feat(assertion): Add .toBeUndefined() matcher (#64)

src/lib/Assertion.ts

@@ -128,6 +128,28 @@ export class Assertion<T> {
     });
   }
 
+  /**
+   * Check if the value is `undefined`
+   *
+   * @returns the assertion instance
+   */
+  public toBeUndefined(): this {
+    const error = new AssertionError({
+      actual: this.actual,
+      message: `Expected <${this.actual}> to be undefined`
+    });
+    const invertedError = new AssertionError({
+      actual: this.actual,
+      message: "Expected the value NOT to be undefined"
+    });
+
+    return this.execute({
+      assertWhen: this.actual === undefined,
+      error,
+      invertedError
+    });
+  }
+
   /**
    * Check if the value is `null`.
    *

test/lib/Assertion.test.ts

@@ -164,6 +164,32 @@ describe("[Unit] Assertion.test.ts", () => {
     });
   });
 
+  describe(".toBeUndefined", () => {
+    context("when the value is undefined", () => {
+      it("returns the assertion instance", () => {
+        const test = new Assertion(undefined);
+
+        assert.deepStrictEqual(test.toBeUndefined(), test);
+        assert.throws(() => test.not.toBeUndefined(), {
+          message: "Expected the value NOT to be undefined",
+          name: AssertionError.name
+        });
+      });
+    });
+
+    context("when the value is NOT undefined", () => {
+      it("throws an assertion error", () => {
+        const test = new Assertion("foo");
+
+        assert.throws(() => test.toBeUndefined(), {
+          message: "Expected <foo> to be undefined",
+          name: AssertionError.name
+        });
+        assert.deepStrictEqual(test.not.toBeUndefined(), test);
+      });
+    });
+  });
+
   describe(".toBeNull", () => {
     context("when the value is null", () => {
       it("returns the assertion instance", () => {