Environment

Node version: v18.2.0
npm version: 8.10.0
Local ESLint version: 8.16.0
Global ESLint version: none
Operating System: Ubuntu 22.04 LTS

What parser are you using?

Default (Espree)

What did you do?

{
    "parserOptions": {
        "ecmaVersion": "latest",
        "sourceType": "module"
    },
    "rules": {
        "key-spacing": [2, { "align": "value" }]
    }
}

const foo = {
    "a": "bar",
    "𐌘": "baz" // U+10318 https://en.wikipedia.org/wiki/Old_Italic_(Unicode_block)
};

Object.keys(foo).forEach((key) => {
    console.log(key, key.length, [...key].length);
});
// a 1 1
// 𐌘 2 1

What did you expect to happen?

No error reported.

What actually happened?

$ npx eslint index.js

/home/regseb/testcase/index.js
  2:10  error  Missing space before value for key 'a'  key-spacing

✖ 1 problem (1 error, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

Participation

• I am willing to submit a pull request for this issue.

Additional comments

JavaScript uses UTF-16 for String:

• A character before U+FFFF is encoded as one code unit.
• A character after U+FFFF is encoded as two code units.

String.length counts code units instead of characters.

I think we need to change the function getKeyWidth().