no-undef-init incorrectly autofixes some var declarations

[ljqx]

Tell us about your environment

• ESLint Version: 4.6.1
• Node Version: 6.9.1
• npm Version: 3.9.3

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

Please show your full configuration:

Configuration

{
  "env": {
    "browser": true,
    "node": true,
    "amd": true,
    "mocha": true
  },

  "rules": {
    //Possible Errors
    //The following rules point out areas where you might have made mistakes.

    "comma-dangle": [2, "always-multiline"], //disallow or enforce trailing commas (recommended)
    "no-cond-assign": [2], //disallow assignment in conditional expressions (recommended)
    "no-console": [2], //disallow use of console in the node environment (recommended)
    "no-constant-condition": [2], //disallow use of constant expressions in conditions (recommended)
    "no-control-regex": [2], //disallow control characters in regular expressions (recommended)
    "no-debugger": [2], //disallow use of debugger (recommended)
    "no-dupe-args": [2], //disallow duplicate arguments in functions (recommended)
    "no-dupe-keys": [2], //disallow duplicate keys when creating object literals (recommended)
    "no-duplicate-case": [2], //disallow a duplicate case label. (recommended)
    "no-empty-character-class": [2], //disallow the use of empty character classes in regular expressions (recommended)
    "no-empty": [2], //disallow empty statements (recommended)
    "no-ex-assign": [2], //disallow assigning to the exception in a catch block (recommended)
    "no-extra-boolean-cast": [2], //disallow double-negation boolean casts in a boolean context (recommended)
    "no-extra-parens": [2, "all", { "nestedBinaryExpressions": false }], //disallow unnecessary parentheses
    "no-extra-semi": [2], //disallow unnecessary semicolons (recommended)
    "no-func-assign": [2], //disallow overwriting functions written as function declarations (recommended)
    "no-inner-declarations": [2, "functions"], //disallow function or variable declarations in nested blocks (recommended)
    "no-invalid-regexp": [2], //disallow invalid regular expression strings in the RegExp constructor (recommended)
    "no-irregular-whitespace": [2], //disallow irregular whitespace outside of strings and comments (recommended)
    "no-negated-in-lhs": [2], //disallow negation of the left operand of an in expression (recommended)
    "no-obj-calls": [2], //disallow the use of object properties of the global object (Math and JSON) as functions (recommended)
    "no-prototype-builtins": [0], //Disallow use of Object.prototypes builtins directly
    "no-regex-spaces": [2], //disallow multiple spaces in a regular expression literal (recommended)
    "no-sparse-arrays": [2], //disallow sparse arrays (recommended)
    "no-unexpected-multiline": [2], //Avoid code that looks like two expressions but is actually one
    "no-unreachable": [2], //disallow unreachable statements after a return, throw, continue, or break statement (recommended)
    "no-unsafe-finally": [2], //disallow control flow statements in finally blocks
    "use-isnan": [2], //disallow comparisons with the value NaN (recommended)
    "valid-jsdoc": [2], //Ensure JSDoc comments are valid
    "valid-typeof": [2], //Ensure that the results of typeof are compared against a valid string (recommended)

    //Best Practices
    //These are rules designed to prevent you from making mistakes. They either prescribe a better way of doing something or help you avoid footguns.

    "accessor-pairs": [0], //Enforces getter/setter pairs in objects
    "array-callback-return": [0], //Enforces return statements in callbacks of array's methods
    "block-scoped-var": [2], //treat var statements as if they were block scoped
    "complexity": [0], //specify the maximum cyclomatic complexity allowed in a program
    "consistent-return": [2], //require return statements to either always or never specify values
    "curly": [2], //specify curly brace conventions for all control statements
    "default-case": [2], //require default case in switch statements
    "dot-notation": [2], //encourages use of dot notation whenever possible
    "dot-location": [2, "property"], //enforces consistent newlines before or after dots
    "eqeqeq": [2, "smart"], //require the use of === and !== (fixable)
    "guard-for-in": [0], //make sure for-in loops have an if statement
    "no-alert": [2], //disallow the use of alert, confirm, and prompt
    "no-caller": [2], //disallow use of arguments.caller or arguments.callee
    "no-case-declarations": [2], //disallow lexical declarations in case clauses
    "no-div-regex": [2], //disallow division operators explicitly at beginning of regular expression
    "no-else-return": [0], //disallow else after a return in an if
    "no-empty-function": [2], //disallow use of empty functions
    "no-empty-pattern": [2], //disallow use of empty destructuring patterns
    "no-eq-null": [0], //disallow comparisons to null without a type-checking operator
    "no-eval": [2], //disallow use of eval()
    "no-extend-native": [2], //disallow adding to native types
    "no-extra-bind": [2], //disallow unnecessary function binding
    "no-extra-label": [2], //disallow unnecessary labels
    "no-fallthrough": [2], //disallow fallthrough of case statements (recommended)
    "no-floating-decimal": [2], //disallow the use of leading or trailing decimal points in numeric literals
    "no-implicit-coercion": [0], //disallow the type conversions with shorter notations
    "no-implicit-globals": [2], //disallow var and named functions in global scope
    "no-implied-eval": [2], //disallow use of eval()-like methods
    "no-invalid-this": [2], //disallow this keywords outside of classes or class-like objects
    "no-iterator": [2], //disallow usage of __iterator__ property
    "no-labels": [2], //disallow use of labeled statements
    "no-lone-blocks": [2], //disallow unnecessary nested blocks
    "no-loop-func": [2], //disallow creation of functions within loops
    "no-magic-numbers": [0], //disallow the use of magic numbers
    "no-multi-spaces": [0], //disallow use of multiple spaces
    "no-multi-str": [2], //disallow use of multiline strings
    "no-native-reassign": [0], //disallow reassignments of native objects
    "no-new": [0], //disallow use of the new operator when not part of an assignment or comparison
    "no-new-func": [2], //disallow use of new operator for Function object
    "no-new-wrappers": [2], //disallows creating new instances of String,Number, and Boolean
    "no-octal": [2], //disallow use of octal literals (recommended)
    "no-octal-escape": [2], //disallow use of octal escape sequences in string literals, such as var foo = "Copyright \251";
    "no-param-reassign": [2], //disallow reassignment of function parameters
    "no-proto": [2], //disallow usage of __proto__ property
    "no-redeclare": [2], //disallow declaring the same variable more than once (recommended)
    "no-return-assign": [2], //disallow use of assignment in return statement
    "no-script-url": [2], //disallow use of javascript urls.
    "no-self-assign": [2], //disallow assignments where both sides are exactly the same
    "no-self-compare": [1], //disallow comparisons where both sides are exactly the same
    "no-sequences": [2], //disallow use of the comma operator
    "no-throw-literal": [2], //restrict what can be thrown as an exception
    "no-unmodified-loop-condition": [2], //disallow unmodified conditions of loops
    "no-unused-expressions": [2], //disallow usage of expressions in statement position
    "no-unused-labels": [2], //disallow unused labels
    "no-useless-call": [2], //disallow unnecessary .call() and .apply()
    "no-useless-concat": [2], //disallow unnecessary concatenation of literals or template literals
    "no-useless-escape": [2], //disallow unnecessary escape characters
    "no-void": [0], //disallow use of the void operator
    "no-warning-comments": [0], //disallow usage of configurable warning terms in comments //e.g. TODO or FIXME
    "no-with": [0], //disallow use of the with statement
    "radix": [0], //require use of the second argument for parseInt()
    "vars-on-top": [0], //require declaration of all vars at the top of their containing scope
    "wrap-iife": [2, "inside"], //require immediate function invocation to be wrapped in parentheses
    "yoda": [2], //require or disallow Yoda conditions

    //Strict Mode
    //These rules relate to using strict mode.

    "strict": [0], //controls location of Use Strict Directives

    //Variables
    //These rules have to do with variable declarations.

    "init-declarations": [0], //enforce or disallow variable initializations at definition
    "no-catch-shadow": [2], //disallow the catch clause parameter name being the same as a variable in the outer scope
    "no-delete-var": [2], //disallow deletion of variables (recommended)
    "no-label-var": [2], //disallow labels that share a name with a variable
    "no-restricted-globals": [0], //disallow specified global variables
    "no-shadow": [2], //disallow declaration of variables already declared in the outer scope
    "no-shadow-restricted-names": [2], //disallow shadowing of names such as arguments
    "no-undef": [2], //disallow use of undeclared variables unless mentioned in a /*global */ block (recommended)
    "no-undef-init": [2], //disallow use of undefined when initializing variables
    "no-undefined": [0], //disallow use of undefined variable
    "no-unused-vars": [2], //disallow declaration of variables that are not used in the code (recommended)
    "no-use-before-define": [2], //disallow use of variables before they are defined

    //Node.js and CommonJS
    //These rules are specific to JavaScript running on Node.js or using CommonJS in the browser.

    "callback-return": [1], //enforce return after a callback
    "global-require": [1], //disallow require() outside of the top-level module scope
    "handle-callback-err": [1], //enforce error handling in callbacks
    "no-mixed-requires": [1], //disallow mixing regular variable and require declarations
    "no-new-require": [1], //disallow use of new operator with the require function
    "no-path-concat": [1], //disallow string concatenation with __dirname and __filename
    "no-process-env": [2], //disallow use of process.env
    "no-process-exit": [1], //disallow process.exit()
    "no-restricted-modules": [1], //restrict usage of specified node modules
    "no-sync": [1], //disallow use of synchronous methods

    //Stylistic Issues
    //These rules are purely matters of style and are quite subjective.

    "array-bracket-spacing": [2, "never"], //enforce spacing inside array brackets
    "block-spacing": [2, "always"], //disallow or enforce spaces inside of single line blocks
    "brace-style": [2], //enforce one true brace style
    "camelcase": [2], //require camel case names
    "comma-spacing": [2], //enforce spacing before and after comma
    "comma-style": [2], //enforce one true comma style
    "computed-property-spacing": [2], //require or disallow padding inside computed properties
    "consistent-this": [2, "self"], //enforce consistent naming when capturing the current execution context
    "eol-last": [2], //enforce newline at the end of file, with no multiple empty lines
    "func-names": [0], //require function expressions to have a name
    "func-style": [0], //enforce use of function declarations or expressions
    "id-blacklist": [2], //blacklist certain identifiers to prevent them being used
    "id-length": [0], //this option enforces minimum and maximum identifier lengths (variable names, property names etc.)
    "id-match": [0], //require identifiers to match the provided regular expression
    "indent": [2, 2], //specify tab or space width for your code
    "jsx-quotes": [2, "prefer-double"], //specify whether double or single quotes should be used in JSX attributes
    "key-spacing": [2], //enforce spacing between keys and values in object literal properties
    "keyword-spacing": [2], //enforce spacing before and after keywords
    "linebreak-style": [0], //disallow mixed 'LF' and 'CRLF' as linebreaks
    "lines-around-comment": [0], //enforce empty lines around comments
    "max-depth": [0], //enforce a maximum depth that blocks can be nested
    "max-len": [0], //enforce a maximum line length
    "max-lines": [0], //enforce a maximum depth that callbacks can be nested
    "max-nested-callbacks": [0], //enforce a maximum depth that callbacks can be nested
    "max-params": [0], //enforce a maximum number of parameters in function definitions
    "max-statements": [0], //enforce a maximum number of statements allowed in function blocks
    "max-statements-per-line": [0, {
      "max": 2
    }], //enforce a maximum number of statements allowed per line
    "new-cap": [2], //require a capital letter for constructors
    "new-parens": [2], //disallow the omission of parentheses when invoking a constructor with no arguments
    "newline-after-var": [2], //require or disallow an empty newline after variable declarations
    "newline-before-return": [0], //require an empty line before return statements
    "newline-per-chained-call": [0], //enforce newline after each call when chaining the calls
    "no-array-constructor": [2], //disallow use of the Array constructor
    "no-bitwise": [2], //disallow bitwise operators
    "no-continue": [2], //disallow use of the continue statement
    "no-inline-comments": [0], //disallow comments inline after code
    "no-lonely-if": [2], //disallow if as the only statement in an else block
    "no-mixed-operators": [2], //disallow mixes of different operators
    "no-mixed-spaces-and-tabs": [2], //disallow mixed spaces and tabs for indentation (recommended)
    "no-multiple-empty-lines": [2], //disallow multiple empty lines
    "no-nested-ternary": [2], //disallow nested ternary expressions
    "no-negated-condition": [2], //disallow negated conditions
    "no-new-object": [2], //disallow the use of the Object constructor
    "no-plusplus": [0], //disallow the unary operators ++ and --
    "no-restricted-syntax": [2], //disallow use of certain syntax in code
    "no-spaced-func": [2], //disallow space between function identifier and application
    "no-ternary": [0], //disallow the use of ternary operators
    "no-trailing-spaces": [2], //disallow trailing whitespace at the end of lines
    "no-underscore-dangle": [0], //disallow dangling underscores in identifiers
    "no-unneeded-ternary": [2], //disallow the use of ternary operators when a simpler alternative exists
    "no-whitespace-before-property": [2], //disallow whitespace before properties
    "object-curly-newline": [0], //require or disallow line breaks inside braces
    "object-curly-spacing": [0], //require or disallow padding inside curly braces
    "object-property-newline": [2, {
      "allowMultiplePropertiesPerLine": true
    }], //enforce placing object properties on separate lines
    "one-var": [0], //require or disallow one variable declaration per function
    "one-var-declaration-per-line": [2, "always"], //require or disallow an newline around variable declarations
    "operator-assignment": [0], //require assignment operator shorthand where possible or prohibit it entirely
    "operator-linebreak": [2, "after"], //enforce operators to be placed before or after line breaks
    "padded-blocks": [0], //enforce padding within blocks
    "quote-props": [0], //require quotes around object literal property names
    "quotes": [2, "single"], //specify whether backticks, double or single quotes should be used (fixable)
    "require-jsdoc": [0], //Require JSDoc comment
    "semi-spacing": [2], //enforce spacing before and after semicolons
    "sort-imports": [0], //sort import declarations within module
    "semi": [2, "always"], //require or disallow use of semicolons instead of ASI (fixable)
    "sort-vars": [0], //sort variables within the same declaration block
    "space-before-blocks": [2], //require or disallow a space before blocks
    "space-before-function-paren": [0], //require or disallow a space before function opening parenthesis
    "space-in-parens": [0], //require or disallow spaces inside parentheses
    "space-infix-ops": [2], //require spaces around operators
    "space-unary-ops": [2], //require or disallow spaces before/after unary operators
    "spaced-comment": [0], //require or disallow a space immediately following the // or /* in a comment
    "wrap-regex": [2], //require regex literals to be wrapped in parentheses

  }

}

What did you do? Please include the actual source code causing the issue.
Auto fix no-undef-init which changes behavior of code

function fun() {
  for (var p in [0, 1, 2]) {
    var selected = undefined; // auto fix will remove `= undefined`

    for (var i in [0, 1, 2]) {
      if (!selected) {
        console.log('selected');
      }
      selected = i;
    }
  }
}

fun();

What did you expect to happen?
Auto fix should not fix this part as it changes behavior of code.

What actually happened? Please include the actual, raw output from ESLint.
Before fix, the code logs 3 times 'selected'; after fix, the code only logs 1 time.

[platinumazure]

I think I see the problem- the declaration is in a loop, so there is actually a reinitializion going on which the autofix removes.

Simple fix is to just remove the autofix functionality. Less simple is to avoid autofixing when the variable declaration is in a loop of any kind. Honestly, we've removed autofix for less in other rules, so I would be okay with removing it here.

@eslint/eslint-team Any thoughts?

[not-an-aardvark]

Would it work to remove the autofix for var declarations, but keep it for let? I don't think let variables can be redeclared like that.

One solution would be to use code path analysis to check whether a declaration can ever be reached twice, but that might not be worth the implementation effort.