Overrides

Overview

The --overrides flag (CLI) and .overrides() builder method (library) deep-merge a JSON object into every generated resource. Fields you supply replace the generated values; fields you omit remain generated.

CLI: inline JSON

fhir-test-data generate patient --locale us \
  --overrides '{"meta":{"profile":["http://hl7.org/fhir/us/core/StructureDefinition/us-core-patient"]}}'

CLI: via stdin

Pipe JSON into generate as stdin. The --overrides flag, if also present, wins on key conflicts.

echo '{"active": true}' | fhir-test-data generate patient --locale us

Combine stdin base with --overrides for a two-layer merge:

cat base-overrides.json | fhir-test-data generate patient \
  --overrides '{"status":"active"}'

Library: .overrides() method

import { createPatientBuilder } from "fhir-test-data";

const patients = createPatientBuilder()
  .locale("us")
  .count(3)
  .seed(42)
  .overrides({
    meta: {
      profile: ["http://hl7.org/fhir/us/core/StructureDefinition/us-core-patient"],
    },
  })
  .build();

Merge semantics

Deep-merge means nested objects are merged recursively. Arrays are replaced entirely (not concatenated). For example:

• Override { "meta": { "profile": ["..."] } } adds meta.profile without removing other meta fields.
• Override { "identifier": [{ "system": "...", "value": "..." }] } replaces the entire identifier array.