Parentheses removed falsely when transforming nullish-coalescing-operator

Bug Report

Current Behavior When transforming an AST containing a nullish coalescing operator, parentheses are removed falsely, leading to different behavior.

Input Code

const babel = require('@babel/core')
const { readFileSync, writeFileSync } = require('fs')

const code = readFileSync('src/index.js')
// src/index.js:
// const foo = 'test'
// console.log((foo ?? '') == '')

const babelConfig = {
  plugins: [
    '@babel/plugin-proposal-nullish-coalescing-operator',
  ],
}

// this works as expected
const ast = babel.parseSync(code, babelConfig)

// this removes the parentheses
const { code: transformedCode } = babel.transformFromAstSync(ast, code)

writeFileSync('dist/index.js', transformedCode)
// dist/index.js:
// const foo = 'test';
// console.log(foo ?? '' == '');

const babelRegister = require('@babel/register')
babelRegister(babelConfig)

require('./dist')
// outputs 'test' instead of false

See also: https://github.com/dword-design/test-babel-missing-parentheses

Expected behavior/code The resulting file should still contain the parentheses.

Environment

• Babel version(s): 7.7.7
• Node/npm version: Node 11.15.0/npm 6.7.0
• OS: macOS 10.14.6
• Monorepo: no
• How you are using Babel: core, register

[sidntrivedi012]

Hey, I would like to work on it. Thanks.

[nicolo-ribaudo]

Its precedence should be the same as ?? or lower.

In the spec, they are represented at the same level: https://tc39.es/ecma262/#sec-binary-logical-operators

ShortCircuitExpression:
  LogicalORExpression
  CoalesceExpression

Also, how to find which function in parentheses.js implements operations on ?? operator ?

??, && and || are represented using a LogicalExpression node: you can see it on ASTExplorer. demo You need to create a LogicalExpression function, which must return true when the node needs parentheses.