feat(lens.spec): Add the lens array length test.

src/lens.spec.ts

@@ -1,19 +1,19 @@
 import { idLens } from "./lens";
 
-const testObj = {
-  a: {
-    b: {
-      c: "d",
-    },
-    e: "f",
-  },
-  g: "h",
-};
-const testLens = idLens<typeof testObj>();
-const deepLens = testLens.thenKey("a").thenKey("b").thenKey("c");
-
 describe("with deep lens and object", () => {
 
+  const testObj = {
+    a: {
+      b: {
+        c: "d",
+      },
+      e: "f",
+    },
+    g: "h",
+  };
+  const testLens = idLens<typeof testObj>();
+  const deepLens = testLens.thenKey("a").thenKey("b").thenKey("c");
+
   test("get a value deep", () => {
     expect(deepLens.get(testObj)).toBe("d");
   });
@@ -41,3 +41,14 @@ describe("with deep lens and object", () => {
 
   });
 });
+
+describe("with array", () => {
+
+  const testObj = [0, 1];
+  const testLens = idLens<typeof testObj>();
+  const deepLens = testLens.thenKey("length");
+
+  test("get a length from list", () => {
+    expect(deepLens.get(testObj)).toBe(2);
+  });
+});