fix(std/testing): Provide message and diff for assertStrictEq (#5417)

closes #5416

std/testing/asserts.ts

@@ -156,7 +156,8 @@ export function assertEquals(
       actualString.split("\n"),
       expectedString.split("\n")
     );
-    message = buildMessage(diffResult).join("\n");
+    const diffMsg = buildMessage(diffResult).join("\n");
+    message = `Values are not equal:\n${diffMsg}`;
   } catch (e) {
     message = `\n${red(CAN_NOT_DISPLAY)} + \n\n`;
   }
@@ -205,24 +206,41 @@ export function assertStrictEq(
   expected: unknown,
   msg?: string
 ): void {
-  if (actual !== expected) {
-    let actualString: string;
-    let expectedString: string;
-    try {
-      actualString = String(actual);
-    } catch (e) {
-      actualString = "[Cannot display]";
-    }
-    try {
-      expectedString = String(expected);
-    } catch (e) {
-      expectedString = "[Cannot display]";
-    }
-    if (!msg) {
-      msg = `actual: ${actualString} expected: ${expectedString}`;
-    }
-    throw new AssertionError(msg);
+  if (actual === expected) {
+    return;
   }
+
+  let message: string;
+
+  if (msg) {
+    message = msg;
+  } else {
+    const actualString = format(actual);
+    const expectedString = format(expected);
+
+    if (actualString === expectedString) {
+      const withOffset = actualString
+        .split("\n")
+        .map((l) => `     ${l}`)
+        .join("\n");
+      message = `Values have the same structure but are not reference-equal:\n\n${red(
+        withOffset
+      )}\n`;
+    } else {
+      try {
+        const diffResult = diff(
+          actualString.split("\n"),
+          expectedString.split("\n")
+        );
+        const diffMsg = buildMessage(diffResult).join("\n");
+        message = `Values are not strictly equal:\n${diffMsg}`;
+      } catch (e) {
+        message = `\n${red(CAN_NOT_DISPLAY)} + \n\n`;
+      }
+    }
+  }
+
+  throw new AssertionError(message);
 }
 
 /**

std/testing/asserts_test.ts

@@ -7,6 +7,7 @@ import {
   assertArrayContains,
   assertMatch,
   assertEquals,
+  assertStrictEq,
   assertThrows,
   AssertionError,
   equal,
@@ -278,7 +279,13 @@ test({
     assertThrows(
       (): void => assertEquals(1, 2),
       AssertionError,
-      [...createHeader(), removed(`-   1`), added(`+   2`), ""].join("\n")
+      [
+        "Values are not equal:",
+        ...createHeader(),
+        removed(`-   1`),
+        added(`+   2`),
+        "",
+      ].join("\n")
     );
   },
 });
@@ -289,7 +296,12 @@ test({
     assertThrows(
       (): void => assertEquals(1, "1"),
       AssertionError,
-      [...createHeader(), removed(`-   1`), added(`+   "1"`)].join("\n")
+      [
+        "Values are not equal:",
+        ...createHeader(),
+        removed(`-   1`),
+        added(`+   "1"`),
+      ].join("\n")
     );
   },
 });
@@ -301,6 +313,7 @@ test({
       (): void => assertEquals([1, "2", 3], ["1", "2", 3]),
       AssertionError,
       [
+        "Values are not equal:",
         ...createHeader(),
         removed(`-   [ 1, "2", 3 ]`),
         added(`+   [ "1", "2", 3 ]`),
@@ -317,6 +330,7 @@ test({
       (): void => assertEquals({ a: 1, b: "2", c: 3 }, { a: 1, b: 2, c: [3] }),
       AssertionError,
       [
+        "Values are not equal:",
         ...createHeader(),
         removed(`-   { a: 1, b: "2", c: 3 }`),
         added(`+   { a: 1, b: 2, c: [ 3 ] }`),
@@ -325,3 +339,51 @@ test({
     );
   },
 });
+
+test({
+  name: "strict pass case",
+  fn(): void {
+    assertStrictEq(true, true);
+    assertStrictEq(10, 10);
+    assertStrictEq("abc", "abc");
+
+    const xs = [1, false, "foo"];
+    const ys = xs;
+    assertStrictEq(xs, ys);
+
+    const x = { a: 1 };
+    const y = x;
+    assertStrictEq(x, y);
+  },
+});
+
+test({
+  name: "strict failed with structure diff",
+  fn(): void {
+    assertThrows(
+      (): void => assertStrictEq({ a: 1, b: 2 }, { a: 1, c: [3] }),
+      AssertionError,
+      [
+        "Values are not strictly equal:",
+        ...createHeader(),
+        removed("-   { a: 1, b: 2 }"),
+        added("+   { a: 1, c: [ 3 ] }"),
+        "",
+      ].join("\n")
+    );
+  },
+});
+
+test({
+  name: "strict failed with reference diff",
+  fn(): void {
+    assertThrows(
+      (): void => assertStrictEq({ a: 1, b: 2 }, { a: 1, b: 2 }),
+      AssertionError,
+      [
+        "Values have the same structure but are not reference-equal:\n",
+        red("     { a: 1, b: 2 }"),
+      ].join("\n")
+    );
+  },
+});