Tell us about your environment

• ESLint Version: 6.8.0
• Node Version: 8.15
• npm Version: 6.4.1

What parser (default, @babel/eslint-parser, @typescript-eslint/parser, etc.) are you using?
default

Please show your full configuration:

module.exports = {
    "env": {
        "browser": true,
        "es6": true
    },
    "extends": "eslint:recommended",
    "globals": {
        "Atomics": "readonly",
        "SharedArrayBuffer": "readonly"
    },
    "parserOptions": {
        "ecmaVersion": 2018,
        "sourceType": "module"
    },
    "rules": {
        "prefer-const": "error",
        "no-undef-init": "warn"
    }
};

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

let foo= undefined; //foo is not reasigned so it will try to transform to const and keep initialization as undefined;

Eslint command:

node node_modules/eslint/bin/eslint.js --fix testfile.js

What did you expect to happen?
transform variable to constant but not remove the undefined init as it will become a constant.

What actually happened? Please include the actual, raw output from ESLint.
I have runned an autofix in a legacy code with more than 10k files. When it finds a variable that should be a constant (because it is not reasigned) and is initialized as undefined it fix the code transforming it to const foo; causing errors over the code. I tried to change the rules order to first turns it to a constant and them check the undefined init so it should be fixed (as per: https://eslint.org/docs/rules/no-undef-init). But is still happening.

Are you willing to submit a pull request to fix this bug?
Yes, need some guidance but I can do this :)