False positive for no-implicit-coercion

[LJNeon]

Tell us about your environment

• ESLint Version: 4.11.0
• Node Version: 9.2.0
• npm Version: 5.5.1

What parser (default, Babel-ESLint, etc.) are you using? ECMA2017 Please show your full configuration:

Configuration

```js { "env": { "es6": true, "node": true }, "extends": "eslint:recommended", "rules": { "accessor-pairs": 2, "array-bracket-newline": [2, "consistent"], "array-bracket-spacing": 2, "array-callback-return": 2, "array-element-newline": [2, "never"], "arrow-body-style": 2, "arrow-parens": [2, "as-needed"], "arrow-spacing": 2, "block-scoped-var": 2, "block-spacing": [2, "never"], "brace-style": 2, "comma-dangle": 2, "comma-spacing": 2, "comma-style": 2, "computed-property-spacing": 2, "curly": 2, "dot-notation": 2, "dot-location": [2, "property"], "eol-last": 2, "eqeqeq": 2, "for-direction": 2, "func-call-spacing": 2, "func-name-matching": 2, "func-style": 2, "function-paren-newline": 2, "getter-return": 2, "key-spacing": 2, "keyword-spacing": [2, { "before": false, "after": true, "overrides": { "of": {"before": true}, "break": {"after": false}, "case": {"after": false}, "catch": {"after": false}, "continue": {"after": false}, "debugger": {"after": false}, "default": {"after": false}, "do": {"after": false}, "else": {"after": false}, "extends": {"before": true}, "finally": {"after": false}, "for": {"after": false}, "if": {"after": false}, "in": {"before": true}, "instanceof": {"before": true}, "super": {"after": false}, "switch": {"after": false}, "this": {"after": false}, "while": {"after": false} } }], "line-comment-position": 2, "lines-between-class-members": [2, "never"], "linebreak-style": 2, "multiline-comment-style": [2, "bare-block"], "multiline-ternary": [2, "never"], "newline-per-chained-call": [2, {"ignoreChainWithDepth": 3}], "new-parens": 2, "no-array-constructor": 2, "no-bitwise": 2, "no-catch-shadow": 2, "no-else-return": 2, "no-empty-function": [2, {"allow": ["arrowFunctions"]}], "no-extend-native": 2, "no-extra-bind": 2, "no-extra-boolean-cast": 2, "no-extra-label": 2, "no-extra-parens": 2, "no-extra-semi": 2, "no-floating-decimal": 2, "no-global-assign": 2, "no-implicit-coercion": 2, "no-implicit-globals": 2, "no-implied-eval": 2, "no-inline-comments": 2, "no-invalid-this": 2, "no-label-var": 2, "no-lone-blocks": 2, "no-lonely-if": 2, "no-loop-func": 2, "no-mixed-spaces-and-tabs": 2, "no-multiple-empty-lines": [2, { "max": 0, "maxEOF": 1 }], "no-multi-spaces": 2, "no-multi-str": 2, "no-negated-condition": 2, "no-new": 2, "no-new-wrappers": 2, "no-octal-escape": 2, "no-path-concat": 2, "no-param-reassign": 2, "no-plusplus": [2, {"allowForLoopAfterthoughts": true}], "no-return-await": 2, "no-self-assign": [2, {"props": true}], "no-self-compare": 2, "no-shadow": 2, "no-shadow-restricted-names": 2, "no-template-curly-in-string": 2, "no-trailing-spaces": 2, "no-undef-init": 2, "no-unmodified-loop-condition": 2, "no-unneeded-ternary": 2, "no-unused-expressions": 2, "no-useless-call": 2, "no-useless-computed-key": 2, "no-useless-concat": 2, "no-useless-constructor": 2, "no-useless-rename": 2, "no-useless-return": 2, "no-use-before-define": 2, "no-var": 2, "no-void": 2, "no-whitespace-before-property": 2, "no-with": 2, "object-curly-newline": [2, {"consistent": true}], "object-curly-spacing": 2, "object-shorthand": 2, "one-var": 2, "one-var-declaration-per-line": 2, "operator-assignment": 2, "operator-linebreak": [2, "before"], "padded-blocks": [2, "never"], "padding-line-between-statements": [2, { "blankLine": "never", "prev": "*", "next": "*" }], "prefer-arrow-callback": 2, "prefer-const": 2, "prefer-destructuring": 2, "prefer-promise-reject-errors": 2, "prefer-rest-params": 2, "prefer-spread": 2, "quote-props": [2, "as-needed"], "quotes": [2, "double"], "radix": 2, "rest-spread-spacing": 2, "require-await": 2, "semi": [2, "always"], "semi-spacing": 2, "semi-style": 2, "sort-keys": [2, "asc", { "caseSensitive": false, "natural": true }], "sort-vars": [2, {"ignoreCase": true}], "space-before-blocks": 2, "space-before-function-paren": [2, { "anonymous": "never", "asyncArrow": "always", "named": "never" }], "space-in-parens": 2, "space-infix-ops": 2, "space-unary-ops": [2, { "words": true, "nonwords": false }], "spaced-comment": 2, "strict": [2, "global"], "symbol-description": 2, "wrap-iife": 2, "yoda": 2 }, "parserOptions": {"ecmaVersion": 2017} } ```

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

for(let i = 0; i < this.guessed.length; i++) {
    const amount = 3 - 1 * i, // line 88
    {id} = this.guessed[i];
    if(this.points.hasOwnProperty(id)) {
        this.points[id] += amount;
    }else{
        this.points[id] = amount;
    }
}

eslint src/**

What did you expect to happen? No errors would be thrown as i is already a number. What actually happened? Please include the actual, raw output from ESLint.

  88:28  error  use `Number(i)` instead  no-implicit-coercion

[LJNeon]

And yes I am fully aware that the 1 * part isn't needed and that I could do 3 - i. My point here was that i is a number yet it still errors.

[not-an-aardvark]

Thanks for the report. This is happening because it's not possible in general to statically determine what types your variables will have at runtime.

For example, if your code looked like this:

const i = someUnknownFunction() ? 'a string' : 5;

console.log(1 * i);

...i would possibly be a string and possibly be a number, and ESLint wouldn't be able to statically determine which is the case. So instead the rule assumes that 1 * x is always used to coerce a value to a number, because it wouldn't be useful to do 1 * x if x was already a number.

[LJNeon]

Couldn't it check for conditionals like that statically? If there's any uncertainty on whether or not a variable is a number assume that it isn't, otherwise it's fine.

[not-an-aardvark]

Type checking is generally out of scope for a linter -- if you want to enforce static type checking, you should use a dedicated type checker like TypeScript. In this case I don't think implementing type checking would be worth the very minor benefit that it would provide for the no-implicit-coercion rule.

[ilyavolodin]

Closing this issue as this is working as intended. Unfortunately, ESLint is not capable of checking types and validating the example provided.