type error when rewire'ing const

[justinrush]

I'm still seeing #144 which has been closed

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.

[justinrush]

You can see this in a unit test on the main branch by:

• adding an export to testLib/someOtherModule.js: exports.meh = () => {return "meh";}
• import it in testLib/constModule.js: const { meh } = require("./someOtherModule");
• add this test case to testLib/sharedTestCases.js:

    it("should be possible to re-assign const required methods", function () {
        var test = rewire("./constModule.js");

        test.__set__({meh: () => {return "bleh"; }});
        const val = test.meh();

        expect(val).to.be("bleh");
    });

[georgezlei]

I have an interesting finding.

If you rewire a js module below, a type error will be thrown saying the constant variable cannot be assigned.

target.js

const a = 1;
a = 2;

function handler() {}

However, a slight change will make the error disappear.

const a = 1;
a = 2;

async function handler() {}

I haven't checked the rewire source yet and have no idea which behaviour is correct. But it seems that the keyword async triggers another parsing system that changes the constant to a normal variable.

[kswanny]

@georgezlei + 1 on this, found this out today too. I think the latter is incorrect, becasue it means tests will pass gracefully if you accidently re-assign a const.

[georgezlei]

Disagree with the conclusion. The constant re-assignment can be easily detected by the lint tool and it doesn't have to be the responsibility of the unit test. On the other hand, enabling constant re-assignment can help to mock the dependencies of the target module. Think of the below example in your target module:

const dep = require('dependency');