`test.each` should perform string interpolation with object properties

🚀 Feature Proposal

When giving an array of objects to the test.each overload that accepts an array, and not a template strings table, jest should interpolate the object’s properties into the test name.

Motivation

The current interpolation of template strings works great, but it doesn’t provide any hook for type-safety. This is problematic when scaling up to large and complex test cases, and in TypeScript, it allows any to sneak into your test functions. It also is more intuitive, it left me wondering for a while why a transformation to a type-safe version of a test wasn’t causing interpolation of the test case.

Example

We can do something like this right now:

test.each`
  a    | b    | expectedResult
  ${1} | ${1} | ${2}
  ${2} | ${2} | ${4}
`('add($a, $b) === $expectedResult', ({ a, b, expectedResult }) => {
  expect(a + b).toEqual(expectedResult);
});

Which will output the test cases: √ add(1, 1) === 2 √ add(2, 2) === 4

In order to achieve type-safety on the same test, I would make this transformation:

type AddExample = {
  a: number;
  b: number;
  expectedResult: number;
};

test.each<AddExample>([
  { a: 1, b: 1, expectedResult: 2 },
  { a: 2, b: 2, expectedResult: 4 },
])('add($a, $b) === $expectedResult', ({ a, b, expectedResult }) => {
  expect(a + b).toEqual(expectedResult);
});

Which I would expect should output the same test cases as above, but instead: √ add($a, $b) === $expectedResult √ add($a, $b) === $expectedResult

Pitch

It is isomorphic to functionality in the core library, but I believe it cannot be implemented as an extension.

[mattphillips]

This has been something I’ve wanted to add for a while but not got around to it and I think we should add parity with the tagged template table with $ syntax for sure 👍

[github-actions[bot]]

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