Babel-eslint: Use of Nullish Coalescing crashes ESLINT (stops linting completely in VS Code)

[babakness]

If I use ?? anywhere in a file, babel-eslint gives an obscure "unreachable" error and stops working. Removing the Nullish Coalescing from the file brings it back to life.

Using babel-eslint 8.2.3

[aplb]

Same for me
code

const object = {
  foo: null,
};

console.log('----------', object.foo ?? 'default');

deps

"devDependencies": {
    "@babel/cli": "^7.0.0-beta.46",
    "@babel/core": "^7.0.0-beta.46",
    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.0.0-beta.46",
    "babel-eslint": "^8.2.3",
    "eslint": "^4.19.1"
  }

.babelrc

{
  "plugins": ["@babel/plugin-proposal-nullish-coalescing-operator"]
}

.eslintrc

{
  "parser": "babel-eslint",
  "root": true,

  "plugins": [],
  "rules": {
    "comma-dangle": 2
  }
}

[lehni]

I am seeing the same problem. The problem is not linked to VScode, it also happens when running the linter in the command line:

$ eslint --ext .js,.vue src
unreachable
Error: unreachable
    at CodePathState.popChoiceContext (…/node_modules/eslint/lib/code-path-analysis/code-path-state.js:444:23)
    at processCodePathToExit (…/node_modules/eslint/lib/code-path-analysis/code-path-analyzer.js:434:19)
    at CodePathAnalyzer.leaveNode (…/node_modules/eslint/lib/code-path-analysis/code-path-analyzer.js:627:9)
    at CodePathAnalyzer.overrideLeaveNode (…/node_modules/eslint-plugin-node/lib/rules/process-exit-as-throw.js:130:27)
    at Traverser.leave [as _leave] (…/node_modules/eslint/lib/linter.js:868:28)
    at Traverser._traverse (…/node_modules/eslint/lib/util/traverser.js:155:18)
    at Traverser._traverse (…/node_modules/eslint/lib/util/traverser.js:147:30)
    at Traverser._traverse (…/node_modules/eslint/lib/util/traverser.js:144:34)
    at Traverser._traverse (…/node_modules/eslint/lib/util/traverser.js:147:30)
    at Traverser._traverse (…/node_modules/eslint/lib/util/traverser.js:147:30)
error Command failed with exit code 1.

[lehni]

BTW, if I modify this line:

node_modules/eslint/lib/code-path-analysis/code-path-state.js:444:23:

        switch (context.kind) {
            case "&&":
            case "||":

to:

        switch (context.kind) {
            case "&&":
            case "||":
            case "??":

then it works.

[lehni]

As a temporary fix, I created a fork with this change and a couple of others to support the nullish-coalescing-operator:

https://github.com/lehni/eslint-nullish-coalescing
https://github.com/lehni/eslint-nullish-coalescing/commit/ab24b4ba4a7daee71ae7bffc2be7f66355803caf

It's available on NPM as eslint-nullish-coalescing:

https://www.npmjs.com/package/eslint-nullish-coalescing

To prevent peerDependency error messages and make it work with VSCode, it's best to install it as an alias to the normal eslint:

yarn add --dev eslint@npm:eslint-nullish-coalescing

[lehni]

Another update:

The VSCode JS grammar package (ms-vscode.typescript-javascript-grammar) struggles with these operators, but both joshpeng.sublime-babel-vscode and ms-vscode.js-atom-grammar appear to work well with it. So a temporary fix is to install one of these packages in VSCode.

[Hypnosphi]

@lehni can you please open a pull request from your fork to eslint repo?

[lehni]

@Hypnosphi I didn't do that so far because I was under the impression that this is just a quick fix, not the proper way to go about it... What do you think?

[Hypnosphi]

I think the eslint maintainers know better =) And the best way to ask is to send a PR which they will review

[lehni]

What's keeping me from doing so is all the requirements for a PR, see: https://raw.githubusercontent.com/eslint/eslint/master/.github/PULL_REQUEST_TEMPLATE.md

I simply don't have the time for it.

But here's the branch that could be used as a base:
https://github.com/lehni/eslint-nullish-coalescing/tree/nullish-coalescing

The actual changes are tiny:
https://github.com/lehni/eslint-nullish-coalescing/commit/345033232f9f6940c055ef561009c7d1716d3a09

[lehni]

Also, my changes are against the v4.19.1 version, not against master, which is heading towards v5 as we speak.

[Hypnosphi]

@lehni Can you please at least sign their CLA so that your changes could be used in my PR?

https://cla.js.foundation/eslint/eslint?pullRequest=10399

[lehni]

@Hypnosphi done! thanks for taking your time to do this.

[charleschenster]

for anyone else running into this issue, I was running an older version of eslint (4.19.1). upgrading to the latest version 6.1.0 unblocked me

[lehni]

Should this issue be closed then?

[volaix]

Is anyone else still having this issue? I've upgraded to 6.1.0 and also tried the latest of 6.4.0. Both get the same 'unreachable' error.
[Error - 9:57:20 am] ESLint stack trace: [Error - 9:57:20 am] Error: unreachable at CodePathState.popChoiceContext (./node_modules/eslint/lib/code-path-analysis/code-path-state.js:444:23)

[Haaxor1689]

I'm having slightly different issue with the same exact problem. I'm running eslint v6.5.1 and when ?? is used in a file vscode extension reports a crash with different error:

[Error - 6:58:03 PM] TypeError: Cannot read property 'range' of null

[Haaxor1689]

@lerayne this looks like an error related to optional chaining operator which I was getting before installing the @babel/plugin-proposal-optional-chaining package. Are you using that package?