eval() in code being tested not working in Jest.

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

eval() inside code being tested with Jest doesn’t work. Does it have to do with vm.runInContext? (see https://github.com/facebook/jest/issues/439)

reproduction

I have some code like this in a source file of mine which some tests are triggering:

            let Ctor

            if ( className ) eval( `Ctor = function ${ className }() {}` )
            else {
                // anonymous even in ES6+
                Ctor = ( () => function() {} )()
            }

            Ctor.prototype = { __proto__: Object.prototype, constructor: Ctor }

            return Ctor

Reproduction: https://github.com/trusktr/lowclass/tree/jest-issue-5936

Check out that branch, then:

To see the error run

npm i
npm test

To see the code working fine under normal execution, run

node src/test.js

Note, npm test runs Jest with builder,

To run the Jest test manually (for ease debugging), you can run this command:

❯ node --inspect-brk ./node_modules/.bin/jest --config ./node_modules/builder-js-package/config/jest.config.js

There is already a debugger statement on the line before the problem, you’ll have to click play a few times until the first time that the code steps into the part of the conditional when className is truthy, and the value of className will be "Foo".

You can also place a conditional breakpoint on that line if you’re familiar with Chrome inspector (right click the line number, click “Add conditional breakpoint…”, then input "className === "Foo")

What is the expected behavior?

My test file currently does not use any libraries, only console.assert(), so I can run it with node test.js.

When I run it with node test.js, it works great!

When I run the test with Jest, it fails:

The eval() should work. But when the code reaches that part, the eval() seems to be running in some other context (other scope).

So what I see happen is this:

            let Ctor

            // Ctor is undefined here

            eval( `Ctor = function ${ className }() {}` )

            // Ctor is still undefined here

I was expecting Ctor to be assigned a function, and therefore no error in the execution of the test file.

Again, this all works fine under normal Node execution of the file, and in browsers, just not in Jest.

Please provide your exact Jest configuration

module.exports = {
    rootDir: process.cwd(),

    transform: {
        '^.+\\.js$': path.resolve(__dirname, 'babel-jest'),
    },

    // a great environment that supports Custom Elements
    testEnvironment: '@skatejs/ssr/jest',
}

Run npx envinfo --preset jest in your project directory and paste the results here

❯ npx envinfo --preset jest
npx: installed 1 in 2.41s

  System:
    OS: Linux 3.18
    CPU: x64 Intel(R) Core(TM) m3-6Y30 CPU @ 0.90GHz
  Binaries:
    Node: 9.9.0
    Yarn: 1.3.2
    npm: 5.8.0

❯ cat node_modules/jest/package.json | grep version
  "version": "22.4.3"

Jest is not installed globally

[trusktr]

To fix the problem on my end, I changed the code so it uses Function instead of eval:

            if ( className ) Ctor = new Function( `return function ${ className }() {}` )()

This seems to be nicer anyways because I don’t need anything from outer scope, and because the code I am evaluating is my own code in my own scope (not third party code) I can easily pass arguments into the new Function if needed. Well, I can easily pass specific arguments in for third party code too. So, Function seems better overall and more secure.

[github-actions[bot]]

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