Iterate over headers
for (const [key, value] of response.headers.entries()) {
  console.log(\`\${key}: \${value}\`)
}