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Cannot read property 'ecmaFeatures' of undefined

See original GitHub issue


Parsing error: Cannot read property 'ecmaFeatures' of undefined

Tell us about your environment

  • ESLint Version: 5.1.0
  • Node Version: 10.6.0
  • **npm Version:**6.1.0"

my eslintrc

  node: true
  es6: true
  jquery: false
  browser: false

  - eslint:recommended

  curly: off

Issue Analytics

  • State:closed
  • Created 5 years ago
  • Reactions:7
  • Comments:16 (12 by maintainers)

github_iconTop GitHub Comments

not-an-aardvarkcommented, Aug 1, 2018

The root cause of this issue is that two different versions of espree are being loaded simultaneously with the same version of acorn. espree works by adding a plugin to the acorn parser, and acorn’s plugin API seems to require setting global state on the acorn object. As a result, espree@4 is setting some state by adding listeners to acorn, and then espree@3 is getting loaded and is overriding the state that was set by espree@4. This is causing espree@3’s listeners to unexpectedly get called even though the running version of ESLint is using espree@4, which is causing espree@3 to throw an error.

To me, it seems like the best way to fix this would be for acorn to allow registering a plugin without modifying global state on the acorn package. Another possibility would be for espree to include acorn as a bundled dependency to ensure that it never shares its version of acorn with any other packages.

regsebcommented, Jul 31, 2018

@not-an-aardvark This bug comes when eslint@5 and eslint@4 are used in project. It’s not specific at standard ( and

A test case with only ESLint:

  • package.json:
    "dependencies": {
        "eslint": "5.2.0",
        "other": "file:./other"
  • testcase.js:
#!/usr/bin/env node

var Linter = require("eslint").Linter;
require("other"); // Comment me !
var linter = new Linter();

var messages = linter.verify("var foo;", {
    rules: {
        semi: 2
}, { filename: "foo.js" });

  • other/package.json:
    "name": "other",
    "main": "index",
    "dependencies": {
        "eslint": "4.19.1"
  • other/index.js:

To test, decompress testcase.tar.gz. Execute npm install and ./testcase.js. This JSON is displayed in console:

[ { ruleId: null,
    fatal: true,
    severity: 2,
     'Parsing error: Cannot read property \'ecmaFeatures\' of undefined',
    line: undefined,
    column: undefined } ]

If you comment line 4 of testcase.js (require("other");), the output is:

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