.eslintrc

{
    "ecmaFeatures": {
        "modules": true,
        "arrowFunctions": true
    },
    "env": {
        "es6": true
    },
    "rules": {
        "semi": [2, "always"]
    }
}

test.js

class TestClass
{
  constructor() {
    this.x = 4;
      
    this.foo = (y) -> y + 1;
  //^ Unexpected token line 6 column 5
      
  }
}