Required message header

Lint rule that requires the message header segment (MSH) to be the first segment of an HL7v2 message.

What it does

Walks the parsed tree and checks that the first segment node is named MSH. If a different segment appears first, the rule reports one message naming that segment; if the message has no segments, it reports the message as empty. HL7v2 messages must begin with MSH because it carries the metadata required for routing and interpretation.

Install

npm install @glion/lint-required-message-header

Use

import { hl7v2Parser } from "@glion/parser";
import hl7v2LintRequiredMessageHeader from "@glion/lint-required-message-header";
import { unified } from "unified";
import { reporter } from "vfile-reporter";

const message =
  "MSH|^~\\&|SENDER|FAC|RECV|RFAC|20250601120000||ADT^A01^ADT_A01|MSG00001|P|2.5";

const file = await unified()
  .use(hl7v2Parser)
  .use(hl7v2LintRequiredMessageHeader)
  .process(message);

console.error(reporter([file]));

API

unified().use(hl7v2LintRequiredMessageHeader)

A unified lint rule plugin. Takes no options.

The plugin visits the tree and finds the first segment node (descending through any nested groups). If it is not named MSH, or the tree has no segments, the plugin reports a message. The message's severity is the one the rule is configured with; it does not throw.

import type { Plugin } from "unified";
import type { Root } from "@glion/ast";

declare const hl7v2LintRequiredMessageHeader: Plugin<[], Root>;
export default hl7v2LintRequiredMessageHeader;

What it checks

The message must have at least one segment, and the first must be MSH.

Valid

MSH comes first, followed by any other segments:

MSH|^~\&|SENDER|FAC|RECV|RFAC|20250601120000||ADT^A01^ADT_A01|MSG00001|P|2.5
PID|1||PATID1234^^^HOSP^MR||DOE^JANE||19800101|F
OBX|1|TX|...

Invalid

A non-MSH segment appears first:

PID|1||PATID1234^^^HOSP^MR||DOE^JANE||19800101|F

Reported message:

The first segment is not the message header segment; a message must start with the message header segment (`MSH`).

A segment with an empty Segment ID first, such as an input that starts with the field separator:

|A|B

Reported message:

The first segment is not the message header segment; a message must start with the message header segment (`MSH`).

An empty input (""), which parses to a tree with no segments, is reported as:

The message has no segments; it must start with the message header segment (`MSH`).

The rule reports at most one message per tree. Each message sets expected to ['MSH'] and, when the message has a first segment, actual to its Segment ID.