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.

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