TypeScript Jest Matchers functions are typed any

[bozhanglab49]

Jest doesn't type some of its expect functions in Matchers. For example, it doesn't generate compiling errors for following code:

  expect('123').toEqual(123);
   expect(() => '123').toHaveReturnedWith(123);

Here is how toEqual and toHaveReturnedWith are declared:

 toEqual<E = any>(expected: E): R;
   toHaveReturnedWith<E = any>(expected: E): R;

Obviously, parameters of both are typed any. Even though it allows you to specify a type, it is not bound with the actual value. They can be improved like this:

interface Matchers<R, T = {}> {
      toEqual(expected: T): R;
      toHaveReturnedWith(expected: ReturnType<T>): R;
   }

Now TypeScript complains:

    expect('123').toEqual(123); // type number is not assignable to type string
    expect(() => '123').toHaveReturnedWith(123); // type number is not assignable to type

My question is why jest chooses not to type them like above which is obviously better?

• Jest doesn't really care about type safety since those are unit tests and won't pass any way?
• Backward compatible?
• etc.

[kulshekhar]

In tests in a dynamically typed language like javascript (typescript is just for compile time), you'd ideally want to test for different types of values as well. For example, if a function accepts only a number, you might want to test its behavior when it receives a string to ensure that that case is handled well.

[bozhanglab49]

But if we are using TypeScript and everything is well typed, we wouldn't want to test a string for a number. If we allow a string passed to a number at runtime dynamically, it defeats the purpose of using TypeScript. So we should let the function accept both number and string by typing it number | string in the first place.

Even if we want JavaScript's dynamic feature, we can always turn off type checking by casing to any or for function parameters using expect.anything().

[kulshekhar]

Typing in typescript doesn't prevent wrong types being sent in during runtime. Think fetching something from an API, database, etc. Those are the scenarios you also want to test but would have to do extra work (casting to any) if the tests were strongly typed.

[ahnpnl]

Jest typing is defined in @types/jest, it’s a package which is contributed by several people. You can also make changes there and discuss with others

[bozhanglab49]

Currently in my project, we have to create a utiltiy expectThat function to swap out the untyped functions.