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  • License MIT

Type-safe FHIR query builder inspired by Kysely

Package Exports

  • @fhir-dsl/core

Readme

@fhir-dsl/core

A fully type-safe FHIR query builder for TypeScript, inspired by Kysely.

Construct FHIR REST queries with compile-time safety for resource types, search parameters, includes, and profiles. No more guessing parameter names or hoping your query string is valid.

Install

npm install @fhir-dsl/core @fhir-dsl/types

Setup

1. Generate your FHIR types

Before using the query builder, generate typed definitions for your FHIR version and profiles using @fhir-dsl/cli:

npx @fhir-dsl/cli generate \
  --version r4 \
  --ig hl7.fhir.us.core@6.1.0 \
  --out ./src/fhir

This generates a FhirSchema type along with typed resources, search parameters, profiles, and a pre-configured client — everything the query builder needs for compile-time safety.

2. Create a client

The CLI emits a pre-bound createClient for your generated schema — that's the recommended entry point. It calls createFhirClient<FhirSchema> under the hood with the schema, search params, includes, and validators all pre-wired:

import { createClient } from "./fhir/r4"; // generated in step 1

const fhir = createClient({
  baseUrl: "https://hapi.fhir.org/baseR4",
  auth: { type: "bearer", credentials: "your-token" },
});

If you prefer to wire the schema yourself (e.g. when bundling for a web client), createFhirClient is exported directly:

import { createFhirClient } from "@fhir-dsl/core";
import type { FhirSchema } from "./fhir";

const fhir = createFhirClient<FhirSchema>({
  baseUrl: "https://hapi.fhir.org/baseR4",
  auth: { type: "bearer", credentials: "your-token" },
});

Usage

Search resources

const result = await fhir
  .search("Patient")
  .where("family", "eq", "Smith")
  .where("birthdate", "ge", "1990-01-01")
  .sort("birthdate", "desc")
  .count(10)
  .execute();

// result.data     - Patient[] (typed)
// result.total    - total count from server
// result.included - included resources (typed)
// result.link     - pagination links
// result.raw      - raw Bundle response

Every method is fully typed. where("family", ...) only accepts valid search parameters for Patient, and the operator is constrained to what makes sense for that parameter type (string, token, date, etc.).

const result = await fhir
  .search("Patient")
  .where("name", "eq", "Smith")
  .include("general-practitioner")
  .execute();

// result.data     - Patient[]
// result.included - Practitioner[] (inferred from include target)

The include parameter is typed to only accept valid include paths for the resource, and the included resource type is automatically inferred.

Narrow returned fields

Use .select() to request only specific top-level fields. The result type narrows to match, and the query compiles to FHIR's _elements search parameter:

const result = await fhir
  .search("Patient")
  .where("family", "eq", "Smith")
  .select(["id", "name", "birthDate"])
  .execute();

// result.data[0]: { resourceType: "Patient"; id?: string; name?: HumanName[]; birthDate?: FhirDate }

Only top-level element names are accepted (matches the _elements spec). resourceType is always preserved. Calling .select() twice replaces the earlier selection.

Profile-aware queries

const vitals = await fhir
  .search("Observation", "http://hl7.org/fhir/us/core/StructureDefinition/us-core-vital-signs")
  .where("patient", "eq", "Patient/123")
  .where("status", "eq", "final")
  .execute();

// vitals.data is typed as USCoreVitalSignsProfile[]

When you pass a profile URL as the second argument, the result type narrows to that profile's interface.

Read a single resource

const patient = await fhir.read("Patient", "123").execute();
// patient is typed as Patient

Transactions

const result = await fhir
  .transaction()
  .create({
    resourceType: "Patient",
    name: [{ family: "Doe", given: ["Jane"] }],
  })
  .update({
    resourceType: "Patient",
    id: "456",
    name: [{ family: "Smith" }],
  })
  .delete("Observation", "789")
  .execute();

Batch operations

const result = await fhir
  .batch()
  .create({
    resourceType: "Patient",
    name: [{ family: "Doe", given: ["Jane"] }],
  })
  .delete("Observation", "789")
  .execute();

Server-side _filter via FHIRPath

.filter() accepts a raw FHIRPath string or a built FhirPathExpr (anything with .compile(): string) and emits the FHIR _filter search param. Compose typed expressions with @fhir-dsl/fhirpath for end-to-end type safety:

import { fhirpath } from "@fhir-dsl/fhirpath";

const officialFamilyIsSmith = fhirpath<"Patient">("Patient")
  .name.where(($) => $.use.eq("official"))
  .family.eq("Smith");

const result = await fhir
  .search("Patient")
  .filter(officialFamilyIsSmith)
  .execute();
// GET /Patient?_filter=Patient.name.where(use%20%3D%20%27official%27).family%20%3D%20%27Smith%27

Not every server supports _filter. Check CapabilityStatement.rest.searchParam before relying on it. See the FHIRPath + Query Builder guide for the full pattern catalogue (server _filter, post-fetch projection, client filtering, aggregates).

Errors

Every error from this package extends FhirDslError — pattern-match on kind, read structured context, and serialise with toJSON():

Class kind When
FhirRequestError core.request Non-2xx HTTP response (context.status, context.statusText, context.operationOutcome)
AsyncPollingTimeoutError core.async_polling_timeout async polling exceeded maxAttempts
ValidationError core.validation A resource failed Standard Schema validation; context.issues lists every failure
ValidationUnavailableError core.validation_unavailable .validate() called without a SchemaRegistry 
import { isFhirDslError } from "@fhir-dsl/utils";
import { FhirRequestError } from "@fhir-dsl/core";

try {
  await fhir.read("Patient", "missing").execute();
} catch (err) {
  if (err instanceof FhirRequestError) {
    console.error(err.context.status, err.context.operationOutcome);
  } else if (isFhirDslError(err)) {
    console.error(err.kind, err.context);
  } else {
    throw err;
  }
}

Or with the Result toolkit, no try/catch:

import { tryAsync } from "@fhir-dsl/utils";
const r = await tryAsync(() => fhir.read("Patient", "missing").execute());
if (!r.ok) console.error(r.error.kind);

Compile without executing

Every builder has a compile() method that returns the raw query representation without hitting the server:

const query = fhir
  .search("Observation")
  .where("status", "eq", "final")
  .count(50)
  .compile();

// {
//   method: "GET",
//   path: "Observation",
//   params: [
//     { name: "status", value: "final" },
//     { name: "_count", value: 50 }
//   ]
// }

Design

  • Immutable builders - Every method returns a new builder instance. Safe to store, fork, and compose.
  • Schema-driven - All type safety comes from the FhirSchema generic, which is generated from actual FHIR StructureDefinitions.
  • No runtime magic - The builder compiles to plain objects. Bring your own HTTP client or use the built-in fetch executor.

Search Parameter Operators

Param Type Operators
string eq, contains, exact
token eq, not, in, not-in, text, above, below, of-type
date eq, ne, gt, ge, lt, le, sa, eb, ap
number eq, ne, gt, ge, lt, le
quantity eq, ne, gt, ge, lt, le, sa, eb, ap
reference eq
uri eq, above, below

License

MIT