.eslintrc.yaml

parser: babel-eslint

extends:
  - airbnb
  - plugin:jest/recommended
  - ./tools/formatting.eslintrc.yaml

plugins:
 - jest
 - flowtype
 # - react-native  # see commented-out rules config below

env:
  browser: true
  mocha: true
  node: true
  jest: true

globals: # true/false means mutable/const
  __DEV__: false
  __TEST__: false

settings:
  # This setting is shared by the rules `import/*`.  For details, see:
  #   https://github.com/benmosher/eslint-plugin-import#resolvers
  #   https://www.npmjs.com/package/eslint-import-resolver-node
  import/resolver:
    node:
      extensions: [.js, .ios.js, .android.js, .json]

rules:

  #
  # Airbnb rule repeals / adjustments
  #
  # Our settings for a rule supersede Airbnb's, so we duplicate some
  # of Airbnb's settings where we don't want a full repeal.
  #

  # Tricky code.
  no-bitwise: off
  no-confusing-arrow: off
  no-continue: off
  no-plusplus: off
  no-nested-ternary: off

  # Allow multiple classes per file.
  #
  # It's best to have just one class per file, but there are cases
  # where it's useful to have some tiny helper React components
  # declared in the same file as a main component.
  max-classes-per-file: off

  # Don't forbid prop spreading (it works fine for us)
  react/jsx-props-no-spreading: off

  # Don't enforce linebreaks in curly braces in JSX
  #
  # The difference this would make is minor, and leaving this rule off
  # better aligns with our practices in JS proper (not JSX).
  react/jsx-curly-newline: off

  # Enforce `static` class property syntax.
  react/static-property-placement:
  - error
  - static public field

  # Enforce state initialization style (never in constructor)
  react/state-in-constructor:
  - error
  - never

  # Airbnb uses four selectors under 'no-restricted-syntax':
  #
  # ForInStatement, ForOfStatement, LabeledStatement, WithStatement.
  # https://github.com/airbnb/javascript/blob/dee4f17/packages/eslint-config-airbnb-base/rules/style.js#L334.
  no-restricted-syntax:
  - error

  # ForInStatement: We agree to forbid 'for..in' loops.
  - selector: ForInStatement
    message: 'for..in loops iterate over the entire prototype chain, which
      is virtually never what you want. Use Object.{keys,values,entries}, and
      iterate over the resulting array.'
      # Further to Airbnb's explanation: They're also slightly buggy
      # in JSC [1], on which react-native is built... and,
      # surprisingly, their semantics weren't fully nailed down until
      # as late as 2019-12 [2]. But the above is reason enough.
      # [1]: https://bugs.webkit.org/show_bug.cgi?id=38970
      # [2]: https://github.com/tc39/proposals/commit/cb9c6e50

  # ForOfStatement: We don't agree to forbid 'for..of' loops.
  #
  # Airbnb's style guide discourages language-level loops in favor of
  # array iterations. We don't share this controversial view; see it
  # expressed at https://github.com/airbnb/javascript/issues/1103.

  # LabeledStatement: We don't agree to forbid labeled statements.
  #
  # We don't really use them, but Airbnb's reasoning flows from their
  # anti-loop position, mentioned above, which we don't share.

  # WithStatement: Agree to forbid with statements, but not this way.
  #
  # ESLint provides a built-in `no-with` rule with the same effect,
  # which we choose, instead of this circumlocution.
  no-with: error

  #
  # Formatting and naming
  #

  camelcase: off

  no-await-in-loop: off # Promise.all is not always desirable

  # Permit dangling underscores in class property names, to denote "private"
  # fields. (These should be replaced with true private fields per the TC39
  # class fields proposal [1], once that's available to us.)
  #
  # [1]: https://github.com/tc39/proposal-class-fields
  no-underscore-dangle: ["error", { allowAfterThis: true } ]
  # Disallow double underscores and trailing underscores.
  id-match: ["error", "^_?([a-zA-Z0-9$]+_?)*$", { "properties": true }]

  # Disable a bunch of rules that should be taken care of by prettier.
  arrow-parens: off
  comma-dangle: off
  function-paren-newline: off
  indent: off
  lines-between-class-members: off
  max-len: off
  no-mixed-operators: off
  object-curly-newline: off
  quote-props: off

  # For more formatting rules, see tools/formatting.eslintrc.yaml .

  # Repeal some rather absurd rules that make some code impossible to write in
  # the most reasonable way.
  no-else-return: off
  prefer-destructuring:
  - error
  - AssignmentExpression: {array: false, object: false}

  # Likely-wrong code
  no-unused-vars: [warn, {vars: local, args: none}]
  # no-param-reassign seems well-intentioned... but fires on common uses
  # of Array#reduce.  Too noisy.
  no-param-reassign: off

  # Imports
  import/prefer-default-export: off
  import/no-extraneous-dependencies:
  - error
  - devDependencies: ['**/__tests__/**/*.js', tools/**]
  no-restricted-imports:
  - error
  - patterns: ['**/__tests__/**']
  import/no-cycle: off  # This would be nice to fix; but isn't easy.
  import/export: off  # This is redundant with Flow, and buggy.

  # Jest
  # This rule could be useful if it fired only on new instances; but we
  # don't have a way to do that, and it doesn't make sense to keep these
  # out of the repo.
  jest/no-disabled-tests: off
  
  # Only using string literals for test names, under the heading
  # `titleMustBeString`, [1] is silly and should probably not be a
  # thing [2]. We wish we could get all of this rule except that part.
  # In fact, it's possible to ignore it *for describe blocks
  # only* [3], but there are no plans to allow turning it off for
  # `test` blocks.
  # 
  # [1] https://github.com/jest-community/eslint-plugin-jest/blob/master/docs/rules/valid-title.md
  # [2] https://github.com/jest-community/eslint-plugin-jest/issues/203.
  # [3] https://github.com/jest-community/eslint-plugin-jest/issues/470
  jest/valid-title: off

  # React
  react/jsx-filename-extension: off  # Like RN upstream, we call the files *.js.
  react/destructuring-assignment: off  # The opposite is often much cleaner.
  react/no-multi-comp: off  # This often just forces making code worse.
  react/no-unused-prop-types: off  # This can be perfectly appropriate.
  react/default-props-match-prop-types: off  # We handle this better with types.
  react/prop-types: off  # We handle this better with types.
  react/require-default-props: off  # We handle this better with types.
  # These two could be good to fix.
  react/prefer-stateless-function: off
  react/sort-comp: off

  # React Native.
  #
  # This plugin isn't included in the airbnb config, and it doesn't
  # itself enable any rules by default, so this config is it.
  #
  # (From the docs as of 2020-03, these are the rules we'd like to use.
  # Plugin disabled only because the rules are; bring it back when
  # reviving either of them.)
  #
  # react-native/no-unused-styles: error  # This is buggy on the `this.styles` pattern.
  # react-native/no-color-literals: error  # TODO eliminate these and enable.

  # Flow
  flowtype/boolean-style: [error, boolean]
  flowtype/define-flow-type: warn
  flowtype/delimiter-dangle: off
  flowtype/generic-spacing: [error, never]
  flowtype/no-dupe-keys: error
  flowtype/no-primitive-constructor-types: error
  flowtype/no-types-missing-file-annotation: error
  ################# flowtype/no-weak-types: error
  # flowtype/require-parameter-type: error
  flowtype/require-readonly-react-props: error
  # flowtype/require-return-type: [error, always, {annotateUndefined: never}]
  flowtype/require-valid-file-annotation: error
  flowtype/semi: [error, always]
  flowtype/space-before-generic-bracket: [error, never]
  flowtype/space-before-type-colon: [error, never]
  # For more formatting rules, see tools/formatting.eslintrc.yaml.
  flowtype/type-id-match: [error, '^([A-Z][a-z0-9]+)+$']
  flowtype/union-intersection-spacing: [error, always]
  flowtype/use-flow-type: warn
  flowtype/valid-syntax: warn

overrides:

# Test file overrides
- files: ['**/__tests__/**']
  rules:
    no-restricted-imports: off

# Maintain code style of redux-persist as we received it.
- files: ['src/third/redux-persist/**']
  rules:
    semi: off # semicolons are omitted
    curly: off # one-line `if`s and `else`s without semicolons
    nonblock-statement-body-position: off # same reason as `curly`
    space-before-function-paren: off # `function myFunc (arg1) { ... }`
    arrow-body-style: off # `() => { return 1; }` used over `() => 1`
    object-curly-spacing: off # `{foo: 'bar'}` used over { foo: 'bar' }
    no-underscore-dangle: off # various identifiers beginning with '_'
    import/order: off # imports not ordered a particular way
    func-names: off # anonymous non-arrow functions sometimes used
    object-shorthand: off # ES6 object method syntax not used

    # function declarations are made (we favor function expressions
    # with `const`), so these functions may be used above the
    # declaration because they are hoisted. Don't change them to our
    # style without addressing this!
    no-use-before-define: off

    no-console: off
    prefer-const: off # `let` is sometimes used unnecessarily
    consistent-return: off # defense against accidental `undefined` return
    no-unused-expressions: off # `myFn && myFn()` used
    no-var: off # var is rarely, but sometimes, used (avoid new uses)

    # a few `var`s not *quite* at the top of function scope (they're
    # very near to it); no bugs known to result from this
    vars-on-top: off

    # a few variables already declared in upper scope; no bugs known
    # to result from this
    no-shadow: off

    # foo.hasOwnProperty("bar") used instead of
    # Object.prototype.hasOwnProperty.call(foo, "bar")
    no-prototype-builtins: off