HTTP caching headers (ETag, Cache-Control) in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the ETag HTTP header?

The ETag header provides a unique identifier for a specific version of a resource. It helps browsers and servers check if the resource has changed, so the browser can use a cached version if it hasn't.

beginner

Explain the role of the Cache-Control header in HTTP caching.

The Cache-Control header tells browsers and proxies how to store and reuse responses. It can specify how long to keep a resource, whether to revalidate it, or if it should not be cached at all.

intermediate

How does Express help you set ETag headers automatically?

Express automatically generates ETag headers for responses by default, based on the response body. This helps with efficient caching without extra code.

beginner

What does the Cache-Control: no-store directive do?

It tells browsers and proxies not to store any part of the response. This means the resource will always be fetched fresh from the server.

beginner

Why is it important to use caching headers like ETag and Cache-Control in web applications?

Using caching headers improves performance by reducing unnecessary data transfer. It saves bandwidth, speeds up page loads, and reduces server load by letting browsers reuse cached resources when possible.

What does the ETag header represent?

A The size of the resource in bytes

B A unique version identifier for a resource

C The expiration time of a cached resource

D The server's IP address

ETag is a unique identifier for a resource version, used to check if the cached copy is still valid.

Which Cache-Control directive tells the browser to always fetch a fresh copy?

A no-cache

B max-age=3600

C public

D private

no-cache means the browser must revalidate with the server before using the cached resource.

In Express, how can you disable automatic ETag generation?

A app.etag = false

B app.disable('etag')

C app.use(etag(false))

D app.set('etag', false)

Use app.set('etag', false) to turn off automatic ETag generation in Express.

What does Cache-Control: max-age=0 mean?

A Cache forever

B Do not cache at all

C Resource is stale immediately and must be revalidated

D Cache for 1 hour

max-age=0 means the cached resource is immediately stale and requires revalidation.

Which header does the browser send to check if the cached resource matches the server's version?

A If-None-Match

B ETag

C Cache-Control

D If-Modified-Since

The browser sends If-None-Match with the ETag value to ask if the resource has changed.

Describe how ETag and Cache-Control headers work together to improve web performance.

Think about how the browser knows when to reuse cached data or ask the server for updates.

Explain how to set caching headers in an Express app to cache a response for 1 hour.

Remember Cache-Control max-age is in seconds.