test.each(): Accept a readonly (as const) table properly

[Peeja]

Summary

Sometimes the inferred types in a test.each() table are too wide for the test that uses them. For instance, consider this test in which letterCase has to be specifically 'upper' or 'lower':

const changeCase = (letterCase: 'upper' | 'lower', str: string) => {
  if (letterCase === 'upper') {
    return str.toUpperCase();
  } else {
    return str.toLowerCase();
  }
};

test.each([
  ['upper', 'abc', 'ABC'],
  ['lower', 'ABC', 'abc'],
])('some test', (letterCase, str, expected) => {
  expect(changeCase(letterCase, str)).toBe(expected);  // Argument of type 'string' is not assignable to parameter of type '"upper" | "lower"'
});

You can keep the literal types of the table values by giving it as const, but that causes a different (stranger) error:

test.each([
  ['upper', 'abc', 'ABC'],
  ['lower', 'ABC', 'abc'],
] as const)('some test', (letterCase, str, expected) => {
//                        ^-----------------------^
// Argument of type '(letterCase: "upper" | "lower", str: "abc" | "ABC", expected: "abc" | "ABC") => void' is not assignable to parameter of type '(...args: readonly ["upper", "abc", "ABC"] | readonly ["lower", "ABC", "abc"]) => TestReturnValue | TestReturnValueGenerator'.
//   Types of parameters 'letterCase' and 'args' are incompatible.
//     Type 'readonly ["upper", "abc", "ABC"] | readonly ["lower", "ABC", "abc"]' is not assignable to type '[letterCase: "upper" | "lower", str: "abc" | "ABC", expected: "abc" | "ABC"]'.
//       The type 'readonly ["upper", "abc", "ABC"]' is 'readonly' and cannot be assigned to the mutable type '[letterCase: "upper" | "lower", str: "abc" | "ABC", expected: "abc" | "ABC"]'.
  expect(changeCase(letterCase, str)).toBe(expected);
});

TypeScript appears to infer the type of the argument list as mutable, and then tries to assign the readonly argument tuple to it, which is an error. But we don't actually need the argument list to be mutable, so we want to tell TypeScript to consider it readonly as well. This type change does that, and makes the error go away.

Test plan

I believe as long as this test (and the rest of the suite) passes, it's safe. To verify, see that the test fails without this type change, and that everything passes with it.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Peeja / name: Petra Jaros (ba20f48, a9a8e61)
• ✅ login: SimenB / name: Simen Bekkhus (21d25a8, c4cb26b)

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[SimenB]

thanks!

[SimenB]

https://github.com/jestjs/jest/releases/tag/v30.0.0-alpha.1

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