Jest: Test Intl.datetimeformat

I would like to test a filter function I wrote which return a date formatted using Intl.DateTimeFormat('en-GB', options): // module 'date.js' export default function (dateISOString

Solution 1:

The only solution that I managed to find to this problem was to install full-icu which seemed to provide the right locales to node during the testing process.

That package is needed because node by default only ships with a limited set of locales, explained here: https://nodejs.org/docs/latest-v9.x/api/intl.html

With that package installed, the additional step that I had to take was to change my test command to use:

"test": "NODE_ICU_DATA=node_modules/full-icu jest --config jest.config.js"

I ran into this problem in a couple of different environments. When running the tests locally on Mac OS and also when running the tests inside a Docker container during CI.

Interestingly, I don't need to use the export when running the tests via WebStorm's Jest integration. It definitely seems like the behaviour of the Intl library is far from stable in node.

Solution 2:

You can use polyfill, as described here

importIntlPolyfillfrom'intl';
import'intl/locale-data/jsonp/ru';

if (global.Intl) {
    Intl.NumberFormat = IntlPolyfill.NumberFormat;
    Intl.DateTimeFormat = IntlPolyfill.DateTimeFormat;
} else {
    global.Intl = IntlPolyfill;
}

Solution 3:

Just found a solution if you want to change the zone per test.

The benefit here is minimal change to the Intl object since we are just using the normal api to set a default

const timezoneMock = function(zone: string) {
    constDateTimeFormat = Intl.DateTimeFormat
    jest
    .spyOn(global.Intl, 'DateTimeFormat')
    .mockImplementation((locale, options) =>newDateTimeFormat(locale, {...options, timeZone: zone}))
  }

  afterEach(() => {
    jest.restoreAllMocks();
  })

and then

describe('when Europe/London', () => {
    it('returns local time', () => {
      timezoneMock('Europe/London')
   //etc.... 

Solution 4:

In package.json add the intl package

"intl":"*",

In the jest.config.js

module.exports = {
    moduleNameMapper: {
        Intl: '<rootDir>/node_modules/intl/'
    }
};

Then in Date.spec.js

describe(`Date by locale`, () => {
     beforeAll(() => {  global.Intl = require('intl'); });
    // Add your tests here. // Add a temporary console.log to verify the correct output
 }