How to mock Cookie.get('language') in JEST

Question

I have a function which sets a language in cookie and gets it for some functionality. How do I set the value of language correctly for testing in JEST

function resolveLanguage () {   // browser cookie is checked to see if there is a value for language   const lang = Cookie.get('language')    // If no lang, it is defaulted to en    return lang || 'en' } 

Now I want to test it in JEST

it('can resolve the language using a cookie', () => {       Cookie.__defineGetter__("language", function() {         return 'fr'       })       // console.log(Cookie.language) //returns fr       // console.log(Cookie.get('language')) //returns fr       expect(module.resolveLanguage()).toBe('fr') ---FAILS because it returns en     }) 

Answer 1

Don't have to download any deps, just use:

Object.defineProperty(window.document, 'cookie', {   writable: true,   value: 'myCookie=omnomnom', }); 

(writable needs to be set to true)

Answer 2

You need to mock js-cookie using jest to set the language set you want.

import Cookie from 'js-cookie'  jest.mock('js-cookie', ()=> jest.fn())  Cookie.setMockImplementation(()=>({get: () => 'fr'})) 

Or if you only need to set it once

jest.mock('js-cookie', ()=>({get: () => 'fr'})) 

Note that this solution would always return 'fr' for all Cookie.get calls. If you need support multiple values on get you could do something like this:

jest.mock('js-cookie', ()=>({get: key => {    language: 'fr',     somethingElse: 'someThing' }[key]}))