no-unused-vars false positive for variable declared in one scope but only used in some subscope(s)

*OSX

• **ESLint Version: ** 4.19.1
• **Node Version: ** 8.9.4
• **npm Version: ** 5.6.0

What parser (default, Babel-ESLint, etc.) are you using? Default

Please show your full configuration:

Configuration

module.exports = {
    root: true,
    parserOptions: {
        ecmaVersion: 2017,
        sourceType: 'module'
    },
    extends: 'eslint:recommended',
    env: {
        'node': true,
        'es6': true
    },
    rules: {
        // Additional rules
        //  http://eslint.org/docs/rules

        // ESLint Possible Errors
        "no-template-curly-in-string": "error",
        "valid-jsdoc": [ "error", {
            "prefer": { "arg": "param", "argument": "param", "class": "constructor", "return": "returns", "virtual": "abstract" },
            "preferType": { "boolean": "Boolean", "number": "Number", "object": "Object", "string": "String" },
        }],

        // ESLint best practices
        "block-scoped-var": "error",
        "consistent-return": "error",
        "default-case": "error",
        "guard-for-in": "error",
        "no-caller": "error",
        "no-div-regex": "error",
        "no-eval": "error",
        "no-empty-function": "error",
        "no-eq-null": "error",
        "no-extend-native": "error",
        "no-floating-decimal": "error",
        "no-implicit-coercion": "error",
        // "no-invalid-this": "error",
        "no-iterator": "error",
        "no-labels": "error",
        "no-loop-func": "error",
        "no-multi-str": "error",
        "no-new": "error",
        "no-new-func": "error",
        "no-new-wrappers": "error",
        "no-octal-escape": "error",
        "no-param-reassign": "error",
        "no-proto": "error",
        "no-return-assign": "error",
        "no-return-await": "error",
        "no-script-url": "error",
        "no-self-compare": "error",
        "no-sequences": "error",
        "no-throw-literal": "error",
        "no-unmodified-loop-condition": "error",
        "no-unused-expressions": "error",
        "no-useless-call": "error",
        "no-useless-concat": "error",
        "no-useless-escape": "error",
        "no-void": "error",
        "no-warning-comments": [ "warn", { "terms": ["todo", "fixme"], "location": "anywhere" } ],
        "no-with": "error",
        "prefer-promise-reject-errors": "error",
        "radix": "error",
        "require-await": "error",
        "wrap-iife": "error",

        // ESLint Strict Mode rules
        "strict": "error",

        // ESLint Variables rules
        "init-declarations": "error",
        "no-catch-shadow": "error",
        "no-label-var": "error",
        "no-restricted-globals": "error",
        "no-shadow": "error",
        "no-shadow-restricted-names": "error",
        "no-undef-init": "error",
        "no-undefined": "error",
        "no-use-before-define": "error",

        // ESLint Stylistic Issues
        "indent": "error",
        "keyword-spacing": "error",
        "no-tabs": "error",
        "semi": "error",
        "sort-imports": [ "error", { "ignoreCase": true }],
        "space-unary-ops": "error",

        // ECMAScript 6
        "arrow-parens": "error",
        "no-confusing-arrow": "error",
        "no-var": "error",
        "prefer-const": "warn",
        "prefer-numeric-literals": "error",
        "prefer-rest-params": "error",
        "prefer-spread": "error",
        "prefer-template": "error",

        // Project specific
        "no-console": 0
    }
};

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

const generateBinaryHeader = (uuidStr, {num1, num2, num3}, someObj) => {
    const header = Buffer.alloc(30);
    let i = 0;
    {
        const begin = Buffer.from([0xAA, 0x00, 0x00, 0x88, 0x00, 0x00]); // Magic number
        i = i + begin.copy(header, i);
    }

    {
        const uuidBuf = uuidStr2Buf(uuidStr);
        i = i + uuidBuf.copy(header, i);
    }

    {
        const versionBuf = Buffer.from([num1, num2, num3, 0x00, someObj.num, 0x00 ,0xEF, 0xEF]);
        i = i + versionBuf.copy(header, i);
    }

    return header;
};

npm start

// package.json
// ...
  "scripts": {
    "start": "eslint .; ./my_start_script",
  },
// ...

What did you expect to happen? Variable i is used in the sub scope. Sub-scope defined for code clarity. This example is simple, but elsewhere in our code things are more complex and the above pattern still exists. The variable is both read and written to, therefore it should be considered used. What actually happened? Please include the actual, raw output from ESLint.

> eslint .; ./tfp_pkg


/Users/me/projects/project_name/src/code.js
  192:9   error  'i' is assigned a value but never used  no-unused-vars
  253:17  error  'i' is assigned a value but never used  no-unused-vars

✖ 2 problems (2 errors, 0 warnings)

[not-an-aardvark]

In the function call, you are using the handler variable declared here:

btn.addEventListener('click', (event, handler) => subscribe(event, handler));
                                      ^^^^^^^

However, you are not using the handler variable declared here:

const handler = 'x';
      ^^^^^^^

You might have intended to make your code look like this instead:

const handler = 'x';
btn.addEventListener('click', (event) => subscribe(event, handler));

function subscribe(event, handler) {
 ...
}

[not-an-aardvark]

Hi, thanks for the report. However, this is working as intended.

@TuckerDowns In your example, the last assignment to i is unused, because i is never used after that. You could just replace that line with versionBuf.copy(header, i);.

@vitaly-zdanevich Your example is working as intended because the handler variable is never actually used. You are creating two functions that shadow the handler variable, but if you use handler inside those functions it will refer to the shadowed variable and not the variable that you declared at the top of your file.