How to filter outgoing request body

[CodeSmith32]

In our app (like most apps I'm sure) we need to pass Date values between the client and server.

As there might be other special types of objects that need to be passed around, I've set up a system for serializing these kinds of classes safely, using an object with a special field (that wouldn't ever be used otherwise):

{
    "@type": "date",
    "time": 124804183002
}

Server to Client

For server-to-client responses, the server filters its data and transforms dates into objects of this form. Then, when received by the client, I've overridden customFetch to respond with a Response object with a replaced .json method. When this method is called, the request body is parsed as json, and then further filtered to transform these objects back into Dates.

This works great!

Client to Server

However, when going from client to server, I'd like to transform Date objects into objects of this form automatically as well. I have a function for doing this (it accepts any object, and deeply converts dates to this format). Now I'm looking to inject it into the Api class generated by swagger-typescript-api.

As far as I can tell, the individual endpoint methods call request and pass the data argument to body - unchanged - which then gets passed to one of the contentFormatters - in this case json - which calls JSON.stringify on the data, irreversibly converting dates into strings.

That is, there doesn't appear to be a safe place to inject my function for filtering outgoing data (unless I override JSON.stringify itself, which would likely break most other things).

Our currrent build command looks something like,

npx swagger-typescript-api -p ./api/swagger.json -o ./api -n api.ts

How can we modify our build process arguments such that we can somehow inject a filter function before JSON stringifying the data going up? Or, otherwise, what's the proper method for adding our own value for contentFormatters["application/json"] in place of the default one? (contentFormatters is marked as private, though I imagine that doesn't prevent us from force-casting it public and injecting a method manually.)

The other issue here is dealing with TypeScript, which is currently claiming that these dates are string fields when - because of this transform process - they are in fact real dates. I'd like to resolve the above issue first, though, as it's a bit more critical.

TIA!

[CodeSmith32]

Ok, I think I figured this out!

The power of swagger-typescript-api primarily exists in its ability to allow us to override templates. It took a lot of reconfiguration and work, but I think it's working very well now.

I started by switching from a command-line build process to a script. I now have a command that looks like, tsx ./api/generateApi.ts which then imports generateApi from swagger-typescript-api and calls it.

Then I used the generateApi templates parameter to provision a custom template for the HttpClient class (http-client.ejs) by copy/pasting the template from the swagger-typescript-api package's templates/base/http-clients/fetch-http-client.ejs file. In the copied template, I added requestBodyFilter / responseBodyFilter options as function inputs to the constructor and set them up to be called and passed the body, directly before uploading, and directly after being downloaded and decoded.

This allowed me to resolve the first issue - making the uploaded / downloaded data automatically get filtered - by then simply passing functions to the new requestBodyFilter / responseBodyFilter options:

const apiService = new Api({
    // ...
    requestBodyFilter: (body) => encode(body),
    responseBodyFilter: (body) => decode(body),
    // ...
});

Lastly, to fix the typing, I used a trick directly provided by the docs to specify that the "date-time" string values should be given a "Date" type, instead of a plain "string". This is done in the generateApi call which, in the end, now looks something like this:

import path from "node:path";
import { generateApi } from "swagger-typescript-api";

generateApi({
  input: path.resolve(process.cwd(), "./api/swagger.json"),
  output: path.resolve(process.cwd(), "./api"),
  name: "api.ts",
  primitiveTypeConstructs: (struct) => ({
    ...struct,
    string: {
      ...(typeof struct.string === "string"
        ? { $default: struct.string }
        : struct.string),
      "date-time": "Date",
    },
  }),
  templates: path.resolve(process.cwd(), "./api/templates"),
});

Hopefully this helps someone! It was a bit of work, but it seems to be running smoothly now.