1. Home
2. Learn to build on the Fastly platform
3. Compute@Edge services

JavaScript on Compute@Edge

Compute@Edge supports application code written in JavaScript bundled into a WebAssembly (Wasm) binary. It is a great SDK to get started with on Compute@Edge if you are used to writing browser-based JavaScript or Node.js applications.

Quick access

Project layout

If you don't yet have Node.js and Compute@Edge service set up, start by getting set up.

At the end of the initialization process, the current working directory will contain a file tree resembling the following:

├── README.md
├── fastly.toml
├── node_modules
├── package-lock.json
├── package.json
├── webpack.config.js
└── src
    └── index.js

The most important file to work on is src/index.js, which contains the logic you'll run on incoming requests. If you initialized your project from the default starter kit, the contents of this file should match the one in the starter kit's repo. The other files include:

• npm metadata: package.json and package-lock.json describe the dependencies of your package, managed using npm, Node's package manager.
• Fastly metadata: The fastly.toml file contains metadata required by Fastly to deploy your package to a Fastly service. It is generated by the fastly compute init command. Learn more about fastly.toml.
• Project dependencies: The node_modules directory contains the dependencies listed in package.json.
• Module bundling: The webpack.config.js contains webpack configuration options.

Main interface

Incoming requests trigger an event handler function with a fetch event. This may look familiar if you've used the Service Worker API.

A fetch event will be dispatched for each request that Fastly receives that's routed to your service, and any associated EventListeners must call event.respondWith with a valid response to send to the client. The client Request object can be accessed through the event.request property.

L1 : L2
3function handler(event) {
4  // Get the request from the client.
5  const req = event.request;
6
7  return fetch(req, {
8    backend: "example_backend"
9  });
10}
11
12addEventListener("fetch", event => {
13  // Send the backend response back to the client.
14  return event.respondWith(handler(event));
15});

The @fastly/js-compute module provides the core event target interface, and Request and Response classes referenced in this guide.

Communicating with backend servers and the Fastly cache

A Request can be forwarded to any backend defined on your service. If you specify a backend hostname as part of completing the fastly compute deploy wizard, it will be named the same as the hostname or IP address, but with . replaced with _ (e.g., 151_101_129_57). It's a good idea to define backend names as constants:

const backendName = "my_backend_name";

And then reference them when you want to forward a request to a backend:

L1 : L2
3import { CacheOverride } from "fastly:cache-override";
4const backendName = "my_backend_name";
5
6function handler(event) {
7  // Create a cache override.
8  let cacheOverride = new CacheOverride("override", { ttl: 60 });
9
10  return fetch(event.request, {
11    backend: backendName,
12    cacheOverride
13  });
14}
L15 : L16

JavaScript also supports dynamic backends and allows them to be defined implicitly:

/// <reference types="@fastly/js-compute" />
import { allowDynamicBackends } from "fastly:experimental";
allowDynamicBackends(true);
async function app() {
  // For any request, return the fastly homepage -- without defining a backend!
  return fetch('https://www.fastly.com/');
}
addEventListener("fetch", event => event.respondWith(app(event)));

Requests forwarded to a backend will transit the Fastly cache, and the response may come from cache. Where a request doesn't find a matching result in cache, it will be sent to the origin, and the response will be cached based on the freshness rules determined from its HTTP response headers (unless overridden, as in the example above, by CacheOverride).

In a future release, it will be possible to interact with the cache and the network separately.

Composing requests and responses

In addition to the Request referenced by event.request and Response objects returned from the fetch() function, requests and responses can also be constructed. This is useful if you want to make an arbitrary API call that is not derived from the client request, or if you want to make a response to the client without making any backend fetch at all.

To compose a request from scratch, instantiate a new Request:

3 lines
  // Create some headers for the request to origin
  let upstreamHeaders = new Headers({ "some-header": "someValue" });

  // Create a POST request to our origin using the custom headers
  let upstreamRequest = new Request("https://example.com/", {
    method: "POST",
    headers: upstreamHeaders,
  });
7 lines

Similarly, responses can be created by instantiating a Response:

L1 : L3
4  const headers = new Headers();
5  headers.set('Content-Type', 'text/plain');
6
7  return new Response("Hi from the edge", {
8    status: 200,
9    headers,
10    url: event.request.url
11  })
L12 : L14

Parsing and transforming responses

Requests and responses in Compute@Edge are streams, which allows large payloads to move through your service without buffering or running out of memory. Conversely, running methods such as text on a Response will force the stream to be consumed entirely into memory. This can be appropriate where a response is known to be small or needs to be complete to be parsable.

This example will read a backend response into memory, replace every occurrence of "cat" with "dog" in the body, and then create a new body with the transformed string:

3 lines
  let backendResponse = await fetch("https://host/api/checkAuth", {
    method: "POST",
    backend: "example_backend",
  });

  // Take care! .text() will consume the entire body into memory!
  let bodyStr = await backendResponse.text();
  let newBody = bodyStr.replace("cat", "dog");

  return new Response(newBody, {
    status: backendResponse.status,
    headers: backendResponse.headers
  })
3 lines

Parsing JSON responses is available natively in the Fetch API via the json() method of a Response but also requires consuming the entire response into memory:

3 lines
  let backendResponse = await fetch("https://host/api/checkAuth", {
    method: "POST",
    backend: "example_backend",
  });

  // Take care! .json() will consume the entire body into memory!
  let jsonData = await backendResponse.json();
5 lines

It is often better to avoid buffering responses in this way, especially if the response is large, being delivered slowly in multiple chunks, or capable of being rendered progressively by the client. The Fastly JavaScript SDK implements WHATWG streams. In this example, the backend response is capitalized as it's received:

2 lines
// Take a readable stream and return another readable stream that copies
// the content from the first but uppercases it
function capitalizeFilter(sourceStream) {
  const decoder = new TextDecoder();
  const encoder = new TextEncoder();
  const reader = sourceStream.getReader();

  return new ReadableStream({
    pull(controller) {
      return reader.read().then(({value: chunk, done: readerDone}) => {
        const chunkStr = decoder.decode(chunk);
        const transformedChunk = chunkStr.toUpperCase();
        controller.enqueue(encoder.encode(transformedChunk));
        if (readerDone) {
          controller.close();
        }
      });
    }
  });
}

async function handler(event) {
  // Send the client request to the backend.
  const clientReq = event.request;
  const backendResponse = await fetch(clientReq, { backend: "example_backend" });

  // Pass the backend response through a filter, which uppercases all the text.
  const filteredStream = capitalizeFilter(backendResponse.body);

  // Construct a response using the filtered stream and deliver it to the client.
  return new Response(filteredStream, {
    headers: {
      ...backendResponse.headers,
      "cache-control": "private, no-store"
    }
  });
3 lines

Compression

Fastly can compress and decompress content automatically, and it is often easier to use these features than to try to perform compression or decompression within your JavaScript code. Learn more about compression with Fastly.

Using edge data

Fastly allows you to configure various forms of data stores to your services, both for dynamic configuration and for storing data at the edge. The JavaScript SDK exposes the fastly:config-store and fastly:object-store packages to allow access to these APIs.

All edge data resources are account-level, service-linked resources, allowing a single store to be accessed from multiple Fastly services.

Logging

console provides a standardized interface for emitting log messages to STDOUT or STDERR.

To send logs to Fastly real-time logging, which can be attached to many third party logging providers, use the Logger class. Log endpoints are referenced in your code by name:

/// <reference types="@fastly/js-compute" />
import { Logger } from "fastly:logger";

function handler(event) {

  // logs "Hello!" to the "JavaScriptLog" log endpoint
  const logger = new Logger("JavaScriptLog");
  logger.log("Hello!");

  return new Response({ status: 200 });
}

addEventListener("fetch", event => event.respondWith(handler(event)));

If your code errors, output will be emitted to stderr:

3 lines
  // This logs "abort: Oh no! in src/index.js(line:col)" to stderr.
  throw new Error('Oh no!');
3 lines

Debug logging in the JavaScript runtime

Because JavaScript is an interpreted language, our tooling bundles a JavaScript runtime with your Compute@Edge package. This runtime has the ability to provide additional debugging output during your application's execution. To enable this, call the enableDebugLogging method from fastly:experimental when your app initializes:

import { enableDebugLogging } from "fastly:experimental";
enableDebugLogging(true);
11 lines

The output is unstable and subject to change, but this is the kind of data you can expect to see emitted to STDOUT:

Running JS handleRequest function for C@E service version 1
Request handler took 8.548000ms
Running promise reactions
Running promise reactions took 0.028000ms
Done, waited for 214.297000ms
Running promise reactions took 0.750000ms

Logs written to STDOUT/STDERR can be monitored in real time using log tailing or the local testing server.

Using dependencies

Compute@Edge compiles your code to WebAssembly and uses the WebAssembly System Interface (WASI). Because of this, it supports WASI-compatible npm modules, which in practice is most modules that do not use have native platform bindings. Our Fiddle tool allows the use of a subset of modules that we have tested and confirmed will work with Compute@Edge:

• flight-path (1.0.13)
• seedrandom (3.0.5)
• crypto-js (4.1.1)
• ipaddr.js (2.0.1)
• uuid (9.0.0)
• utf8 (3.0.0)
• @borderless/base64 (1.0.1)
• consistent-hash (1.1.1)
• base-64 (1.0.0)
• cookie (0.5.0)
• unix-checksum (4.4.0)
• minimatch (6.1.6)
• @fastly/js-compute (1.4.2)
• date-fns (2.29.3)
• crypto (1.0.1)
• @fastly/expressly (1.2.0)
• @tusbar/cache-control (0.6.1)
• mustache (4.2.0)
• js-cookie (3.0.1)
• @upstash/redis (1.19.3)

This is a tiny fraction of the modules which will work on Compute@Edge, but these are the most commonly useful modules when building applications.

Access to the client request, creating requests to backends, the Fastly cache, and other Fastly features are exposed via Fastly's own public modules:

• @fastly/js-compute - Core event interface and classes, provides access to client request, backend fetches and caching.

Developer experience

For the best experience of developing on Compute@Edge in JavaScript, include the following comment at the top of any file that uses the fastly. interface:

/// <reference types="@fastly/js-compute" />

This will allow your IDE to import the type definitions for the Fastly JavaScript SDK. If you use eslint with a custom eslintrc file, you may also need to add some extensions to recognize the Fastly types:

{
  "parser": "@typescript-eslint/parser",
  "plugins": ["@typescript-eslint"],
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/eslint-recommended",
    "plugin:@typescript-eslint/recommended",
  ],
}

Module bundling

Compute@Edge applications written in JavaScript can be compiled by the Fastly CLI without any bundling, but you can choose to use a module bundler if you want to replace global modules or provide polyfills. The default JavaScript starter kit for Compute@Edge contains a Webpack configuration which sets reasonable defaults and is suitable for most use cases.

You can adapt this configuration to suit your needs.

For example, you may choose to add rules that determine how the different types of modules will be treated:

module.exports = { // ...
  module: {
    rules: [
      // This allows for inlining of svg images, e.g.,
      // import svgStr from './path/to/image.svg'
      {
        test: /\.(svg)$/,
        type: "asset/source",
      },
    ],
  }
};

Shimming and redirecting module requests are useful techniques when your code relies on Node.js builtins, proposals, or newer standards.

module.exports = { // ...
  // Shimming globals
  plugins: [
    new webpack.ProvidePlugin({
        // Polyfill the URL standard
        URL: "core-js/web/url",
        // Polyfill Node.js' buffer (requires https://www.npmjs.com/package/buffer)
        Buffer: ["buffer", "Buffer"],
    }),
  ],
  // Redirecting module requests
  resolve: {
    fallback: {
      crypto: require.resolve("crypto-browserify"),
      stream: require.resolve("stream-browserify"),
    },
  },
};

Testing and debugging

Logging is the main mechanism to debug Compute@Edge programs. Log output from live services can be monitored via live log tailing. The local test server and Fastly Fiddle display all log output automatically. See Testing & debugging for more information about choosing an environment in which to test your program.

Most common logging requirements involve HTTP requests and responses. It's important to do this in a way that doesn't affect the main program logic, since consuming a response body can only be done once. The following example demonstrates a console.log statement for request headers, response headers, request body and response body:

Since the bodies of HTTP requests and responses in Compute@Edge are streams, we are consuming the stream to its end and then logging the resulting data. In JavaScript once the .body property of a request or response has been read it cannot be used by fetch or respondWith, so we use the extracted body data to construct a new Request or Response after logging the body.

Unit testing

You may choose to write unit tests for small independent pieces of your JavaScript code intended for Compute@Edge. However, Compute@Edge apps heavily depend on and interact with Fastly features and your own systems. This can make an integration testing strategy that focusses on a lesser number of high impact tests more valuable.