test: add tests for Intermediate

src/helpers/intermediate.ts

@@ -1,13 +1,12 @@
 import { IntermediateInterface } from "../interfaces";
-import { isIntermediate } from "./is-intermediate";
 
 export class Intermediate<T = {}> implements IntermediateInterface {
   public static of<T>(value: T): Intermediate<T> {
     return new Intermediate(value);
   }
 
   public static isIntermediate(x: unknown): x is IntermediateInterface {
-    return isIntermediate(x);
+    return typeof x == "object" && x != null && "intermediate" in x;
   }
 
   public intermediate: T;

test/intermediate.spec.ts

@@ -0,0 +1,22 @@
+import { Intermediate } from "../src";
+
+describe("Intermediate", (): void => {
+  it("should return an Intermediate", (): void => {
+    expect(Intermediate.of(123)).toBeInstanceOf(Intermediate);
+  });
+
+  it("should wrap given value into IntermediateInterface", (): void => {
+    expect(Intermediate.of(123).intermediate).toEqual(123);
+  });
+});
+
+describe("Intermediate.isIntermediate", (): void => {
+  it("should return true if argument is intermediate", (): void => {
+    expect(Intermediate.isIntermediate({ intermediate: 123 })).toEqual(true);
+  });
+
+  it("should return false if provided argument is not intermediate", (): void => {
+    expect(Intermediate.isIntermediate(123)).toEqual(false);
+    expect(Intermediate.isIntermediate({ key: 123 })).toEqual(false);
+  });
+});