`expect(...).toEqual(...)` fails on strings encoded with `TextEncoder`

🐛 Bug Report

To Reproduce

Steps to reproduce the behavior:

• Create a project: npx create-react-app jest-test1 --typescript
• Modify package.json: "test": "react-scripts test --env=node" (because TextEncoder is not available at all in jsdom)
• Create a test like the one below

Expected behavior

I expect this test…

function encodeFoo() {
  return new TextEncoder().encode("foo");
}

it('string encoded with TextEncoder equals to the expected byte array', () => {
  expect(encodeFoo()).toEqual(new Uint8Array([102, 111, 111])); // fails
});

…to succeed.

Instead it fails with:

  ✕ string encoded with TextEncoder equals to the expected byte array (3ms)

  ● string encoded with TextEncoder equals to the expected byte array

    expect(received).toEqual(expected) // deep equality

    Expected: [102, 111, 111]
    Received: serializes to the same string

      14 |
      15 | it('string encoded with TextEncoder equals to the expected byte array', () => {
    > 16 |   expect(encodeFoo()).toEqual(new Uint8Array([102, 111, 111])); // fails
         |                       ^
      17 | });
      18 |

      at Object.<anonymous> (src/App.test.tsx:16:23)

Link to repl or repo (highly encouraged)

https://github.com/cubuspl42/jest-test1

envinfo

  System:
    OS: macOS Mojave 10.14.5
    CPU: (8) x64 Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
  Binaries:
    Node: 12.12.0 - /usr/local/bin/node
    npm: 6.11.3 - /usr/local/bin/npm

[wsmd]

I’ve done some investigation on this and it seems that the constructor of the Uint8Array returned from TextEncoder.prototype.encode and the global Uint8Array constructor are not the same (causing the assertion to fail via the iterableEquality); note that this only happens in the jest node environment.

// in Jest (using the node environment)
new TextEncoder().encode('').constructor === Uint8Array // false

// outside Jest (regular node runtime)
new TextEncoder().encode('').constructor === Uint8Array // true

I’m not exactly sure why these two are referencing different constructors or where either one of the constructors is referencing a different one, so I’m wondering if anyone has any pointers on this (I’m really curious!).

That said, updating the jest-environment-node’s global to reference the actual Uint8Array (inside of the following block) seems to resolve this issue.

https://github.com/facebook/jest/blob/9ac2dcd55c0204960285498c590c1aa7860e6aa8/packages/jest-environment-node/src/index.ts#L54-L61

~I’m happy to open a PR.~ Update: I opened #9261 so we can continue the conversation there.

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