next.js: Cannot create a simple jest test

True but that wasn’t the point, the issue itself was in the barMock declaration since that’s the line its threw up on. What I would recommend is doing a fly-by on the jest docs and maybe use mock instead of spyOn in this case. It might also help to use a standard folder structure in the project (not sure if this would solve any issues in this case tho but that’s not really the point in this case).

I already tried mock instead of using spyOn. Also, the folder structure is from the nextjs template in codesandbox so I don’t think there’s anything wrong with the folder structure here.

Sure, it is throwing up the error Cannot redefine property: bar in the barMock declaration but why does it only happen if my config is like this:

const nextJest = require("next/jest");
const createJestConfig = nextJest({
  // Provide the path to your Next.js app to load next.config.js and .env files in your test environment
  dir: "./",
});

const config = {
  preset: "ts-jest",
  testEnvironment: "node",
};

module.exports = createJestConfig(config);

But the tests pass without errors if my config is like this:

const config = {
  preset: "ts-jest",
  testEnvironment: "node",
};

module.exports = config;

You can try it yourself. The test is simple. If you can check whether bar is being called at least once inside foo using jest regardless of what you use (spy/mock) and without using DI, I’ll close this issue. Feel free to use a “standard” folder structure if that helps.

After further reading, it seems like swc won’t be fixing this

https://github.com/swc-project/swc/issues/5059

https://github.com/swc-project/swc/issues/5205

For anyone who will stumble upon this, your tests are fine. Next.js is using SWC as their compiler, which treats ts-jest transpiled tests wrong.

You can fix this by doing one of the following:

• If you are using ts-jest, do not use nextJest. Just export your jest config normally
• If you want to still use ts-jest with nextJest, use jest_workaround swc plugin. Add it in your next.config.js
• Use vitest instead

I see. I will try it when I get back. This is how I imported it before 🤔

From the errors I saw and about 2 min of googling it seemed like it had trouble detecting the ran function called “bar” by the way you imported it. Made a small change on it which you can see in the codesandbox below. Hope this helped ya (works on both config variants you mentioned)

https://codesandbox.io/p/sandbox/funny-villani-0x6j4t

@kapitanluffy as mentioned in a previous comment, I’ve forked your codesandbox and included this fix which will make the test actually run.

euhm… not to just to any conclusions but it more likely seems like your test is just incorrect rather than this being an issue on Next.js 's part.