fix(expect): updated error message for toContain (#4750)

Co-authored-by: Yoshiya Hinosawa <stibium121@gmail.com>

expect/_matchers.ts

@@ -373,13 +373,20 @@ export function toContain(
   // deno-lint-ignore no-explicit-any
   const doesContain = (context.value as any)?.includes?.(expected);
 
+  const fmtValue = format(context.value);
+  const fmtExpected = format(expected);
+
   if (context.isNot) {
     if (doesContain) {
-      throw new AssertionError("The value contains the expected item");
+      throw new AssertionError(
+        `The value ${fmtValue} contains the expected item ${fmtExpected}`,
+      );
     }
   } else {
     if (!doesContain) {
-      throw new AssertionError("The value doesn't contain the expected item");
+      throw new AssertionError(
+        `The value ${fmtValue} doesn't contain the expected item ${fmtExpected}`,
+      );
     }
   }
 }
@@ -399,13 +406,29 @@ export function toContainEqual(
     }
   }
 
+  const prettyStringify = (js: unknown) =>
+    JSON.stringify(js, null, "\t")
+      .replace(/\"|\n|\t/g, "")
+      .slice(0, 100);
+
+  const fmtValue = prettyStringify(context.value);
+  const fmtExpected = prettyStringify(expected);
+
   if (context.isNot) {
     if (doesContain) {
-      throw new AssertionError("The value contains the expected item");
+      throw new AssertionError(
+        `The value contains the expected item.
+Value: ${fmtValue}
+Expected: ${fmtExpected}`,
+      );
     }
   } else {
     if (!doesContain) {
-      throw new AssertionError("The value doesn't contain the expected item");
+      throw new AssertionError(
+        `The value doesn't contain the expected item.
+Value: ${fmtValue}
+Expected: ${fmtExpected}`,
+      );
     }
   }
 }

expect/_to_contain_equal_test.ts

@@ -9,11 +9,23 @@ Deno.test("expect().toContainEqual()", () => {
 
   expect(arr).not.toContainEqual({ bar: 42 });
 
-  assertThrows(() => {
-    expect(arr).toContainEqual({ bar: 42 });
-  }, AssertionError);
+  assertThrows(
+    () => {
+      expect(arr).toContainEqual({ bar: 42 });
+    },
+    AssertionError,
+    `The value doesn't contain the expected item.
+Value: [{foo: 42},{bar: 43},{baz: 44}]
+Expected: {bar: 42}`,
+  );
 
-  assertThrows(() => {
-    expect(arr).not.toContainEqual({ bar: 43 });
-  }, AssertionError);
+  assertThrows(
+    () => {
+      expect(arr).not.toContainEqual({ foo: 42 });
+    },
+    AssertionError,
+    `The value contains the expected item.
+Value: [{foo: 42},{bar: 43},{baz: 44}]
+Expected: {foo: 42}`,
+  );
 });

expect/_to_contain_test.ts

@@ -15,14 +15,22 @@ Deno.test("expect().toContain()", () => {
   assertThrows(() => {
     expect(arr).toContain(4);
   }, AssertionError);
-  assertThrows(() => {
-    expect("foobarbaz").toContain("qux");
-  }, AssertionError);
+  assertThrows(
+    () => {
+      expect("foobarbaz").toContain("qux");
+    },
+    AssertionError,
+    `The value "foobarbaz" doesn't contain the expected item "qux"`,
+  );
 
   assertThrows(() => {
     expect(arr).not.toContain(2);
   }, AssertionError);
-  assertThrows(() => {
-    expect("foobarbaz").not.toContain("bar");
-  }, AssertionError);
+  assertThrows(
+    () => {
+      expect("foobarbaz").not.toContain("bar");
+    },
+    AssertionError,
+    'The value "foobarbaz" contains the expected item "bar"',
+  );
 });