Using args

Pass JSON Args into a function from its instance and read them at runtime to change behavior without editing code.

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Read a value from the Args of the function instance instead of hardcoding it in the function. Use this pattern when the same code runs on several instances and only a few values change between them, such as an identifier, a feature flag, or a destination hostname.

JavaScript
01 async function handleRequest(request, v) {
02 return new Response(v, {
03 headers: new Headers([
04 ["X-Custom-Header", "something defined on JS"],
05 ]),
06 status: 200,
07 });
08 }
09 addEventListener("fetch", (event) => {
10 event.respondWith(handleRequest(event.request, event.args.value));
11 });

The Args set on the function instance that runs this code:

JavaScript
01{
02 "value": "hello_world"
03}

How it works

The fetch handler reads event.args.value and passes it to handleRequest, which returns that value in the body of a new Response, along with a custom header set through new Headers([...]) and an explicit status: 200. The value key comes from the JSON object above, so changing that object changes what the function returns.

Args are not part of the code. You set them on the function instance that binds the function to an application or a firewall, which is why the same function can behave differently on each instance without being edited or redeployed. How it works covers the rest of the chain, from the function to the instance to the Rules Engine rule that runs it.

JavaScript examples

Browse the other ready-to-use examples you can copy into a function.

Function instances

Create the instance that carries the Args this function reads.

FetchEvent

Reference for the fetch event that exposes event.args.

Functions limits

The ceilings a single invocation of this function runs inside, and the ones that vary by plan.

The function fails at instantiation

What an Args object above the ceiling looks like from the outside.

Args and environment variables

Why API keys, credentials, and tokens belong in an environment variable rather than in Args.