Edge Runtime

The Next.js Edge Runtime is based on standard Web APIs, which is used by Middleware.

Runtime APIs

Base64

atob: Decodes a string of data which has been encoded using base-64 encoding
btoa: Creates a base-64 encoded ASCII string from a string of binary data

Encoding

TextEncoder: Takes a stream of code points as input and emits a stream of bytes (UTF8)
TextDecoder: Takes a stream of bytes as input and emit a stream of code points

Environment

process.env: Holds an object with all environment variables for both production and development in the exact same way as any other page or API in Next.js

Fetch

The Web Fetch API can be used from the runtime, enabling you to use Middleware as a proxy, or connect to external storage APIs

A potential caveat to using the Fetch API in a Middleware function is latency. For example, if you have a Middleware function running a fetch request to New York, and a user accesses your site from London, the request will be resolved from the nearest Edge to the user (in this case, London), to the origin of the request, New York. There is a risk this could happen on every request, making your site slow to respond. When using the Fetch API, you must make sure it does not run on every single request made.

Streams

TransformStream: Consists of a pair of streams: a writable stream known as its writable side, and a readable stream, known as its readable side. Writes to the writable side, result in new data being made available for reading from the readable side. Support for web streams is quite limited at the moment, although it is more extended in the development environment
ReadableStream: A readable stream of byte data
WritableStream: A standard abstraction for writing streaming data to a destination, known as a sink

Timers

setInterval: Schedules a function to execute every time a given number of milliseconds elapses
clearInterval: Cancels the repeated execution set using setInterval()
setTimeout: Schedules a function to execute in a given amount of time
clearTimeout: Cancels the delayed execution set using setTimeout()

Web

Headers: A WHATWG implementation of the headers API
URL: A WHATWG implementation of the URL API.
URLSearchParams: A WHATWG implementation of URLSearchParams

Crypto

Crypto: The Crypto interface represents basic cryptography features available in the current context
crypto.randomUUID: Lets you generate a v4 UUID using a cryptographically secure random number generator
crypto.getRandomValues: Lets you get cryptographically strong random values
crypto.subtle: A read-only property that returns a SubtleCrypto which can then be used to perform low-level cryptographic operations

Logging

console.debug: Outputs a message to the console with the log level debug
console.info: Informative logging of information. You may use string substitution and additional arguments with this method
console.clear: Clears the console
console.dir: Displays an interactive listing of the properties of a specified JavaScript object
console.count: Log the number of times this line has been called with the given label
console.time: Starts a timer with a name specified as an input parameter

Unsupported APIs

The Edge Runtime has some restrictions including:

Native Node.js APIs are not supported. For example, you can't read or write to the filesystem
Node Modules can be used, as long as they implement ES Modules and do not use any native Node.js APIs
Calling require directly is not allowed. Use ES Modules instead

The following JavaScript language features are disabled, and will not work: