False positive in no-side-effects-in-computed-properties

[lgnap]

Tell us about your environment
I try using 5f4f3ce (last master) from this repos

• ESLint Version: ^3.19.0

• eslint-plugin-eslint-plugin: ^0.8.0

• eslint-plugin-vue-libs: ^1.2.0

• mocha: ^3.2.0

• eslint-plugin-vue Version: version: 3.13.1

• Node Version: 8.8.1

Please show your full configuration:
Not relevant

What did you do? Please include the actual source code causing the issue.

I write a unit test case but I can't solve the issue myself:

'use strict'

// ------------------------------------------------------------------------------
// Requirements
// ------------------------------------------------------------------------------

const rule = require('../../../lib/rules/no-side-effects-in-computed-properties')
const RuleTester = require('eslint').RuleTester

const parserOptions = {
  ecmaVersion: 6,
  sourceType: 'module',
  ecmaFeatures: { experimentalObjectRestSpread: true }
}

// ------------------------------------------------------------------------------
// Tests
// ------------------------------------------------------------------------------

const ruleTester = new RuleTester()
ruleTester.run('no-side-effects-in-computed-properties', rule, {
  valid: [
    {
      code: `Vue.component('test', {
        computed: {
          test9 () {
            let computedResult = {}
            const index = ''
            computedResult[index] = this.images[index]
            return computedResult
          },    
        }
      })`,
      parserOptions
    }
  ],
  invalid: [
    {
      code: `Vue.component('test', {
        computed: {
          test5 () {
            let computedResult = {}
            const index = ''
            this.images[index] = computedResult[index]
            return computedResult
          },     
        }
      })`,
      parserOptions,
      errors: [{
        line: 6,
        message: 'Unexpected side effect in "test5" computed property.'
      }]
    }
  ]
})

What did you expect to happen?
The first test must succeed (I don't update this.images, just read it), the second must fail

What actually happened? Please include the actual, raw output from ESLint.
I got an Unexpected side effect in both case.

The section of parser lib/rules/no-side-effects-in-computed-properties.js relevant for my case is the following:

      // this.xxx <=|+=|-=>
      'AssignmentExpression > MemberExpression' (node) {
        if (utils.parseMemberExpression(node)[0] === 'this') {
          forbiddenNodes.push(node)
        }
      },

The grammar parser ( AssignmentExpression > MemberExpression) see independent nodes and a console log will show the two part separately, it shouldn't.
A console.log will show the following:

[ 'computedResult', 'index' ]
[ 'this', 'images', 'index' ]

[michalsnik]

Hey @lgnap, thanks for posting this issue. Impressive breakdown! I can dig right into implementation :)

[michalsnik]

Ok I got it, it's waiting for code review now @lgnap

[lgnap]

Ho thanks, I see (but AST is not my domain -- this is why I wrote this bug instead of PR) that you could use the :first-child selector (cfr https://eslint.org/docs/developer-guide/selectors).
'AssignmentExpression:first-child'
What do you thing?

[michalsnik]

It doesn't work unfortunately @lgnap. I'm going to merge and realese new version with current solution :)

[lgnap]

thanks