The enforceConst option of the no-magic-numbers rule incorrectly reports a simple for loop

Tell us about your environment

• ESLint Version: v4.19.1
• Node Version: v8.9.4
• npm Version: 6.0.0

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

Please show your full configuration:

Configuration

{
    "env": {
        "browser": true,
        "es6": true,
        "greasemonkey": true
    },
    "extends": "eslint:recommended",
    "globals": {
        "min": false
    },
    "rules": {
        "array-callback-return": "error",
        "block-scoped-var": "error",
        "curly": "error",
        "dot-notation": "error",
        "eol-last": [
            "error",
            "always"
        ],
        "eqeqeq": "error",
        "guard-for-in": "error",
        "indent": [
            "error",
            4, {
                "SwitchCase": 1
            }
        ],
        "linebreak-style": [
            "error",
            "unix"
        ],
        "no-case-declarations": "error",
        "no-else-return": "error",
        "no-empty-function": "error",
        "no-eq-null": "error",
        "no-eval": "error",
        "no-extra-bind": "error",
        "no-floating-decimal": "error",
        "no-implicit-coercion": "error",
        "no-implicit-globals": "error",
        "no-implied-eval": "error",
        "no-invalid-this": "error",
        "no-loop-func": "error",
        "no-lone-blocks": "error",
        "no-magic-numbers": [
            "error", {
                "enforceConst": true,
                "ignoreArrayIndexes": true
            }
        ],
        "no-multiple-empty-lines": [
            "error", {
                "max": 1,
                "maxBOF": 0,
                "maxEOF": 1
            }
        ],
        "no-trailing-spaces": "error",
        "quotes": [
            "error",
            "double"
        ],
        "semi": [
            "error",
            "always"
        ],
        "valid-jsdoc": [
            "error", {
                "requireReturn": false
            }
        ]
    }
}

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

(function () {
    var array = [];

    for (let i = 0; i < array.length; i++) {
        array[i] = i;
    }
})();

./node_modules/.bin/eslint index.js

What did you expect to happen?

Not throw an error.

What actually happened? Please include the actual, raw output from ESLint.

/Users/revolt/Development/index.js
  4:18  error  Number constants declarations must use 'const'  no-magic-numbers

✖ 1 problem (1 error, 0 warnings)

[not-an-aardvark]

I think we’re straying from the point of the enforceConst option. Imagine the code looked like this:

for (let i = 4124; i < arr.length; i++) {
    // ...
}

I would argue that the use of 4124 is a code smell, because it’s a “magic number” and provides no readable info about what that particular value is for.

The correct fix for the issue would be to do something like this:

const INITIAL_NUM_WIDGETS = 4124; // (e.g.)

for (let i = INITIAL_NUM_WIDGETS; i < arr.length; i++) {
    // ...
}

The enforceConst option on the no-magic-numbers rule controls whether the declaration of INITIAL_NUM_WIDGETS must be declared with const, or whether var is also acceptable. It’s not related to the initialization of i.

If the purpose of the rule is to give a name to special constants, then I don’t think it makes sense to accept a loop iteration variable as the “name” of the constant, because loop iteration variables change their value, so they would not consistently refer to that constant.

For example, the following correction to the original code would not make sense:

// rename `i` to `INITIAL_NUM_WIDGETS`
for (let INITIAL_NUM_WIDGETS = 4124; INITIAL_NUM_WIDGETS < arr.length; INITIAL_NUM_WIDGETS++) {
    // ...
}

I would argue that the above code still has the same code smell as the original code, because it’s still not clear where the value of 4124 comes from. The name INITIAL_NUM_WIDGETS doesn’t clarify the situation, because it refers to a value that changes within the loop, rather than always referring to the value 4124.

[revolter]

@not-an-aardvark, Does my last comment make sense?