feat(std/testing): Add assertExists assertion (#7874)

std/testing/asserts.ts

@@ -329,6 +329,23 @@ export function assertNotStrictEquals(
   );
 }
 
+/**
+ * Make an assertion that actual is not null or undefined. If not
+ * then thrown.
+ */
+export function assertExists(
+  actual: unknown,
+  msg?: string,
+): void {
+  if (actual === undefined || actual === null) {
+    if (!msg) {
+      msg =
+        `actual: "${actual}" expected to match anything but null or undefined`;
+    }
+    throw new AssertionError(msg);
+  }
+}
+
 /**
  * Make an assertion that actual includes expected. If not
  * then thrown.

std/testing/asserts_test.ts

@@ -4,6 +4,7 @@ import {
   assert,
   assertArrayIncludes,
   assertEquals,
+  assertExists,
   AssertionError,
   assertMatch,
   assertNotEquals,
@@ -160,6 +161,34 @@ Deno.test("testingNotEquals", function (): void {
   assertEquals(didThrow, true);
 });
 
+Deno.test("testingAssertExists", function (): void {
+  assertExists("Denosaurus");
+  assertExists(false);
+  assertExists(0);
+  assertExists("");
+  assertExists(-0);
+  assertExists(0);
+  assertExists(NaN);
+  let didThrow;
+  try {
+    assertExists(undefined);
+    didThrow = false;
+  } catch (e) {
+    assert(e instanceof AssertionError);
+    didThrow = true;
+  }
+  assertEquals(didThrow, true);
+  didThrow = false;
+  try {
+    assertExists(null);
+    didThrow = false;
+  } catch (e) {
+    assert(e instanceof AssertionError);
+    didThrow = true;
+  }
+  assertEquals(didThrow, true);
+});
+
 Deno.test("testingAssertStringContains", function (): void {
   assertStringIncludes("Denosaurus", "saur");
   assertStringIncludes("Denosaurus", "Deno");