Question

73

How to mock an exported const in jest

rated 0 times [ 76] [ 3] / answers: 1 / hits: 185562 / 7 Years ago, thu, march 23, 2017, 12:00:00

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 to prevent some behavior if downstream services request it.

I am not sure how to mock a const variable in Jest so that I can change its value for testing the true and false conditions.

Example:

//constants module

export const ENABLED = true;



//allowThrough module

import { ENABLED } from './constants';



export function allowThrough(data) {

  return (data && ENABLED === true)

}



// jest test

import { allowThrough } from './allowThrough';

import { ENABLED } from './constants';



describe('allowThrough', () => {

  test('success', () => {

    expect(ENABLED).toBE(true);

    expect(allowThrough({value: 1})).toBe(true);

  });



  test('fail, ENABLED === false', () => {

    //how do I override the value of ENABLED here?



    expect(ENABLED).toBe(false) // won't work because enabled is a const

    expect(allowThrough({value: 1})).toBe(true); //fails because ENABLED is still true

  });

});

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jarod

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answered 7 Years ago maya · Accepted Answer

This example will work if you compile ES6 modules syntax into ES5, because in the end, all module exports belong to the same object, which can be modified.

import { allowThrough } from './allowThrough';

import { ENABLED } from './constants';

import * as constants from './constants';



describe('allowThrough', () => {

    test('success', () => {

        constants.ENABLED = true;



        expect(ENABLED).toBe(true);

        expect(allowThrough({ value: 1 })).toBe(true);

    });



    test('fail, ENABLED === false', () => {

        constants.ENABLED = false;



        expect(ENABLED).toBe(false);

        expect(allowThrough({ value: 1 })).toBe(false);

    });

});

Alternatively, you can switch to raw commonjs require function, and do it like this with the help of jest.mock(...):

const mockTrue = { ENABLED: true };

const mockFalse = { ENABLED: false };



describe('allowThrough', () => {

    beforeEach(() => {

        jest.resetModules();

    });



    test('success', () => {

        jest.mock('./constants', () => mockTrue)

        const { ENABLED } = require('./constants');

        const { allowThrough } = require('./allowThrough');



        expect(ENABLED).toBe(true);

        expect(allowThrough({ value: 1 })).toBe(true);

    });



    test('fail, ENABLED === false', () => {

        jest.mock('./constants', () => mockFalse)

        const { ENABLED } = require('./constants');

        const { allowThrough } = require('./allowThrough');



        expect(ENABLED).toBe(false);

        expect(allowThrough({ value: 1 })).toBe(false);

    });

});