Compatibility issue with typescript, rewire & jest

Tell us about your environment

• ESLint Version: 4.19.1
• Node Version: 10.2.1
• npm Version: 6.4.1

What parser (default, Babel-ESLint, etc.) are you using?: tslint ¯_(ツ)_/¯

Please show your full configuration: None, really

Using typescript with rewire(https://github.com/jhnns/rewire) and jest(https://github.com/facebook/jest) for testing ended up with the following error:

Cannot find module '../conf/replacements' from 'rules.js'
    at Resolver.resolveModule (node_modules/jest-resolve/build/index.js:221:17)
    at Object.<anonymous> (node_modules/rewire/node_modules/eslint/lib/rules.js:14:26)

This occurs because rewire requires eslint as a dependency and jest tries to do something with all imported modules, but seemingly can’t handle .json files

It can be 'solved ’ by adding the .json extension in rules.js ruleReplacements import

const ruleReplacements = require("../conf/replacements.json").rules; 

and the following in a typescript project’s package.json:

    "jest": {
        "transform": {
			"\\.json$": "<rootDir>/your/dummy/emptyFile.js"
		},

I hope this might help some people in the future

[El-Fitz]

@platinumazure You’re welcome ! And yes, you’re definitely right. In order to run tests in Typescript, I used ts-jest and had to customise jest’s "moduleFileExtensions" parameter in my package.json, to add typescript files extensions, like so:

"moduleFileExtensions": [
			"ts",
			"tsx",
			"js"
		]

when I should have updated it like this:

"moduleFileExtensions": [
			"ts",
			"tsx",
			"js",
			"json"
		]

You see, by default it has .json in there, but setting it overwrites the defaults completely. And without .json in there, jest will fail when a file requires a .json file.

[El-Fitz]

No, you are not. Jest apparently doesn’t bother checking wether it is a JS module or not. And I have no idea how much work it would take on their side to do that. On the other hand, simply adding the .json extension, while not a real fix for Jest’s mishandling, helps fixing it.

I’ve just opened an issue on jest’s. Because it shouldn’t crash like this when some dependeny’s random dependency requires a .json (see https://github.com/facebook/jest/issues/7158)