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

CHANGELOG.md

@@ -26,6 +26,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 - fix: Non-nullish, non-string label values are coerced to strings when a combination is first stored, so exposition escapes them; the store also keeps its own copy, so mutating the caller's object after recording no longer changes the stored series
 - fix: Label-less summaries report `labels: {}` in `getMetricsAsJSON()`, like other metrics (Fixes [#838](https://github.com/prometheus/client_js/issues/838))
 - chore: update benchmark baseline to v0.16.1
+- fix: TypeScript `LabelValues` no longer distributes over a union of label names, so labels read back from `get()` keep every declared key (Fixes [#861](https://github.com/prometheus/client_js/issues/861))
 
 ### Added
 

index.d.ts

@@ -332,7 +332,10 @@ export type MetricValueWithName<T extends string> = MetricValue<T> & {
 	metricName?: string;
 };
 
-type LabelValues<T extends string> = T extends NoLabelNameType
+// The tuple wrapping stops the conditional from distributing over a union of
+// label names, which would turn LabelValues<'a' | 'b'> into a union of
+// single-key objects instead of one object with both keys.
+type LabelValues<T extends string> = [T] extends [NoLabelNameType]
 	? Partial<Record<string, never>>
 	: Partial<Record<T, string | number>>;
 

test/typescript.ts

@@ -73,6 +73,28 @@ async function metricObjectTypesAreExported() {
 }
 void metricObjectTypesAreExported;
 
+// LabelValues must not distribute over a union of label names: labels read
+// back from Metric#get() carry every declared key (#861).
+async function labelValuesKeepEveryLabelName() {
+	const labelled = new Counter({
+		name: 'typescript_test_labelled_counter',
+		help: 'TypeScript test counter with labels',
+		labelNames: ['a', 'b'] as const,
+		registers: [registry],
+	});
+	labelled.inc({ a: '1', b: 2 });
+
+	const { values } = await labelled.get();
+	const a: string | number | undefined = values[0].labels.a;
+	const b: string | number | undefined = values[0].labels.b;
+	void a;
+	void b;
+
+	// @ts-expect-error only declared label names are allowed.
+	void values[0].labels.c;
+}
+void labelValuesKeepEveryLabelName;
+
 // MetricType matches the runtime strings reported in metric metadata, so a
 // reported type can be compared against a literal and narrowed without casts.
 // The switch fails to compile if the union and this member list ever drift