Bug: Label_2P_FM3 / FM4 / FM5 missing-header guard is dead (split.length is never 0); FM3 then throws on header[1].length, FM4/FM5 emit undefined fields

[kevinelliott]

Summary

All three Label_2P_FM* plugins try to detect a missing FM3 / FM4 / FM5 preamble with if (header.length == 0) after splitting parts[0]. String.prototype.split always returns at least one element, so the guard is never true — it's dead code. When the preamble token actually is missing, header[1] is undefined:

• FM3 dereferences header[1].length → throws TypeError: Cannot read properties of undefined (reading 'length'). Because the dispatch in MessageDecoder.decode does not wrap individual plugins in try/catch, the exception propagates out and aborts the whole decode.
• FM4 and FM5 silently pass undefined to ResultFormatter.departureAirport, producing a "successfully decoded" result with departureAirport: { value: undefined }.

All three plugins are reachable from MessageDecoder for label 2P (see MessageDecoder.ts:40-42), and any 2P message whose comma-count matches the plugin's expectation but whose parts[0] doesn't actually contain the FMx token hits this path.

This is not a duplicate of #459 (different file, different handlers).

Affected code

Same shape in three files. FM3:

// lib/plugins/Label_2P_FM3.ts:24-40
if (parts.length === 7) {
  const header = parts[0].split('FM3 ');
  if (header.length == 0) {                          // never true
    ResultFormatter.unknown(decodeResult, message.text);
    decodeResult.decoded = false;
    decodeResult.decoder.decodeLevel = 'none';
    return decodeResult;
  }
  if (header[0].length > 0) {
    ResultFormatter.unknown(decodeResult, header[0].substring(0, 4));
    ResultFormatter.flightNumber(decodeResult, header[0].substring(4));
  }
  if (header[1].length === 4) { ... }                // throws when header[1] is undefined

FM4 (lib/plugins/Label_2P_FM4.ts:23-37) and FM5 (lib/plugins/Label_2P_FM5.ts:23-34) have the same dead guard and then pass header[1] straight into ResultFormatter.departureAirport without checking.

Note FM3 and FM5 split on 'FM3 '/'FM5 ' (with trailing space) while FM4 splits on 'FM4' (no space). The fix is the same shape in all three.

Reproduction

const decoder = new MessageDecoder();

// FM3 throws
decoder.decode({
  label: '2P',
  text: 'noFMheader,1454,N 45.206,E 17.726,34030, 440,98',
});
// → TypeError: Cannot read properties of undefined (reading 'length')

// FM4 silently produces undefined fields (10-field text, no 'FM4' token)
decoder.decode({
  label: '2P',
  text: 'noFMheader,KORD,2114000,1518,N 45.206,E 17.726,34030,180,foo,bar',
});
// → decoded: true, but result includes departureAirport.value === undefined

// FM5 silently produces undefined fields (12-field text, no 'FM5 ' token)
decoder.decode({
  label: '2P',
  text: 'noFMhdr,KORD,142000,1518,N 45.206,E 17.726,34030,a,b,c,FLT123,d',
});
// → decoded: true, but result includes departureAirport.value === undefined

Suggested fix

Change the guard in each plugin to check for the case the comment actually describes — the FMx token wasn't present:

if (header.length < 2) {       // or:  if (parts[0].indexOf('FM3 ') === -1)
  ResultFormatter.unknown(decodeResult, message.text);
  decodeResult.decoded = false;
  decodeResult.decoder.decodeLevel = 'none';
  return decodeResult;
}

(Replace 'FM3 ' / 'FM4' / 'FM5 ' accordingly per file.)

Test coverage

The existing Label_2P_FM3 test only covers well-formed FM3 … input, so the FM3 throw is uncovered. Adding a "no preamble" case per plugin that asserts the result has decoded === false and (for FM3) that the call does not throw would lock the fix in.