diff --git a/lib/lc/client.ts b/lib/lc/client.ts
index 95d803f..c7435b3 100644
--- a/lib/lc/client.ts
+++ b/lib/lc/client.ts
@@ -1,46 +1,24 @@
-import type { Question } from "lc-dailies/lib/api/mod.ts";
+import type {
+  LCClientInterface,
+  LCQuestion,
+  LCSubmission,
+} from "./client_interface.ts";
 import { makeQuestionURL } from "./urls.ts";
 import { gql } from "./gql.ts";
 
-/**
- * LCQuestion is an alias interface for a Leetcode question.
- */
-export type LCQuestion = Question;
-
-/**
- * LCSubmission is the representation of Leetcode's recent submission per user.
- */
-export interface LCSubmission {
-  /**
-   * id is the id details of the submission.
-   */
-  id: string;
-
-  /**
-   * name is the name of the question of the submission.
-   */
-  name: string;
-
-  /**
-   * title is the title of the question of the submission.
-   */
-  title: string;
-
-  /**
-   * timestamp is the time the submission was submitted.
-   */
-  timestamp: string;
-}
-
 /**
  * LCClient is the client for Leetcode.
  */
-export class LCClient {
+export class LCClient implements LCClientInterface {
+  constructor(
+    private readonly fetch: typeof window.fetch = window.fetch.bind(window),
+  ) {}
+
   /**
    * verifyUser verifies the user by username.
    */
   public async verifyUser(username: string): Promise<boolean> {
-    const response = await fetch(`https://leetcode.com/${username}/`);
+    const response = await this.fetch(`https://leetcode.com/${username}/`);
     return response.status === 200;
   }
 
@@ -142,7 +120,7 @@ export class LCClient {
        */
       .then((json) =>
         json.data.recentAcSubmissionList
-          .map((
+          ?.map((
             acSubmission: {
               id: string;
               title: string;
@@ -154,7 +132,7 @@ export class LCClient {
             name: acSubmission.titleSlug,
             title: acSubmission.title,
             timestamp: acSubmission.timestamp,
-          }))
+          })) ?? []
       );
   }
 }
diff --git a/lib/lc/client_interface.ts b/lib/lc/client_interface.ts
new file mode 100644
index 0000000..7a67d74
--- /dev/null
+++ b/lib/lc/client_interface.ts
@@ -0,0 +1,64 @@
+import type { Question } from "lc-dailies/lib/api/mod.ts";
+
+/**
+ * LCQuestion is an alias interface for a Leetcode question.
+ */
+export type LCQuestion = Question;
+
+/**
+ * LCSubmission is the representation of Leetcode's recent submission per user.
+ */
+export interface LCSubmission {
+  /**
+   * id is the id details of the submission.
+   */
+  id: string;
+
+  /**
+   * name is the name of the question of the submission.
+   */
+  name: string;
+
+  /**
+   * title is the title of the question of the submission.
+   */
+  title: string;
+
+  /**
+   * timestamp is the time the submission was submitted.
+   */
+  timestamp: string;
+}
+
+/**
+ * LCClientInterface is the client interface for Leetcode.
+ */
+export interface LCClientInterface {
+  /**
+   * verifyUser verifies the user by username.
+   */
+  verifyUser(username: string): Promise<boolean>;
+
+  /**
+   * getDailyQuestion gets the daily question from Leetcode.
+   */
+  getDailyQuestion(): Promise<LCQuestion>;
+
+  /**
+   * listDailyQuestions gets the last `amount` of daily questions from Leetcode since `asOfYear` and `asOfMonth`.
+   */
+  listDailyQuestions(
+    limit: number,
+    asOfYear: number,
+    asOfMonth: number,
+  ): Promise<LCQuestion[]>;
+
+  /**
+   * getRecentAcceptedSubmissions gets the recent accepted submissions from
+   * Leetcode by username.
+   */
+  getRecentAcceptedSubmissions(
+    username: string,
+    limit?: number,
+  ): Promise<LCSubmission[]>;
+}
diff --git a/lib/lc/fake_client.ts b/lib/lc/fake_client.ts
index d4d22bc..7775e4d 100644
--- a/lib/lc/fake_client.ts
+++ b/lib/lc/fake_client.ts
@@ -1,4 +1,8 @@
-import type { LCClient, LCQuestion, LCSubmission } from "./client.ts";
+import type {
+  LCClientInterface,
+  LCQuestion,
+  LCSubmission,
+} from "./client_interface.ts";
 
 export const FAKE_LC_USERNAME = "fake_lc_username";
 export const FAKE_LC_QUESTION_NAME = "fake_lc_question_name";
@@ -29,7 +33,7 @@ export const FAKE_RECENT_SUBMISSIONS: LCSubmission[] = [
 /**
  * FakeLCClient is a fake implementation of LCClient.
  */
-export class FakeLCClient implements LCClient {
+export class FakeLCClient implements LCClientInterface {
   public verifyUser(username: string): Promise<boolean> {
     return Promise.resolve(username === FAKE_LC_USERNAME);
   }
diff --git a/lib/lc/mod.ts b/lib/lc/mod.ts
index a786f1b..77239a1 100644
--- a/lib/lc/mod.ts
+++ b/lib/lc/mod.ts
@@ -1,2 +1,3 @@
+export * from "./client_interface.ts";
 export * from "./client.ts";
 export * from "./urls.ts";
diff --git a/lib/leaderboard/denokv/denokv_leaderboard_client.ts b/lib/leaderboard/denokv/denokv_leaderboard_client.ts
index 82fa03d..275fd23 100644
--- a/lib/leaderboard/denokv/denokv_leaderboard_client.ts
+++ b/lib/leaderboard/denokv/denokv_leaderboard_client.ts
@@ -2,7 +2,7 @@ import { DAY, ulid, WEEK } from "lc-dailies/deps.ts";
 import type * as api from "lc-dailies/lib/api/mod.ts";
 import type { LeaderboardClient } from "lc-dailies/lib/leaderboard/mod.ts";
 import { sync } from "lc-dailies/lib/leaderboard/mod.ts";
-import { LCClient } from "lc-dailies/lib/lc/mod.ts";
+import type { LCClientInterface } from "lc-dailies/lib/lc/mod.ts";
 
 /**
  * DenoKvLeaderboardClient is the client for the leaderboard.
@@ -16,7 +16,7 @@ export class DenoKvLeaderboardClient implements LeaderboardClient {
     /**
      * lc is the Leetcode client.
      */
-    private readonly lc: LCClient,
+    private readonly lc: LCClientInterface,
     /**
      * restartMS is the milliseconds to restart the leaderboard.
      *
diff --git a/lib/leaderboard/sync.ts b/lib/leaderboard/sync.ts
index 5ded446..6b7e179 100644
--- a/lib/leaderboard/sync.ts
+++ b/lib/leaderboard/sync.ts
@@ -1,6 +1,6 @@
 import { SECOND, WEEK } from "lc-dailies/deps.ts";
 import type * as api from "lc-dailies/lib/api/mod.ts";
-import type { LCClient } from "lc-dailies/lib/lc/mod.ts";
+import type { LCClientInterface } from "lc-dailies/lib/lc/mod.ts";
 import {
   calculateScores,
   makeDefaultCalculateScoresOptions,
@@ -23,7 +23,7 @@ export interface SyncOptions {
   /**
    * lcClient is the Leetcode client.
    */
-  lcClient: LCClient;
+  lcClient: LCClientInterface;
 
   /**
    * questionsFetchAmount is the amount of questions to fetch from Leetcode.