Allow the cli to use a string for --maxWorkers

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 ### Fixes
 
+- `[jest-cli]` fix --maxWorkers work again with % input ([#8559](https://github.com/facebook/jest/pull/8565))
 - `[babel-plugin-jest-hoist]` Expand list of whitelisted globals in global mocks ([#8429](https://github.com/facebook/jest/pull/8429)
 - `[jest-core]` Make watch plugin initialization errors look nice ([#8422](https://github.com/facebook/jest/pull/8422))
 - `[jest-snapshot]` Prevent inline snapshots from drifting when inline snapshots are updated ([#8492](https://github.com/facebook/jest/pull/8492))

packages/jest-cli/src/__tests__/cli/args.test.ts

@@ -50,10 +50,15 @@ describe('check', () => {
   it('raises an exception if maxWorkers is specified with no number', () => {
     const argv = ({maxWorkers: undefined} as unknown) as Config.Argv;
     expect(() => check(argv)).toThrow(
-      'The --maxWorkers (-w) option requires a number to be specified',
+      'The --maxWorkers (-w) option requires a number or string to be specified',
     );
   });
 
+  it('allows maxWorkers to be a %', () => {
+    const argv = ({maxWorkers: '50%'} as unknown) as Config.Argv;
+    expect(() => check(argv)).not.toThrow();
+  });
+
   it('raises an exception if config is not a valid JSON string', () => {
     const argv = {config: 'x:1'} as Config.Argv;
     expect(() => check(argv)).toThrow(

packages/jest-cli/src/cli/args.ts

@@ -42,8 +42,9 @@ export const check = (argv: Config.Argv) => {
 
   if (argv.hasOwnProperty('maxWorkers') && argv.maxWorkers === undefined) {
     throw new Error(
-      'The --maxWorkers (-w) option requires a number to be specified.\n' +
+      'The --maxWorkers (-w) option requires a number or string to be specified.\n' +
         'Example usage: jest --maxWorkers 2\n' +
+        'Example usage: jest --maxWorkers 50%\n' +
         'Or did you mean --watch?',
     );
   }
@@ -349,7 +350,7 @@ export const options = {
       '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' as 'number',
+    type: 'string' as 'string',
   },
   moduleDirectories: {
     description: