jest.mock module auto-mock error when Function names are numeric

[gregsochanik]

🐛 Bug Report

When using the jest.mock('module'); feature to deep auto-mock an entire module, an error is thrown if the module contains any objects that contain functions with a numeric name.

To Reproduce

Example library kafkajs

require(`kafkajs`)
jest.mock('kafkajs');

Expected behavior

The library should be mocked with no issues.

Actual behaviour

Application exits with the following error thrown:

 FAIL  tests/unit/mock.test.js
  ● Test suite failed to run

    SyntaxError: Unexpected number
        at new Function (<anonymous>)

Which is caused by the following line in jest-mock/index.js (https://github.com/facebook/jest/blob/master/packages/jest-mock/src/index.js#L647)

    const createConstructor = new this._environmentGlobal.Function(
      MOCK_CONSTRUCTOR_NAME,
      body
    );

the error is a standard nodejs error which can be emulated in the repl via

> new Function(1, {})
SyntaxError: Unexpected number

Due to new Function() throwing Unexpected number if a numeric value is passed to it.

Offending object with numeric function names in kafkajs library can be found here:
https://github.com/tulios/kafkajs/blob/master/src/protocol/message/compression/index.js#L14

Run npx envinfo --preset jest

Paste the results here:

System:
    OS: Linux 4.18 Ubuntu 18.10 (Cosmic Cuttlefish)
    CPU: (4) x64 Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz
  Binaries:
    Node: 8.12.0 - ~/.nvm/versions/node/v8.12.0/bin/node
    Yarn: 1.12.3 - ~/.yarn/bin/yarn
    npm: 6.4.1 - ~/.nvm/versions/node/v8.12.0/bin/npm
  npmPackages:
    jest: ^23.6.0 => 23.6.0

[jeysal]

I can confirm this bug.

[SimenB]

Not sure how we should handle this. @thymikee ideas?

[jeysal]

Best handling is probably prefix a $ or something, like we do with other invalid function names. I'll work on a fix for this.
Note that in our case, rather than new Function(1, {}), it's something like

new Function('mockConstructor', 'return function 1() {return mockConstructor.apply(this,arguments);}')

[github-actions]

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