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fix(assert): hint at reference-equality when assertEquals diff is empty (#6878) #7158

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fix(assert): hint at reference-equality when assertEquals diff is empty (#6878) #7158

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ef8a534
fix(assert): hint at reference-equality when diff is empty (#6878)
LeSingh1 May 26, 2026
c379203
fmt: wrap long if-condition per deno fmt
LeSingh1 May 26, 2026
0900058
Merge branch 'main' into fix/6878-asserteq-reference-hint
bartlomieju Jun 30, 2026
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15 changes: 15 additions & 0 deletions assert/equals.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -64,5 +64,20 @@ export function assertEquals<T>(
const diffMsg = buildMessage(diffResult, { stringDiff }, arguments[3])
.join("\n");
message = `${message}\n${diffMsg}`;
// #6878: if the diff shows no removed/added lines, the values stringify
// identically but were still deemed unequal — typically because at least
// one nested property is a function (or Promise / Request / Blob / etc.)
// that gets compared by reference. The empty diff is confusing, so append
// a hint pointing at the likely cause.
if (
!stringDiff &&
diffResult.every((r) => r.type !== "added" && r.type !== "removed")

@bartlomieju bartlomieju Jun 30, 2026

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DiffResult.type is "added" | "removed" | "common", so this reads more directly as diffResult.every((r) => r.type === "common").

) {
message = `${message}\n` +
" Note: values stringify identically but are not structurally equal. " +
"Functions, Promises, Requests, Blobs, and other built-ins are compared by " +
"reference, so two distinct instances are never equal even when their " +
"representations match.";
Comment on lines +76 to +80

@bartlomieju bartlomieju Jun 30, 2026

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Minor: the Note: text hard-codes 4 leading spaces to align with buildMessage's [Diff] block — an implicit coupling to that formatter's indentation. Fine to leave, but a one-line comment noting why the 4 spaces are there would help the next person who touches it.

}
throw new AssertionError(message);
}
49 changes: 48 additions & 1 deletion assert/equals_test.ts
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Original file line number Diff line number Diff line change
@@ -1,5 +1,10 @@
// Copyright 2018-2026 the Deno authors. MIT license.
import { assertEquals, AssertionError, assertThrows } from "./mod.ts";
import {
assertEquals,
AssertionError,
assertStringIncludes,
assertThrows,
} from "./mod.ts";
import {
bold,
gray,
Expand Down Expand Up @@ -207,6 +212,48 @@ Deno.test({
},
});

Deno.test({
name:
"assertEquals() hints at reference-equality when objects with function props stringify identically (#6878)",
fn() {
let caught: AssertionError | undefined;
try {
assertEquals(
{ x: 1, y: () => 2 },
{ x: 1, y: () => 2 },
);
} catch (e) {
caught = e as AssertionError;
}
if (!caught) throw new Error("Expected assertEquals to throw");
assertStringIncludes(caught.message, "Values are not equal");
assertStringIncludes(
caught.message,
"stringify identically but are not structurally equal",
);
assertStringIncludes(caught.message, "compared by reference");
},
Comment on lines +219 to +235

@bartlomieju bartlomieju Jun 30, 2026

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Style nit: this file already imports assertThrows, which returns the thrown error — that's cleaner than the hand-rolled try/catch + if (!caught) throw:

const error = assertThrows(
  () => assertEquals({ x: 1, y: () => 2 }, { x: 1, y: () => 2 }),
  AssertionError,
);
assertStringIncludes(error.message, "Values are not equal");
assertStringIncludes(error.message, "stringify identically but are not structurally equal");
assertStringIncludes(error.message, "compared by reference");

});

Deno.test({
name:
"assertEquals() does not append reference-equality hint when there is a real textual diff",
fn() {
let caught: AssertionError | undefined;
try {
assertEquals({ x: 1 }, { x: 2 });
} catch (e) {
caught = e as AssertionError;
}
if (!caught) throw new Error("Expected assertEquals to throw");
assertEquals(
caught.message.includes("stringify identically"),
false,
"Hint should only fire when the diff is empty",
);
Comment on lines +242 to +253

@bartlomieju bartlomieju Jun 30, 2026

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Same assertThrows-returns-the-error simplification applies here. For the negative check, assert(!error.message.includes("stringify identically")) (or assertNotMatch) is clearer than assertEquals(..., false, msg).

},
});

Deno.test({
name: "assertEquals() matches same Set with object keys",
fn() {
Expand Down
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