How to mock an exported const in jest

Question

I have a file that relies on an exported const variable. This variable is set to true but if ever needed can be set to false manually

Answer 1

Instead of Jest and having trouble with hoisting etc. you can also just redefine your property using "Object.defineProperty"

It can easily be redefined for each test case.

This is a pseudo code example based on some files I have:

From localization file:

export const locale = 'en-US';

In another file we are using the locale:

import { locale } from 'src/common/localization';
import { format } from 'someDateLibrary';

// 'MMM' will be formatted based on locale
const dateFormat = 'dd-MMM-yyyy';

export const formatDate = (date: Number) => format(date, dateFormat, locale)

How to mock in a test file

import * as Localization from 'src/common/localization';
import { formatDate } from 'src/utils/dateUtils';

describe('format date', () => {
        test('should be in Danish format', () => {
            Object.defineProperty(Localization, 'locale', {
                value: 'da-DK'
            });
            expect(formatDate(1589500800000)).toEqual('15-maj-2020');
        });
        test('should be in US format', () => {
            Object.defineProperty(Localization, 'locale', {
                value: 'en-US'
            });
            expect(formatDate(1589500800000)).toEqual('15-May-2020');
        });
});