diff --git a/web/database/worker/db-worker.js b/web/database/worker/db-worker.js
--- a/web/database/worker/db-worker.js
+++ b/web/database/worker/db-worker.js
@@ -23,7 +23,7 @@
 };
 localforage.config(localforageConfig);
 
-let sqliteDb = null;
+let sqliteDb: ?SqliteDatabase = null;
 
 async function initDatabase(sqljsFilePath: string, sqljsFilename: ?string) {
   const content = await localforage.getItem(SQLITE_CONTENT);
diff --git a/web/flow-typed/SQLiteJS.js b/web/flow-typed/SQLiteJS.js
new file mode 100644
--- /dev/null
+++ b/web/flow-typed/SQLiteJS.js
@@ -0,0 +1,72 @@
+// @flow
+
+declare type SqlValue = number | string | Uint8Array | null;
+declare type ParamsObject = { +[key: string]: SqlValue, ... };
+declare type ParamsCallback = (obj: ParamsObject) => void;
+
+declare type BindParams = $ReadOnlyArray<SqlValue> | ParamsObject | null;
+
+declare type QueryExecResult = {
+  +columns: $ReadOnlyArray<string>,
+  +values: $ReadOnlyArray<$ReadOnlyArray<SqlValue>>,
+};
+
+declare type StatementIteratorResult = {
+  +done: boolean,
+  +value: SqliteStatement,
+};
+
+declare type SqlJsStatic = {
+  +Database: SqliteDatabase,
+  +Statement: SqliteStatement,
+};
+
+declare type InitSqlJsStatic = {
+  (config?: Object): Promise<SqlJsStatic>,
+};
+
+declare class SqliteDatabase {
+  constructor(data?: Array<number> | Buffer | null): this;
+  close(): void;
+
+  create_function<T, R>(
+    name: string,
+    func: (...args: $ReadOnlyArray<T>) => R,
+  ): SqliteDatabase;
+  each(
+    sql: string,
+    params: BindParams,
+    callback: ParamsCallback,
+    done: () => void,
+  ): SqliteDatabase;
+  each(sql: string, callback: ParamsCallback, done: () => void): SqliteDatabase;
+  exec(sql: string, params?: BindParams): $ReadOnlyArray<QueryExecResult>;
+  export(): Uint8Array;
+  getRowsModified(): number;
+  handleError(): null | empty;
+  iterateStatements(sql: string): StatementIterator;
+  prepare(sql: string, params?: BindParams): SqliteStatement;
+  run(sql: string, params?: BindParams): SqliteDatabase;
+}
+
+declare class SqliteStatement {
+  bind(values?: BindParams): boolean;
+  free(): boolean;
+  freemem(): void;
+  get(params?: BindParams): $ReadOnlyArray<SqlValue>;
+  getAsObject(params?: BindParams): ParamsObject;
+  getColumnNames(): $ReadOnlyArray<string>;
+  getNormalizedSQL(): string;
+  getSQL(): string;
+  reset(): void;
+  run(values?: BindParams): void;
+  step(): boolean;
+}
+
+declare class StatementIterator implements Iterable<SqliteStatement> {
+  @@iterator: () => Iterator<SqliteStatement>;
+  getRemainingSql(): string;
+  next(): StatementIteratorResult;
+}
+
+declare var initSqlJs: InitSqlJsStatic;