--maxWorkers only allows numbers and no % of cores

[philiiiiiipp]

🐛 Bug Report

Giving --maxWorkers a % instead of a fixed core number does not seem to work.

I looked into it, I found that it seems to expect a number and parsing it sets it to NaN

maxWorkers: NaN,
  w: NaN,
  'max-workers': NaN,
  all: undefined,
  bail: undefined,

in jest-cli it checks the args only for a number

https://github.com/facebook/jest/blob/master/packages/jest-cli/src/cli/args.ts#L345

maxWorkers: {
    alias: 'w',
    description:
      'Specifies the maximum number of workers the worker-pool ' +
      'will spawn for running tests. This defaults to the number of the ' +
      'cores available on your machine. (its usually best not to override ' +
      'this default)',
    type: 'number',
  },

I quickly tried setting it to a string and it works as expected. I am not sure of all the ramifications of this change but it might be the correct type for it.

To Reproduce

run jest with --maxWorkers=20% and it will default to 50%

Expected behavior

It should use only 20% of my cpus

Run npx envinfo --preset jest

npx: installed 1 in 1.01s

  System:
    OS: macOS 10.14.5
    CPU: (12) x64 Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
  Binaries:
    Node: 8.12.0 - /usr/local/opt/node@8/bin/node
    Yarn: 1.12.3 - ~/.npm-packages/bin/yarn
    npm: 6.9.0 - ~/.npm-packages/bin/npm
  npmPackages:
    jest: ^24.8.0 => 24.8.0 

[thymikee]

It should be a string. Can you send a PR fixing this? :)
We'll likely be able to remove the casting after this change:
https://github.com/facebook/jest/blob/43d1cf6ad49b00c077c01560bbedc9fc8c61311d/packages/jest-config/src/getMaxWorkers.ts#L17-L19

[philiiiiiipp]

Ok will do :-)

[philiiiiiipp]

It should be a string. Can you send a PR fixing this? :)
We'll likely be able to remove the casting after this change:

https://github.com/facebook/jest/blob/43d1cf6ad49b00c077c01560bbedc9fc8c61311d/packages/jest-config/src/getMaxWorkers.ts#L17-L19

I looked into that. I think in principle we would actually need to make it a string e.g. parseInt(maxWorkers + '', 10). Js is just not really harsh on us and working even if maxWorkers is a number.

I traced this variable around a bit and it seems as if there are multiple definitions depending on where this is coming from

https://github.com/facebook/jest/blob/43d1cf6ad49b00c077c01560bbedc9fc8c61311d/packages/jest-types/src/Config.ts#L325

and

https://github.com/facebook/jest/blob/43d1cf6ad49b00c077c01560bbedc9fc8c61311d/packages/jest-types/src/Config.ts#L457

I am not sure if we can savely change this to a string everywhere rather than a string | number. I suppose this could be a seperate pull request.