fix(types): stop LabelValues distributing over label-name unions

CHANGELOG.md

@@ -11,6 +11,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 
+- fix: LabelValues no longer distributes over label-name unions, so `get()` labels keep every declared key (Fixes [#861](https://github.com/prometheus/client_js/issues/861))
 - Update Prettier and lint-staged
 - Organized default metrics
 - perf: Histogram rendering builds its export list straight from the store iterator instead of an intermediate array. Faster at high series counts on Node 24 and 26, can be slightly slower on Node 22

index.d.ts

@@ -332,7 +332,7 @@ export type MetricValueWithName<T extends string> = MetricValue<T> & {
 	metricName?: string;
 };
 
-type LabelValues<T extends string> = T extends NoLabelNameType
+type LabelValues<T extends string> = [T] extends [NoLabelNameType]
 	? Partial<Record<string, never>>
 	: Partial<Record<T, string | number>>;
 

test/typescript.ts

@@ -14,6 +14,7 @@
 
 import {
 	Counter,
+	Gauge,
 	Pushgateway,
 	Registry,
 	MetricObject,
@@ -107,3 +108,40 @@ async function metricTypeMatchesRuntimeStrings() {
 	void MetricType.Counter;
 }
 void metricTypeMatchesRuntimeStrings;
+
+// LabelValues must not distribute over a label-name union. A naked
+// `T extends never` splits Counter<'method' | 'status'> into a union of
+// one-key objects, so reading labels back from get() keeps only one key
+// (#861). Tuple-wrapping both sides keeps every declared name, and the
+// no-label `never` case still resolves to Partial<Record<string, never>>.
+const twoLabelCounter = new Counter({
+	name: 'typescript_two_label_counter',
+	help: 'TypeScript two-label counter',
+	labelNames: ['method', 'status'] as const,
+	registers: [registry],
+});
+const twoLabelGauge = new Gauge({
+	name: 'typescript_two_label_gauge',
+	help: 'TypeScript two-label gauge',
+	labelNames: ['method', 'status'] as const,
+	registers: [registry],
+});
+
+async function twoLabelGetKeepsBothKeys() {
+	const counterSnapshot = await twoLabelCounter.get();
+	const method: string | number | undefined =
+		counterSnapshot.values[0].labels.method;
+	const status: string | number | undefined =
+		counterSnapshot.values[0].labels.status;
+	void method;
+	void status;
+
+	const gaugeSnapshot = await twoLabelGauge.get();
+	const gaugeMethod: string | number | undefined =
+		gaugeSnapshot.values[0].labels.method;
+	const gaugeStatus: string | number | undefined =
+		gaugeSnapshot.values[0].labels.status;
+	void gaugeMethod;
+	void gaugeStatus;
+}
+void twoLabelGetKeepsBothKeys;