no-unused-state using functional setState

[cristianfraser]

This code:

class MyComponent extends Component {
  state = {
    foo: true,  <---- here eslint shows error
    bar: [1, 2, 3],
  }

  fooBar = () => {
    this.setState(prevState => ({
      bar: prevState.foo ? prevState.bar.sort() : prevState.bar.reverse(),
      foo: !prevState.foo,
    }))
  }

  render() {
    return (<div onClick={this.fooBar}>{...this.state.bar}</div>)
  }
}

But eslint shows error for foo, no-unused-state is displayed, but it is being used.

[ljharb]

Can you share the error, and your eslint config?

[neiker]

Same here.

onTitleChange = (title) => {
    this.setState({ title, titleChanged: true });
  }

  onTemplateChange = (template) => {
    this.props.navigator.pop();

    this.setState((state => ({
      template,
      title: state.titleChanged ? state.title : getTemplateTitle(template),
    })));
  };

97:28 error Unused state field: 'titleChanged' react/no-unused-state

I'm using eslint-config-airbnb

[ljharb]

Your eslint config must be different, because the airbnb config does not permit class properties to be used.

[cristianfraser]

I'm also using eslint-config-airbnb and get the same error

[ljharb]

@cristianfraser same question; can you share your code and your eslint config?

Again, if you're using class properties, you're violating the airbnb style guide, so if you are, you must be using a modified eslint config.

[neiker]

{
    "parser": "babel-eslint",
    "extends": [
        "airbnb"
    ],
    "env": {
        "mocha": true
    },
    "rules": {
        "no-restricted-properties": "off",
        "complexity": [ "error", 10 ],
        "max-depth": [ "error", 4 ],
        "max-nested-callbacks": [ "error", 2 ],
        "max-params": [ "error", 4 ],
        "max-statements": [ "error", 18 ],
        "no-constant-condition": "off",
        "no-plusplus": "off",

        "no-warning-comments": "error",
        "comma-dangle": [
            "error", 
                {
                "arrays": "always-multiline",
                "objects": "always-multiline",
                "imports": "always-multiline",
                "exports": "always-multiline",
                "functions": "always-multiline"
            }
        ],
        "no-restricted-globals": "off",

        "react/require-default-props": "off",
        "react/no-unused-prop-types": "off",
        "react/jsx-filename-extension": [
            "warn",
            {
                "extensions": [".js"]
            }
        ],
        "react/jsx-no-bind": [
            "error",
            {
                "ignoreRefs": false,
                "allowArrowFunctions": false,
                "allowBind": false
            }
        ],
        "react/no-typos": "off",

        "import/no-named-as-default": "off",
        "import/prefer-default-export": "off",

        "jsx-a11y/href-no-hash": "off",
        "jsx-a11y/anchor-is-valid": "off",

        "react-native/split-platform-components": "off",
        "react-native/no-unused-styles": "error",
        "react-native/no-inline-styles": "error",
        "react-native/no-color-literals": "error"
    },
    "globals": {
        "__DEV__": true,
        "fetch": true,
        "ErrorUtils": true
    },
    "plugins": [
        "react-native"
    ]
}

[cristianfraser]

This is a minimal code that shows no-unused-state for ascending:

import React from 'react'


export default class TestComponent extends React.Component {

  state = {
    list: [1, 2, 3],
    ascending: true,
  }

  onClick = () => {
    this.setState(({ list, ascending }) => {
      if (ascending) {
        return {
          list: list.sort(),
          ascending: !ascending,
        }
      }

      return {
        list: list.reverse(),
        ascending: !ascending,
      }
    })
  }

  render() {
    return (
      <div>
        <button onClick={this.onClick}>Click me</button>
        {this.state.list.map(item => <div>{item}</div>)}
      </div>
    )
  }
}

This is my eslint config:

{
  "extends": "airbnb",
  "parser": "babel-eslint",
  "env": {
    "browser": true,
    "node": true,
    "es6": true,
    "jest": true
  },
  "plugins": [
    "react"
  ],
  "settings": {
    "import/resolver": {
      "webpack": { "config": "webpack/dev.config.js" }
    }
  },
  "rules": {
    "camelcase": 0,
    "semi": [2, "never"],
    "react/require-default-props": 0,
    "react/jsx-filename-extension": 0,
    "jsx-a11y/label-has-for": [2, {
      "components": ["label"],
      "required": { "some": ["nesting", "id"] }
    }],
    "jsx-a11y/click-events-have-key-events": 0,
    "jsx-a11y/anchor-is-valid": ["error", {
      "components": ["Link"],
      "specialLink": ["to"],
      "aspects": ["noHref", "invalidHref", "preferButton"]
    }],
  }
}

Also, I've used class properties with plain config-airbnb with no problems.

[ljharb]

@cristianfraser you can only use it with babel-eslint; the airbnb config uses the default parser. It's likely the react-native plugin is enabling that as well, of course.

[cristianfraser]

What do you mean by I can only use it with babel-eslint? That's what's in my config, that's the wrong parser for that rule? Is what's happening the expected behaviour in this case?

[ljharb]

@cristianfraser eslint core doesn't support parsing class properties; only babel-eslint does - thus if you just had extends: "airbnb" you wouldn't be able to use them.

What's happening seems like a bug in the no-unused-state rule, I'm just trying to gather as much info about it as possible :-)

cc @wbinnssmith @jackyho112

[jackyho112]

@ljharb Thanks for tagging me! This rule doesn't support arrow function definition as class method currently. I think this will add it.

[ljharb]

Fixed with #1411

[Godsenal]

I'm having a same problem with latest verstion.

state = {
    removeQueue: [],  <--- unused state field: 'removeQueue'
}
onUpdate = (date, id) => {
    this.setState(prevState => ({
      removeQueue: [...prevState.removeQueue, { date, id }],
    }));
  }

I'm using removeQueue in functional setState, but eslint shows error.

[ljharb]

Try making onUpdate a constructor-bound instance method (putting arrow functions in class properties is a bad practice) and it should pick it up.