Code examples in JavaScript
These code examples have an implementation in JavaScript. To learn more about using JavaScript with our Compute@Edge platform, see using JavaScript.
NOTE: Some of the code examples here are also available in languages other than JavaScript. Being able to see how the same solution can be achieved in other languages is often useful for migrations or learning, so where a code example has multiple implementations, we show all implementations together.
Verify if a web crawler accessing your server really is Googlebot
An implementation of Google's recommended mechanism for verifying googlebot
Add a new field to a JSON response
Decorate API responses in JSON format with new fields.
Geo-IP API at the edge
Create an API endpoint for fetching GeoIP data for the requesting browser, implemented 100% at the edge. The response should show your current approximate location, but no requests to any origin servers.
Tag requests with geolocation data
Add GeoIP data about the client browser as extra headers in any requests from Fastly to your origin.
Apply feature flags from an origin API
Park request, make a different request first, use the response to annotate the real origin request (or make decisions about how to route it).
Enforce a paywall using an origin API
Use a custom Paywall header to trigger preflight requests to authenticate every article view with a backend paywall service.
Detect leaked passwords
Detect requests that contain submitted passwords and use a service to determine whether the password has leaked before allowing the request to proceed to origin (data from haveibeenpwned).
Add www. to apex hostname and subdomains
Detect requests that don't include a www. prefix, and redirect to the equivalent path on a hostname that starts with www., usually to make sure there's only one canonical location for your content.
Override TTLs for path prefixes
Set TTLs at the edge based on looking up a path prefix in an edge dictionary.
Push prices or breaking news in real time with Server-Sent-Events
Stream responses to the browser while still receiving data from the origin and also saving it to cache. Great for spreading out server-sent-events streams to millions of users from a single source stream.
Map IPv6 addresses to IPv4 address space
Create an identifier that looks like an IPv4 address if the user has made their request using IPv6. Useful for origin servers that require an IPv4 address for some reason.
Capture and aggregate log data from client devices
Collect and aggregate log data submitted from browsers directly into S3 or another log store without having to handle the traffic at your origin.
Use microservices to divide up a domain
Send request to different origin servers based on the URL path.
Replace origin errors with 'safe' responses
Detect specified response statuses from backends and instead serve a precomposed error page or error content generated at the edge.
Answer CORS OPTIONS preflight requests at the edge
Browsers send OPTIONS requests before performing cross-origin POSTs. You can answer these requests directly from the edge.
Apply HTTP basic auth to private endpoints
Store username/password list in an edge dictionary, authorize user at the edge, reject requests that don't have correct credentials.
Serve robots.txt from the edge
Serve full text of robots.txt as a synthetic response to avoid requests hitting your origin.
Load balance randomly across multiple backends
Load balance requests randomly across multiple backends, dropping them automatically if they become unhealthy.
Load balance to optimise upstream caching with hash directors
Map requests to backends consistently, which can be useful to improve your internal cache and replication efficiency.
Sticky sessions using client directors
Map requests to backends based on user ID (a.k.a., "sticky sessions").
Create self-destructing URLs for time-limited access
Make URLs expire after a configurable period.
Enable modern web security headers to all responses
Go from an F to an A grade on securityheaders.io by adding security policy headers to your responses at the edge.
Add, remove or change HTTP headers
Fastly can easily read and write HTTP headers at multiple stages of the request/response cycle.
Rewrite URL path
Receive a request for one path but request a different path from origin, without a redirect.
Filter query string parameters
Add, remove, and sort querystring parameters.
Change request method
Change PUT, DELETE, OPTIONS and others to POST, or vice versa, to help integrate incompatible client and server apps.
Client public IP API at the edge
Quickly fetch the user's public IP from an API endpoint on your own domain, with no origin.
Add or remove cookies
Read individual cookies, set new cookies in response.
Filter cookies or other structured headers
Rewrite headers to keep only keys that you want to allow, similar to `querystring.filter_except` but for headers rather than querystrings.
Remove trailing slashes to normalize URLs
Treat URLs with and without suffixed slashes as equivalent, or redirect URLs with slashes to the version without.
Caching responses to POST requests
By default, Fastly does not cache responses to POST requests. But you can enable this if you wish.
Regular expression capturing patterns
Capture regex pattern groups to use in matching url segments and more.
Ban bad IPs for a fixed period
Block a list of IP addresses from accessing your service and include an expiry time.
Apply CAPTCHA to high risk requests
Intercept suspicious traffic and display a CAPTCHA challenge. If the user passes, allow the request to go to the origin server.
Normalize requests to increase cache efficiency
Improve cache performance by normalizing requests. Filter and reorder query params, convert to lowercase, filter headers, and more.
Search and replace in strings
Use regular expression substitution functions to map paths, strip extraneous slashes, and more.
Google Cloud Storage origin (public)
Use a public GCS bucket as a backend for your Fastly service.
Google Cloud Storage origin (private)
Use AWS compat mode to make authenticated requests to your GCS bucket.
AWS S3 bucket origin (private)
Use AWS authenticated requests (signature version 4) to protect communication between your Fastly service and AWS.
Azure blob storage bucket origin (private)
Use Microsoft Azure authenticated requests to protect communication between your Fastly service and Azure.
Log request data to Google BigQuery
Build raw JSON strings matching your BigQuery table schema to send log data to BigQuery.
Base64 POST body
Access the body of a POST request in Base64-encoded form.
Random integers
Generate random whole numbers in a range.
Extract a substring from a string value
Isolate a portion of a string identified by a range of characters.
POST to GET rewrite using a Base64-encoded querystring
To allow caching of POST requests, consider rewriting them as GET requests at the edge.
Smoke test a new origin
Send a copy of your traffic to a test origin before returning a response from production.
Compute intersection of two lists
Useful for comparing capabilities with required permissions.
Prevent hotlinking of product images by third party websites
Detect and reject requests from third party websites that attempt to embed your images on their pages.
Decompress and read gzipped responses
When you need to work on API and text responses from backends that support gzip.
Prohibit browser caching
Ensure resources are not cached on the front end, while allowing caching within Fastly.
Clean backend responses
Remove headers added by backends that you don't want to emit to the browser, like amz- or goog- headers.
Remove querystring from static assets
Use the new Sec-Fetch-Dest header or URL patterns to identify assets that should not allow querystrings to be part of the cache key.
Set Google Analytics _ga cookie
Due to ITP 2.1 restrictions, cookies set in JavaScript may be limited to a 7-day TTL. Set your Google Analytics cookie on the edge to avoid this.
Auto retry a secondary backend
If primary backend fails, retry with a different backend without caching the failure or reducing cache efficiency.
Anonymize client IPs for logging
Strip the last octet or compute a hash of client IP address for anonymization.
Log the IP version (IPv4/IPv6)
Identify which type of IP address was used by the client connecting to your Fastly service.
Send HTTP Basic Auth in request to origin
Convert a password sent by the client in the querystring into a Authorization header to your origin server.
Base64 URL path segments
Unknown data in URL paths can result in invalid URLs, but base64url is designed to be URL-safe.
Check validity of inputs using a non-crypto hash
Block or identify syntactically invalid requests at the edge by using a hash function of your choice.
Use A/B testing to personalize responses
Serve different responses to separate user cohorts.
Redirect old URLs at the edge
Use a dictionary of URL mappings to serve your redirects at lightning speed.
Add time zone offset to requests
Divide the world into time bands of custom size and forward time zone data to your origin server.
Perform redirects with wildcard patterns using an edge dictionary
Match URL prefixes and make use of configurable response status and querystring preservation.
CenturyLink-compatible token validation
Validate your CenturyLink tokens for access to video stream playlists.
Perform multiple origin requests asynchronously
Handle third-party requests as they are resolved.
Slowing down responses (tarpit)
Force a response to be delivered very slowly to reduce the rate at which an attacker can send requests.
Enrich image responses with EXIF metadata
Use the `exif` Rust crate to decorate a backend response with image metadata.
Generate a QR code
Use open source libraries to dynamically serve a QR code based on query parameters.
Validate domain on query string
Check the query string if it contains a valid domain.
Check for viruses using Virus Total
Sha256 hash a file upload and test the hash against the VirusTotal API
Protect PNGs for Acropalypse
Remove extraneous data from PNG files created using screenshotting tools
Transform a response while streaming it
Streaming transformations avoid buffering a response, reducing latency and memory consumption
Set a timeout on a Request
Configure a timeout for a specific origin request, rather than relying on the configuration settings for that origin
All code on this page is provided under both the BSD and MIT open source licenses.