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

closes #5416

1 files changed, 35 insertions, 17 deletions

diff --git a/std/testing/asserts.ts b/std/testing/asserts.ts
index 6dd3af226..d3f8bb678 100644
--- a/std/testing/asserts.ts
+++ b/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}`;
+  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(msg);
   }
+
+  throw new AssertionError(message);
 }
 
 /**