jest-typed-mock

Make unit tests great again!

jest-typed-mock brings type checking to the wild __mocks__ of Jest. Helping you maintain the correct mocks, matching the real files, they are going to mock. This is like eslint, but for mocks.

Manual mocks are defined by writing a module in a __mocks__/ subdirectory immediately adjacent to the module. For example, to mock a module called user in the models directory, create a file called user.js and put it in the models/__mocks__ directory. https://facebook.github.io/jest/docs/en/manual-mocks.html#content

Usage

Add to your package.json

"scripts": {
  ...
  "jest-typed-mock": "jest-typed-mock flow",       // for Flow 
  "jest-typed-mock": "jest-typed-mock typescript", // for TS
  "jest-typed-mock": "jest-typed-mock javascript", // for JS
  "jest-typed-mock": "jest-typed-mock exports",    // only check names
  ...
}

In some cases (always?) you have to specify babel env.

Type safety

Rewiremock can provide a type safety to dependency mocking, but Jest do had it's own mocking (and sandboxing) system. As long rewiremock still able to work, even under the Jest management - it is now a quite good idea.

jest-typed-mock is not a library. It is just a small tool, which can check mocked file against the real one.

Lets imagine - we have a.js

//a.js
export const function1 = (a:number):number => a**2;
export const function2 = (a:number):number => a*2;
export default (a:string) => a.substr(1)

and __mocks__/a.js

export const function1 = (a:number):number => 1;
export const function2 = (a:number):number => 2;
export default (a:string) => "3"

It is ok. But next you change the real a.js

//a.js
// function 1 is changed
export const function1 = (a:string):number => parseInt(a);
// function 2 is removed
export default (a:string) => a.substr(1)

But your tests are still using the old __mocks__, and they are still green.

jest-typed-mock just matches real files and mocks.

TS:

Property 'function2' is missing in type 'typeof "....a"'.

Flow:

property `function2` of exports of "/__mocks__/a.js". Property not found in const real = () => import('/a.js');

JS

As long there is no types, could only test exported names as their types(object, number, function), and function arguments count. Even this is quite helpful.

jest-typed-mock: mocked export "function2" does not exists in a.js jest-typed-mock will also try to check function agains function, at least argument count, which can break a lot of things. To bypass this checking use jest-typed-mock exports, or specify noFunctionCompare in API options

require('jest-typed-mock/runjs')(__dirname , {noFunctionCompare: true})

Result

As result - you reduce a smell of mocking. Mocks becomes less fake, and tests becomes less flake.

Licence

MIT