test: Add tests.

test/source/constants/transformers/pluralisers/common-slavic.spec.ts

@@ -1,5 +1,5 @@
 import { describe, it } from "bun:test";
-import pluralise from "logos:constants/transformers/pluralisers/common-slavic.ts";
+import pluralise from "logos:constants/transformers/pluralisers/common-slavic";
 import { expect } from "chai";
 
 describe("pluralise()", () => {

test/source/constants/transformers/pluralisers/invariant.spec.ts

@@ -1,5 +1,31 @@
-import { describe } from "bun:test";
+import { describe, it } from "bun:test";
+import { expect } from "chai";
+import pluralise from "logos:constants/transformers/pluralisers/invariant";
 
 describe("pluralise()", () => {
-	// TODO(vxern): Test.
+	const FORMS = { one: "ember" };
+
+	it("returns undefined if the passed object does not contain form `one`.", () => {
+		expect(pluralise("0", {})).to.be.undefined;
+	});
+
+	it("returns the singular form when the quantity is 1.", () => {
+		expect(pluralise("1", FORMS)).to.equal("ember");
+	});
+
+	it("returns the singular form when the quantity is 0.", () => {
+		expect(pluralise("0", FORMS)).to.equal("ember");
+	});
+
+	it("returns the singular form when the quantity is -1.", () => {
+		expect(pluralise("-1", FORMS)).to.equal("ember");
+	});
+
+	it("returns the singular form when the quantity is 2 or more.", () => {
+		expect(pluralise("2", FORMS)).to.equal("ember");
+	});
+
+	it("returns the singular form when the quantity is -2 or less.", () => {
+		expect(pluralise("-2", FORMS)).to.equal("ember");
+	});
 });