Type Error with const

[Sufiane]

I face myself against a Type Error when trying to rewire a private const
I already have the last version 4.0.1

I used the workaround using let but I would prefer to use const as I don't want to change my code only for test purposes

Demo:
index.js:

const myPromisifiedFunction = promisify(someFunct.ToPromisify)
index.test.js:

const myModule = rewire('../myModule')
const myStub = sinon.stub()
const revert = myModule.set('myPromisifiedFunction', myStub)
And what I get when running tests is:
TypeError: Assignment to constant variable. at eval (eval at set (index.js:159:5), :1:19) at Object.set (index.js:159:5) at Object. (test/index.test.js:14:48)

Thanks in advance for any help you could provide !

Edit: I'm aware of the #79 issue, I already commented on it but since it's closed I thought it would be better to open a new one

[Shakakai]

I'd like to provide some additional evidence. The issue appears to happen when using the Jest testing library but not with Mocha. I created a git repo to show the issue: https://github.com/Shakakai/jest-rewire-const-error

Here's a link to the issue in Jest: jestjs/jest#6803
I'm not sure which library is responsible.

[Sufiane]

Thanks @Shakakai ! I was indeed writing tests with jest

[sinkersan]

Any updates here?

[Sufiane]

@sinkersan not on my end

[missmatsuko]

I'm having the same problem, also using Jest!

[tyukesz]

+

[andreisaikouski]

I see this same issue in Mocha. Any suggestions / fixes?

[jhnns]

I've added a test case in the most recent version and I think this is not an issue anymore:

rewire/testLib/sharedTestCases.js

Lines 426 to 432 in f5c655a

	it("should be possible to re-assign consts", function () {
	var test = rewire("./constModule.js");
	test.__set__("j", "some other value");
	expect(test.j()).to.be("some other value");
	});

[justinrush]

I'm still seeing this in rewire v6 / jest v27.4.5.

db.js

function connectDB() {
    console.log('in connectdb');
}
module.exports.connectDB = connectDB;

constMod.js

const { connectDB } = require('./db');

function doIt() {
    console.log('no');
    connectDB();
}

constMod.test.js

const rewire = require('rewire');
const db = require('./db');
const constMod = rewire('./constMod');

db.connectDB = jest.fn();

describe('constMod', () => {
    it('mocks connectDb', () => {
        constMod.__set__({
            connectDB: db.connectDB,
        });

        const doIt = constMod.__get__('doIt');
        doIt();
        expect(db.connectDB).toHaveBeenCalled();
    });
});

Output:

-> % npm run jest       

> [email protected] jest /js-test
> jest

 FAIL  ./constMod.test.js
  constMod
    ✕ mocks connectDb

  ● constMod › mocks connectDb

    TypeError: Assignment to constant variable.

      at eval (eval at __set__ (constMod.js:72:5), <anonymous>:1:11)
      at Object.__set__ (constMod.js:72:5)
      at Object.<anonymous> (constMod.test.js:9:18)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        0.247 s, estimated 1 s
Ran all test suites.

[tyukesz]

I'm still seeing this in rewire v6 / jest v27.4.5.

db.js

function connectDB() {
    console.log('in connectdb');
}
module.exports.connectDB = connectDB;

constMod.js

const { connectDB } = require('./db');

function doIt() {
    console.log('no');
    connectDB();
}

constMod.test.js

const rewire = require('rewire');
const db = require('./db');
const constMod = rewire('./constMod');

db.connectDB = jest.fn();

describe('constMod', () => {
    it('mocks connectDb', () => {
        constMod.__set__({
            connectDB: db.connectDB,
        });

        const doIt = constMod.__get__('doIt');
        doIt();
        expect(db.connectDB).toHaveBeenCalled();
    });
});

Output:

-> % npm run jest       

> [email protected] jest /js-test
> jest

 FAIL  ./constMod.test.js
  constMod
    ✕ mocks connectDb

  ● constMod › mocks connectDb

    TypeError: Assignment to constant variable.

      at eval (eval at __set__ (constMod.js:72:5), <anonymous>:1:11)
      at Object.__set__ (constMod.js:72:5)
      at Object.<anonymous> (constMod.test.js:9:18)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        0.247 s, estimated 1 s
Ran all test suites.

This could be because you are using object destructuring: const { connectDB } = require('./db');

[justinrush]

changing the main function to use const db = require('./db'); and then updating the test to be:

const db = require('./db');
const rewire = require('rewire');
const constMod = rewire('./constMod');

db.connectDB = jest.fn();

describe('constMod', () => {
    it('mocks connectDb', () => {
        const conndb = jest.fn();
        constMod.__set__('db', {
            connectDB: conndb,
        });
        const doIt = constMod.__get__('doIt');
        doIt();
        expect(conndb).toHaveBeenCalled();
    });
});

results in the same error

[vuphu]

Based on #149, I used rewire for rewire module

const rewireRef = rewire('rewire');
const moduleEnvRef = rewire('rewire/lib/moduleEnv.js');
const jsExtension = moduleEnvRef.__get__('jsExtension');
moduleEnvRef.__set__('jsExtension', () => {
    return function (module: any, filename: any) {
        jsExtension(module, filename);
    };
});
rewireRef.__set__('moduleEnv', moduleEnvRef);

const yourPackage = rewireRef('your-package')

I hope it works in your projects.