TypeError Array(...).keys(...).slice is not a function after upgrade from 3.8.3 to 4.4.1

[Kimserey]

Current behavior:

I am using Cypress on an Angular application.

The following test is working on version 3.8.3

it('cy.get() - query DOM elements', () => {
  let x = [...Array(100).keys()].map(x => x + "");
  cy.server().route('/test', x);
  cy.get('[data-cy=landmark]').invoke('text').should('contain', 'Welcome');
})

but throws an error on 4.4.1

TypeError: Array(...).keys(...).slice is not a function
    at Context.<anonymous> (http://localhost:4200/__cypress/tests?p=cypress/integration/examples/querying.spec.js-628:8:35)

Test code to reproduce

Repository demonstrating the problem: https://github.com/Kimserey/cypress-test-issue

• branch master is 3.8.3
• branch upgrade-4 is 4.4.1

1. From the root directory, run ng serve,
2. From the root directory, start cypress npx cypress open,
3. The only test in the integration specs will throw the TypeError.

Versions

Cypress 4.4.1

[jennifer-shehane]

I'm not able to reproduce this solely from the snippet provided.

I am able to reproduce this from the repository provided, so this means it is something related to the environment setup. I suspect that it has something to do with the TypeScript configuration, since this begins failing in 4.4.0, which introduced out of the box TypeScript support in #5906

Indeed, I've confirmed that the test code above does not work when there is a devDependency of typescript. cc @sainthkh

To Reproduce

spec.js

let x = [...Array(100).keys()].map(x => x + "");

package.json

{
  "name": "cypress-test",
  "scripts": {},
  "devDependencies": {
    "cypress": "4.4.1",
    "typescript": "3.8.3"
  }
}

4.3.0

4.4.0

4.4.1

TypeScript version 3.5.3

TypeScript version 3.8.3 (current)

Workaround

Downgrade to Cypress 4.3.0 for now.

[sainthkh]

It happened because I didn't set up downlevelIteration option. It will be fixed in the preprocessor first and added to Cypress.