fhir-client

Lightweight, dependency-free FHIR R4 client SDK for Node.js/TypeScript

Category: FHIR & SMART — Interoperability Libraries · License: Apache-2.0 · Status: alpha

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Table of contents

1. What problem does this solve?
2. Features
3. Installation
4. Quick Start
5. Architecture
6. Example Usage
7. Roadmap
8. Contributing & Testing
9. License
10. About Peerbits

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1. What problem does this solve?

Every FHIR integration project ends up hand-rolling the same client layer: building search URLs, following Bundle pagination links, constructing batch/transaction bundles, and turning OperationOutcome error responses into something a caller can actually branch on. The existing public FHIR clients for TypeScript are either unmaintained or tied to a larger platform SDK you didn't ask for. fhir-client is that thin client layer on its own: strongly typed against the FHIR R4 spec, zero required runtime dependencies, and usable in Node or edge/serverless runtimes since it's built on native fetch.

2. Features

• CRUD on any FHIR R4 resource type: read, vread, create, update, delete
• Typed search builders for Patient, Observation, Encounter, Condition, plus a generic search(resourceType, params) escape hatch for anything else — including chained and composite search parameters
• Bundle handling: typed parsing of searchset results, automatic pagination via nextPage(), and a BundleBuilder for batch/transaction requests
• CapabilityStatement discovery with in-memory TTL caching, and a supportsOperation() helper to check server support before calling an operation
• Typed error handling — non-2xx responses are parsed into a FhirClientError with .issues, .severity, and the original HTTP status, not just a raw HTTP error
• A TokenProvider type (not an implementation) so this client composes with peerbits-smart-launch's token storage, or any other auth mechanism, without a hard dependency on either
• Opt-in retry/backoff for transient failures (network errors, 5xx) — off by default
• TLS enforced by default; http:// requires an explicit, clearly-unsafe opt-out flag for local sandbox testing only

3. Installation

npm install @peerbits/fhir-client

Requires Node.js 20.12+ (global fetch, and the node:util APIs this package's tooling depends on) or any modern runtime that provides a spec-compliant fetch. This package ships ESM only ("type": "module") — consume it with import. From CommonJS: Node.js 22.12+ can require() it directly (Node's native require(esm) support); on older Node, use await import("@peerbits/fhir-client") instead — require() fails there with a clear ERR_REQUIRE_ESM error.

Working from a local clone instead of npm? See Local development under Contributing & Testing for how to build and link it locally.

4. Quick Start

Runs as-is against the public HAPI FHIR test server — no auth, no setup beyond npm install.

import { FhirClient, type Patient } from "@peerbits/fhir-client";

const client = new FhirClient({ baseUrl: "https://hapi.fhir.org/baseR4/" });

const { resources: patients } = await client.searchPatients({ family: "Smith", _count: 5 });
console.log(`Found ${patients.length} patient(s) named Smith`);

if (patients[0]?.id) {
  const patient = await client.read<Patient>("Patient", patients[0].id);
  console.log(patient.name?.[0]?.given?.join(" "), patient.name?.[0]?.family);
}

5. Architecture

src/
  client.ts       FhirClient — config, request(), auth header injection, retry/backoff
  auth.ts         TokenProvider type only — no implementation, no dependency on any token source
  search.ts       Typed search-parameter builders + the generic search escape hatch
  bundle.ts       Bundle parsing, pagination, batch/transaction construction
  capability.ts   CapabilityStatement fetch + TTL cache + supportsOperation()
  errors.ts       OperationOutcome parsing into FhirClientError
  resources/      Typed interfaces: Patient, Observation, Encounter, Condition, + shared datatypes
  index.ts        Public API surface — everything else is an internal implementation detail

Request flow: a client method builds the request URL → if a tokenProvider is configured, it's called and the result becomes the Authorization header → the request is sent via fetch, with retry/backoff applied only when explicitly configured → on success the body is parsed into the typed return shape (a resource, or a parsed Bundle for search/batch) → on failure the response is checked for an OperationOutcome and thrown as a typed FhirClientError, otherwise a generic typed HTTP error is thrown.

Built against FHIR R4 (not R5 — see Roadmap). Resource and search-parameter field names are grounded in the FHIR R4 specification, not inferred from general REST convention.

Configuration reference

Everything FhirClient accepts, via new FhirClient(config):

Option	Type	Default	Notes
baseUrl	string	(required)	No default is provided on purpose — never accidentally points at a real vendor endpoint. Must include a trailing slash or the client normalizes it internally.
tokenProvider	() => string | Promise<string>	none	Called before every request; return value becomes Authorization: Bearer <token>. See auth.ts — needs human security review wherever it's implemented (token storage/refresh isn't this repo's job).
headers	Record<string, string>	none	Merged into every request; use for server-specific headers. Can't override Authorization (set from tokenProvider) or Content-Type/Accept (set internally).
retry	{ attempts, backoffMs?, retryOnStatus? }	undefined (off)	Opt-in only. backoffMs (default 250) doubles per attempt: backoffMs * 2^attemptNumber. retryOnStatus defaults to any 5xx; network errors always count as retryable once retry is set.
allowInsecureHttp	boolean	false	Unsafe. Only set true for local sandbox testing against http://. The constructor throws immediately if baseUrl isn't https:// and this isn't set.
capabilityCacheTtlMs	number	300000 (5 min)	How long fetchCapabilityStatement() caches /metadata before re-fetching.
fetchFn	typeof fetch	global fetch	Inject a custom fetch — for test doubles, runtimes without a global fetch, or wrapping with your own metrics/tracing/timeout logic.
debug	boolean	false	Logs METHOD url -> status via console.debug. Patient-identifying search-parameter values are redacted to [REDACTED]; the Authorization header is never logged, period.

6. Example Usage

Full runnable examples live in docs/examples:

• basic-crud — CRUD lifecycle
• search-and-paginate — typed search + pagination
• batch-transaction — one batch request creating multiple resources
• with-smart-launch — wiring a token from peerbits-smart-launch

Batch/transaction, inline:

import { BundleBuilder, FhirClient } from "@peerbits/fhir-client";

const client = new FhirClient({ baseUrl: "https://hapi.fhir.org/baseR4/" });

const builder = new BundleBuilder()
  .create({ resourceType: "Patient", name: [{ family: "Doe", given: ["Jane"] }] })
  .create({ resourceType: "Patient", name: [{ family: "Doe", given: ["John"] }] });

const results = await client.executeBundle(builder, "batch");
results.forEach((r) => console.log(r.status, r.location));

Composing with peerbits-smart-launch's token output (illustrative — swap getTokenFromSmartLaunch for the real import once that repo is available):

import { FhirClient, type TokenProvider } from "@peerbits/fhir-client";
// import { getStoredToken as getTokenFromSmartLaunch } from "@peerbits/peerbits-smart-launch";

declare function getTokenFromSmartLaunch(): Promise<string>;

const tokenProvider: TokenProvider = () => getTokenFromSmartLaunch();
const client = new FhirClient({ baseUrl: process.env.FHIR_BASE_URL!, tokenProvider });

Handling errors (FR6 — every non-2xx response becomes a typed FhirClientError):

import { FhirClientError } from "@peerbits/fhir-client";

try {
  await client.read("Patient", "does-not-exist");
} catch (error) {
  if (error instanceof FhirClientError) {
    console.error(`FHIR error ${error.status} (${error.severity ?? "unknown"}):`, error.issues);
  } else {
    throw error; // network error, programmer error, etc. — not a FHIR-shaped failure
  }
}

For production deployment guidance — auth handling, retry/timeout tuning, logging, TLS, and a pre-go-live checklist — see docs/PRODUCTION_GUIDE.md. For which real EHR vendors (Epic, Cerner, SMART Health IT) this client has actually been checked against, see docs/VENDOR_COMPATIBILITY.md.

7. Roadmap

• Additional typed resource helpers beyond Patient/Observation/Encounter/Condition (the generic search/read/create escape hatch covers any resource type today)
• R5 support (tracked separately — this client targets R4 only for v1)
• FHIR Subscriptions / websocket / webhook support

Explicitly out of scope for this repo: any client-engagement-specific resource mapping or business logic, and full code-generation for all 150+ R4 resource types (see CONTRIBUTING.md for what's in/out of scope for contributions).

8. Contributing & Testing

See CONTRIBUTING.md for the full contribution workflow. Issues tagged good first issue are a good place to start.

Local development

Check your Node version first — this needs 20.12+ (see §3 Installation):

node -v
# too old? if you use nvm:
nvm install 20 && nvm use 20

Then clone and run the full check:

git clone https://github.com/PeerbitsSolution/fhir-client.git
cd fhir-client
npm install
npm run lint && npm run typecheck && npm test && npm run build

Running the examples in docs/examples locally — they import from "@peerbits/fhir-client", same as a real consumer would, but you don't need npm link or to edit the import to try them from inside this repo: once npm run build has produced dist/, Node's package self-referencing resolves that import to this repo's own dist/ automatically, because docs/examples/* lives inside the package whose own package.json declares that exact name:

npm run build
npx tsx docs/examples/basic-crud/index.ts
npx tsx docs/examples/search-and-paginate/index.ts
npx tsx docs/examples/batch-transaction/index.ts

To try an unreleased local change from a separate project (i.e. before it's published to npm — testing the way an external consumer eventually will, from outside this repo), use npm link instead:

npm run build
npm link
cd /path/to/some-other-project
npm link @peerbits/fhir-client

Testing

Command	What it runs	Network?
npm test	Unit tests (tests/*.test.ts) — client, search, bundle, errors, capability, all against a mocked fetch	No
npm run test:sandbox	Integration tests (tests/e2e.sandbox.test.ts) against the real HAPI FHIR public test server — CRUD, search/pagination, batch/transaction, capability discovery	Yes
npm run lint	ESLint over src/ and tests/	No
npm run typecheck	tsc --noEmit (strict mode)	No
npm run build	Compiles src/ → dist/	No

Sandbox tests are excluded from npm test / CI on purpose (a public sandbox can be temporarily down) but must be run manually and pass before any tagged release.

9. License

Apache License 2.0 — see LICENSE.

10. About Peerbits

fhir-client is part of the PeerbitsSolution HealthTech Open Source initiative — reusable engineering components extracted from our healthcare technology work, published so other teams don't have to solve the same problems from scratch. This repository contains generalized, reusable logic only; it is not tied to any specific client engagement or commercial product.