diff --git a/tests/utils/language.test.ts b/tests/utils/language.test.ts
new file mode 100644
index 0000000..3c3d958
--- /dev/null
+++ b/tests/utils/language.test.ts
@@ -0,0 +1,46 @@
+import faker from "faker";
+import { replaceBoth, retrieveFiltered } from "../../utils/language";
+import { languages, exceptions, mappings } from "../../utils/languages.json";
+
+describe("replaceBoth", () => {
+    it("replaces excepted sources correctly", () => {
+        Object.entries(exceptions.source).forEach(([code, replacement]) => {
+            const { source } = replaceBoth("exception", { source: code, target: "" })
+            expect(source).toBe(replacement);
+        });
+    });
+
+    it("replaces excepted targets correctly", () => {
+        Object.entries(exceptions.target).forEach(([code, replacement]) => {
+            const { target } = replaceBoth("exception", { source: "", target: code })
+            expect(target).toBe(replacement);
+        });
+    });
+
+    it("replaces mapped sources correctly", () => {
+        Object.entries(mappings.source).forEach(([code, replacement]) => {
+            const { source } = replaceBoth("mapping", { source: code, target: "" })
+            expect(source).toBe(replacement);
+        });
+    });
+
+    it("replaces mapped targets correctly", () => {
+        Object.entries(mappings.target).forEach(([code, replacement]) => {
+            const { target } = replaceBoth("mapping", { source: "", target: code })
+            expect(target).toBe(replacement);
+        });
+    });
+});
+
+describe("retrieveFiltered", () => {
+    it("filters by exceptions & by opposite values", () => {
+        const source = faker.random.locale();
+        const target = faker.random.locale();
+        const sourceKeys = Object.keys(languages).filter(code => !Object.keys(exceptions.source).includes(code) && code !== target);
+        const targetKeys = Object.keys(languages).filter(code => !Object.keys(exceptions.target).includes(code) && code !== source);
+
+        const { sourceLangs, targetLangs } = retrieveFiltered(source, target);
+        expect(sourceLangs.map(([code]) => code)).toStrictEqual(sourceKeys);
+        expect(targetLangs.map(([code]) => code)).toStrictEqual(targetKeys);
+    });
+});
diff --git a/tests/utils/reducer.test.ts b/tests/utils/reducer.test.ts
new file mode 100644
index 0000000..13431ed
--- /dev/null
+++ b/tests/utils/reducer.test.ts
@@ -0,0 +1,53 @@
+import faker from "faker";
+import langReducer, { Actions, initialState } from "../../utils/reducer";
+
+it("changes a field value", () => {
+    const query = faker.random.words();
+
+    const res = langReducer(initialState, {
+        type: Actions.SET_FIELD,
+        payload: {
+            key: "query",
+            value: query
+        }
+    });
+    expect(res).toStrictEqual({ ...initialState, query });
+});
+
+it("changes all fields", () => {
+    const state = {
+        source: faker.random.locale(),
+        target: faker.random.locale(),
+        query: faker.random.words()
+    };
+
+    const res = langReducer(initialState, {
+        type: Actions.SET_ALL,
+        payload: { state }
+    });
+    expect(res).toStrictEqual(state);
+});
+
+it("switches the languages", () => {
+    const state = {
+        ...initialState,
+        source: "es",
+        target: "ca"
+    };
+
+    const res = langReducer(state, { type: Actions.SWITCH_LANGS });
+    expect(res.source).toStrictEqual(state.target);
+    expect(res.target).toStrictEqual(state.source);
+});
+
+it("resets the source while switching if they're the same", () => {
+    const state = {
+        ...initialState,
+        source: "eo",
+        target: "eo"
+    };
+
+    const res = langReducer(state, { type: Actions.SWITCH_LANGS });
+    expect(res.source).toStrictEqual(initialState.source);
+    expect(res.target).toStrictEqual(state.source);
+});
diff --git a/utils/language.ts b/utils/language.ts
index 8780c1b..c376c62 100644
--- a/utils/language.ts
+++ b/utils/language.ts
@@ -7,7 +7,7 @@ const checkTypes = {
 
 type CheckType = keyof typeof checkTypes;
 
-export const langTypes = [
+const langTypes = [
     "source",
     "target"
 ] as const;