Incorrect no-return-assign report

[brianwhu]

Tell us about your environment

Windows 10

Node version: v10.16.0
npm version: v6.9.0
Local ESLint version: v6.8.0
Global ESLint version: Not found

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

babel-eslint

Please show your full configuration:

Configuration

module.exports = {
  root: true,

  parserOptions: {
    parser: 'babel-eslint',
    sourceType: 'module'
  },

  env: {
    browser: true
  },

  extends: [
    'standard',
    // Uncomment any of the lines below to choose desired strictness,
    // but leave only one uncommented!
    // See https://eslint.vuejs.org/rules/#available-rules
    'plugin:vue/essential' // Priority A: Essential (Error Prevention)
    // 'plugin:vue/strongly-recommended' // Priority B: Strongly Recommended (Improving Readability)
    // 'plugin:vue/recommended' // Priority C: Recommended (Minimizing Arbitrary Choices and Cognitive Overhead)
  ],

  // required to lint *.vue files
  plugins: [
    'vue'
  ],

  globals: {
    'ga': true, // Google Analytics
    'cordova': true,
    '__statics': true,
    'process': true,
    'Capacitor': true,
    'chrome': true
  },

  // add your custom rules here
  rules: {
    // allow async-await
    'generator-star-spacing': 'off',
    // allow paren-less arrow functions
    'arrow-parens': 'off',
    'one-var': 'off',

    'import/first': 'off',
    'import/named': 'error',
    'import/namespace': 'error',
    'import/default': 'error',
    'import/export': 'error',
    'import/extensions': 'off',
    'import/no-unresolved': 'off',
    'import/no-extraneous-dependencies': 'off',
    'prefer-promise-reject-errors': 'off',

    // allow debugger during development only
    'no-debugger': process.env.NODE_ENV === 'production' ? 'error' : 'off',

    'no-sequences': 'off',
  }
}

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

I have an arrow function that returns a comma expression (i.e. sequence). After I changed the rule to allow comma operator, eslint reported "no-return-assign" error while in fact it is the comma expression that is being returned.

row.reduce((object, value, index) => (object[rs.columns[index]] = value, object), {})

Not sure. ESlint was invoked by quasar/cli

quasar dev

What did you expect to happen?

The code is NOT returning an assignment, but a comma expression where the second operand is a single object. This should be accepted by ESlint

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

ESlint reported no-return-assign error:

error Arrow function should not return assignment no-return-assign

Are you willing to submit a pull request to fix this bug?

No

[ljharb]

Oof. why isn't this => { object[rs.columns[index]] = value; return object; }?

[mdjermanovic]

Hi @brianwhu, thanks for the bug report!

I can confirm this is the intended behavior of this rule (introduced in #6041, issue #5913).

On the other hand, we really don't have any incorrect examples with a nested assignment, and there are also no examples with arrow functions, so I'm marking this as an accepted issue to improve the documentation for this rule.

[mdjermanovic]

As for the provided example, you can wrap object[rs.columns[index]] = value in parens:

/*eslint no-return-assign: error*/

row.reduce((object, value, index) => ((object[rs.columns[index]] = value), object), {})

online demo

[brianwhu]

Hi @mdjermanovic, thanks for the suggestion. But this make the case even more puzzling. Now ESlint would accept

() => (a = b)

But not

() => (a = b, c)

?

[anikethsaha]

Hi @mdjermanovic, thanks for the suggestion. But this make the case even more puzzling. Now ESlint would accept

() => (a = b)

But not

() => (a = b, c)

?

this is because, as per the rule , except-parens (which is enabled by default) allows return assignment if parenthesis around assignment expression only.. so

• () => (a = b)

in this, the parenthesis is around assignment expression. so it's ok

• () => (a = b, c)

in this, parenthesis is around returned expression not around assignment expression, so its an error

[mdjermanovic]

Just to add, correct code without linting errors would be:

/*eslint no-return-assign: "error"*/

() => ((a = b), c)

This rule by default allows parenthesized assignments (parentheses should be directly around the assignment expression).