Custom Code Outbound

Add your own custom policy coded in TypeScript. See below for more details on how to build your own policy.

Configuration

The configuration shows how to configure the policy in the 'policies.json' document.

{
  "name": "my-custom-code-outbound-policy",
  "policyType": "custom-code-outbound",
  "handler": {
    "export": "default",
    "module": "$import(./modules/YOUR_MODULE)",
    "options": {
      "config1": "YOUR_VALUE",
      "config2": true
    }
  }
}

Policy Options

The options for this policy are specified below. All properties are optional unless specifically marked as required.

Using the Policy

The outbound policy will only execute if the response status code is 'ok'

(e.g. response.ok === true or the status code is 200-299) - see response.ok on MDN.

Writing A Policy#

Custom policies can be written to extend the functionality of your gateway. This document is about outbound policies that can intercept the request and, if required, modify it before passing down the chain.

The outbound custom policy is similar to the inbound custom policy but also accepts a Response parameter. The outbound policy must return a valid Response (or throw an error, which will result in a 500 Internal Server Error for your consumer, not recommended).

Tip

Note that both ZuploRequest and Response are based on the web standards Request and Response. ZuploRequest adds a few additional properties for convenience, like user and params.

export type OutboundPolicyHandler<TOptions = any> = (
  response: Response,
  request: ZuploRequest,
  context: ZuploContext,
  options: TOptions,
  policyName: string,
) => Promise<ZuploRequest | Response>;

A common use case for outbound policies is to change the body of the response. In this example, we'll imagine we are proxying the /todos example api at https://jsonplaceholder.typicode.com/todos.

The format of the /todos response looks like this

[
  {
    "userId": 1,
    "id": 1,
    "title": "delectus aut autem",
    "completed": false
  },
  {
    "userId": 1,
    "id": 2,
    "title": "quis ut nam facilis et officia qui",
    "completed": false
  },

We will write an outbound policy that does two things

Removes the userId property
Adds a new outbound header called color

Here's the code:

export default async function (
  response: Response,
  request: ZuploRequest,
  context: ZuploContext,
  options: any,
  policyName: string,
) {
  if (response.status !== 200) {
    // if we get an unexpected response code, something went wrong, just let the response flow
    return response;
  }
 
  const data = (await response.json()) as any[]; // we know this is JSON and an array
  data.forEach((item) => {
    delete item.userId;
  });
 
  // create a new response
  const newResponse = new Response(JSON.stringify(data), {
    status: response.status,
    headers: response.headers,
  });
 
  // let's add an additional header as an example, for good measure
  newResponse.headers.set("color", "yellow");
 
  return newResponse;
}

Tip

Note, that because we're not using the original response here (we just use the new one called newResponse) we didn't need to clone the original response before reading the body with .json(). If you need to read the body and use that same instance you must first clone() to avoid runtime errors such as "Body is unusable".

Adding headers#

Note if you just need to add headers, it more efficient not read the body stream and reuse it, e.g.

export default async function (
  response: Response,
  request: ZuploRequest,
  context: ZuploContext,
  options: any,
  policyName: string,
) {
  // create a new response
  const newResponse = new Response(response.body, {
    status: response.status,
    headers: response.headers,
  });
 
  // let's add an additional header as an example, for good measure
  newResponse.headers.set("color", "yellow");
 
  return newResponse;
}