Handling Conditional Defaults in API Schema Parsing

import * as S from "@effect/schema/Schema";

const Body = S.Struct({
  foo: S.String.pipe(S.optional({ default: () => "foo" })),
});

declare const bodyJson: unknown;

const result = S.decodeUnknownSync(Body)(bodyJson);

// use result.foo in a couple places where the default value is expected if the value was not provided

declare const otherThing: { foo: string | undefined };
declare const condition: boolean;

const finalValue = (() => {
  if (
    condition &&
    typeof bodyJson === "object" &&
    bodyJson !== null &&
    "foo" in bodyJson &&
    typeof bodyJson.foo === "string"
  ) {
    return bodyJson.foo;
  } else if (otherThing.foo) {
    return otherThing.foo;
  } else {
    return result.foo;
  }
})();