feat: ✨️ 添加sum操作符

src/operator/index.ts

@@ -3,3 +3,4 @@ export * from "./toDesc";
 export * from "./toPage";
 export * from "./toOption";
 export * from "./toVirtualPage";
+export * from "./sum";

src/operator/sum.test.ts

@@ -0,0 +1,13 @@
+import { from } from "rxjs";
+import { sum } from "./sum";
+
+describe("sum", function () {
+  it("should be right", function (done) {
+    from([1, 2, 3, 4])
+      .pipe(sum())
+      .subscribe((x) => {
+        expect(x).toBe(10);
+        done();
+      });
+  });
+});

src/operator/sum.ts

@@ -0,0 +1,5 @@
+import { pipe, reduce } from "rxjs";
+
+export function sum() {
+  return pipe(reduce<number, number>((acc, cur) => acc + cur, 0));
+}