jestjs - TypeScript Jest Matchers functions are typed any

Question

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.

Answer 1

等待大神答复