Bug: ResultFormatter.heading / airspeed / groundspeed / mach / day / month / arrivalDay / departureDay don't guard NaN — silently render `"NaN"` values in `formatted.items`

[kevinelliott]

Summary

ResultFormatter.position (lib/utils/result_formatter.ts:51-65), ResultFormatter.altitude (67-78), ResultFormatter.flightNumber (80-91), and ResultFormatter.temperature/totalAirTemp (331-359) all validate their input before pushing to formatted.items. Eight sibling formatters do not — they store whatever number they receive (including NaN) and then interpolate it into the formatted string, producing strings like "NaN knots", "NaN mach", and "NaN" in downstream UI:

Formatter	Line	Output on NaN input
heading	361-369	raw.heading = NaN, value: "NaN"
groundspeed	301-309	raw.groundspeed = NaN, value: "NaN knots"
airspeed	311-319	raw.airspeed = NaN, value: "NaN knots"
mach	321-329	raw.mach = NaN, value: "NaN mach"
day	440-448	raw.day = NaN, value: "NaN"
month	450-458	raw.month = NaN, value: "NaN"
departureDay	460-468	raw.departure_day = NaN, value: "NaN"
arrivalDay	470-478	raw.arrival_day = NaN, value: "NaN"

Because JavaScript's Number(...) returns NaN for common ACARS "unavailable" markers ('***', '---', '****') and any non-numeric text, every caller that passes Number(field) or parseInt(field) straight to one of these formatters propagates NaN all the way to the consumer.

Affected callers (non-exhaustive)

Real call sites already in the codebase that can silently emit NaN:

• lib/plugins/Label_10_Slash.ts:41 — ResultFormatter.heading(decodeResult, Number(parts[5])) — any 10 message with parts[5] === '---' (a common "unknown heading" marker) sends NaN into heading.
• lib/plugins/Label_2P_FM4.ts:53 — ResultFormatter.heading(decodeResult, Number(parts[7])).
• lib/plugins/Label_10_Slash.ts:42 — ResultFormatter.altitude(decodeResult, 100 * Number(parts[6])) — this one is covered because altitude has its own isNaN guard.
• lib/plugins/Label_44_Base.ts:52-53, Label_44_ETA.ts:37-38, Label_44_POS.ts:41-48 — ResultFormatter.month/day receive Number(data[4].substring(0, 2)); if data[4] is empty or non-numeric the result is NaN.
• lib/plugins/Label_4T_ETA.ts:31-33 (before/after the fix for Bug: Label_4T_ETA dereferences etaData[2].substring with no length guard — short ETA payloads throw TypeError and abort MessageDecoder.decode() #482) — ResultFormatter.departureDay(...) / arrivalDay(...) on Number(data[1]) etc. can be NaN.

Reproduction

import { ResultFormatter } from './utils/result_formatter';

const dr: any = { raw: {}, formatted: { items: [] } };
ResultFormatter.heading(dr, Number('---'));
// dr.raw.heading === NaN
// dr.formatted.items[0].value === 'NaN'

ResultFormatter.mach(dr, Number(''));
// dr.raw.mach === 0                            (Number('') → 0, so this one is "silent zero" — see #487)
ResultFormatter.mach(dr, Number('***'));
// dr.raw.mach === NaN, value === 'NaN mach'

ResultFormatter.day(dr, Number('  '));
// dr.raw.day === NaN, value === 'NaN'

End-to-end reproduction via a real plugin (Label_10_Slash needs 17 slash-separated fields; simplest way to hit the heading NaN path):

const decoder = new MessageDecoder();
decoder.decode({
  label: '10',
  text: '/N0000/E0000//-/---/000/KJFK/000000////WPT1/WPT2/000000/WPT3/000000/KLAX',
});
// result.formatted.items includes { code: 'HDG', value: 'NaN' }
// result.raw.heading is NaN, so consumers can't distinguish "unknown" from "0".

Suggested fix

Match the pattern used by altitude (guard isNaN at the top and return without pushing), so callers can pass raw Number(field) results and unknown values disappear from formatted.items rather than surfacing as literal "NaN":

static heading(decodeResult: DecodeResult, value: number) {
  if (isNaN(value)) return;
  decodeResult.raw.heading = value;
  decodeResult.formatted.items.push({
    type: 'heading',
    code: 'HDG',
    label: 'Heading',
    value: `${value}`,
  });
}
// Same shape for airspeed, groundspeed, mach, day, month, departureDay, arrivalDay.

The fuel formatters (currentFuel, burnedFuel, remainingFuel, outFuel, offFuel, onFuel, inFuel, startFuel) have the same shape and are worth guarding while touching this file — some callers do their own isNaN check first (e.g. Label_44_Base.parseFuel, Label_44_ETA:45-48), but not all, and centralising the guard removes the requirement.

Test coverage

Add unit tests for each affected formatter covering:

• Number('---') / Number('***') → no item pushed, raw.<field> left unset
• Number('42') → item pushed with value: '42' (or '42 knots', etc.)
• Number('') — document the current behaviour (silent 0; that's a separate class, see Bug: Label_2P_FM3 else-branch reports position (0, 0) for empty lat/lon fields — silently places aircraft in the Gulf of Guinea #487)