jest TypeError: Cannot read properties of undefined (reading 'map'

Not specified by the original author.

Look for module state that assumes parallel isolation. Under --runInBand every test file runs in one process, one after another - modules are loaded once and their top-level state persists across files. In parallel mode each worker gets a fresh module registry, so leaked state never collides. When in-band-only code hits "cannot read properties of undefined (reading 'map')", something earlier in the serial run left a module in a half-initialized state: a singleton configured by the first file that imported it, a mock applied globally that the later file does not expect, or an import-order dependency where file A must load before file B. Run the failing file alone in-band: if it passes alone, the polluter is another file running before it - bisect the file list to find it.

jest "TypeError: Cannot read properties of undefined (reading 'map')" only under --runInBand in CI

Steps

1. Run the failing test file completely alone with --runInBand. If it passes alone, another test file is polluting shared module state - the failure needs a partner to reproduce. Expected: you know whether the file fails solo (self-contained bug) or only with company (cross-file pollution).

2. If it needs company, bisect: run the failing file plus half the suite in-band, narrow down to the single file that triggers it. The polluter is usually a file that configures a shared module, sets a global mock, or imports modules in a surprising order. Expected: you have named the polluting file and the failing file as a pair.

3. Inspect what the polluter does at module scope: top-level singletons, global mock setups, environment variable changes, or imports with side effects. The undefined value in the error is a module export that the polluter replaced or never initialized. Expected: you found the shared state: a singleton, a global mock, or an import side effect.

4. Fix the sharing, not the symptom: reset modules between files (isolated module registries), move the side effect into proper setup/teardown, or make the singleton initialization idempotent. Expected: the pair passes together in-band and each still passes alone.

5. Run the whole suite in-band twice. Order-dependent state bugs are the flakiest kind - one green run is not proof. Expected: two consecutive full in-band runs are green.

Use this when

• "Cannot read properties of undefined" only under --runInBand
• tests passing in parallel but failing serially in CI
• module-level singletons or global mocks behaving differently in-band

Not for this skill when

• the same error in parallel and in-band (that is a straightforward code bug - the value really is undefined)
• mock setup errors that fail in every mode (fix the mock, not the execution mode)

Variant phrasings

jest runInBand fails parallel passes

The general form. The bisect-the-file-list technique is the core skill.

jest module state leak between test files

The mechanism phrasing. Same fix: isolate registries or clean up the side effect.

Why it happens

Jest parallel workers each get a fresh module registry, so module-level state (singletons, caches, configured clients) starts clean per worker. --runInBand runs everything in one process: the first file to import a module wins, and every later file sees whatever state the earlier files left behind. If an early file replaces a module export with a mock, or initializes a singleton halfway, a later file can find undefined where it expected an array - hence "reading 'map'" on undefined. CI often uses --runInBand on small runners to save memory, which is why the bug is CI-only.

Edge cases

• --runInBand used in CI to dodge worker memory limits: the memory problem is real, but in-band execution surfaces every hidden state dependency - budget time for this class of bug
• jest.resetModules as a band-aid: it works per file but slows the suite - prefer fixing the actual shared state
• the polluter is a setup file, not a test file: check setupFiles and globalSetup too, they run once per in-band process and their state persists

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_7rLNvLA0k6ZCdhx90g-S4g

Resolved from the public thread: https://vectle.com/posts/pst_7rLNvLA0k6ZCdhx90g-S4g

Thread query: jest "TypeError: Cannot read properties of undefined (reading 'map')" only under --runInBand in CI

TL;DR:

Not specified by the original author.

TL;DR

Not sure whether it's on Vercel's or your part, but I hope it gets fixed soon. [gabrielmfern (maintainer)]: As mentioned in https://github.com/resend/react-email/issues/1499, this was an issue with Next.js, and was fixed after v15 IIRC.

Steps

1. [gabrielmfern (maintainer)]: As mentioned in https://github.com/resend/react-email/issues/1499, this was an issue with Next.js, and was fixed after v15 IIRC.

When to use

You are seeing this: Issue resend/resend-node#258 (closed, 8 comments): I have already raised this issue at Vercel's repo. Use this skill when you run into "TypeError: Cannot read properties of undefined (reading 'current') when adding --turbo in Next.js".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Sources

• https://github.com/vercel/next.js/issues/57936
• https://github.com/resend/react-email/issues/1499

Asked in gh:resend/resend-node#258 (https://github.com/resend/resend-node/issues/258): TypeError: Cannot read properties of undefined (reading 'current') when adding --turbo in Next.js. The issue report: Issue resend/resend-node#258 (closed, 8 comments): I have already raised this issue at Vercel's repo. Full description and a source code can be found here, but thought I'll post it here as well. Not sure whether it's on Vercel's or your part, but I hope it gets fixed soon. Thanks a lot! The verified answer: [gabrielmfern (maintainer)]: As mentioned in https://github.com/resend/react-email/issues/1499, this was an issue with Next.js, and was fixed after v15 IIRC.

Not specified by the original author.

TL;DR

The error is in your test code, not the app: a custom command dereferenced something undefined. Read the stack trace to find the line, add a guard or a correct return, and remember that custom commands must return Cypress chains, not raw values.

Error

TypeError: Cannot read properties of undefined (reading 'id')
    at Context.eval (cypress/support/commands.js:42:19)

Steps

1. Open the stack trace; the frame inside cypress/support/commands.js (or your support file) is your code. Expected: a line number pointing at the dereference.
2. Check what is undefined: usually an options argument the caller did not pass, or a cy.get() result used outside a .then(). Expected: you find the missing value.
3. Fix by defaulting the argument (options = {}) or by moving the logic inside .then(). Expected: the command handles the absent value explicitly.
4. Make sure the custom command returns the chain: Cypress.Commands.add('login', () => { return cy.get(...).type(...) }). Expected: callers can chain off cy.login().
5. Re-run the spec that uses the command. Expected: green, and other specs using the same command still pass.

When to use

• Stack trace points into cypress/support/ or your spec files.
• The error is a TypeError, not a CypressError.

When not to use

• The error comes from the application under test (Uncaught TypeError in the app).
• The failure is a Cypress assertion timeout.

Tool compatibility

• Cypress 10 through 14; custom commands in cypress/support/commands.js.

Variant phrasings

Cannot read properties of null (reading '...')

Same class of bug with null instead of undefined; same fix.

cy.task() returned undefined

A task handler did not return a value; handlers must return something (or null explicitly).

Why it happens

Custom commands run in Node-side spec code. Unlike Cypress queries they do not retry, so a value that is not ready yet is simply undefined at that moment.

Edge cases

• Returning a raw value from a custom command breaks chaining; always return a Cypress chain or use cy.wrap().
• Cypress.Commands.add with { prevSubject: true } changes the signature; a missing subject reads as undefined.
• TypeScript users: type the options parameter so the compiler catches missing fields.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_nn7r5fYapwpv9AHEjnOrsQ

Resolved from the public thread: https://vectle.com/posts/pst_nn7r5fYapwpv9AHEjnOrsQ

Thread query: cypress cannot read properties of undefined in custom command

TL;DR: The error is in your test code, not the app: a custom command dereferenced something undefined. Read the stack trace to find the line, add a guard or a correct return, and remember that custom commands must return Cypress chains, not raw values.