Return / resolve values while simultaneously testing expected arguments. Full TypeScript type safety.

Have you ever wanted to setup a Jest mock to return a value only when it received the correct arguments?

Sure, you could access mockVariable.mock.calls and assert its contents besides your original assert, but to my taste that is a pretty low-level API that furthermore makes your test code bloated and noisy.

With jest-setup-mock, you can set up the mock to return / resolve a value only when the arguments match the expected arguments that you specify:

let someFunction: (x: number, y: string) => Promise<boolean>;
let functionMock: FunctionMock<typeof someFunction>;

setupMock(someFunction)
    .expectArguments(42, 'is the answer')
    .resolveValueOnce(true);

The code above will result in the mock resolving true whenever it is called with two arguments matching [42, 'is the answer'], and throwing a descriptive error otherwise.

Furthermore, by typing your mock variable as FunctionMock<ActualFunctionType>, the types of arguments and return values in the above fluent syntax will be inferred correctly, guarding you from type mistakes.

For reference, lodash's isEqual method is used to check for expected / actual argument equality, so deep object / array equality comparison will work.

Pull requests are accepted.