generate() produces incorrect output for arrow function expression

[overlookmotel]

Bug Report

• I would like to work on a fix!

Current behavior
A clear and concise description of the behavior.

generate() produces incorrect code for arrow function expression.

const generate = require('@babel/generator').default;
const node = t.arrowFunctionExpression( [], t.objectExpression( [] ) );
console.log( generate( node ) );

Output:

() => {}

Output should be:

() => ({})

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

No config used. The above is the complete reproduction case.

Environment

System:
	OS: macOS Mojave 10.14.6
Binaries:
	Node: 14.9.0 - ~/.nvm/versions/node/v14.9.0/bin/node
	npm: 6.14.8 - ~/.nvm/versions/node/v14.9.0/bin/npm
npmPackages:
	@babel/core: ^7.11.6 => 7.11.6 
	@babel/generator: ^7.11.6 => 7.11.6 
	@babel/helper-module-transforms: ^7.11.0 => 7.11.0 
	@babel/parser: ^7.11.5 => 7.11.5 
	@babel/plugin-transform-modules-commonjs: ^7.10.4 => 7.10.4 
	@babel/plugin-transform-react-jsx: ^7.10.4 => 7.10.4 
	@babel/register: ^7.11.5 => 7.11.5 
	@babel/traverse: ^7.11.5 => 7.11.5 
	@babel/types: ^7.11.5 => 7.11.5 
	babel-jest: ^26.3.0 => 26.3.0 
	babel-plugin-dynamic-import-node: ^2.3.3 => 2.3.3 
	eslint: ^7.8.1 => 7.8.1 
	jest: ^26.4.2 => 26.4.2

[babel-bot]

Hey @overlookmotel! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite."

[JLHwung]

You can take a look at

babel/packages/babel-generator/src/node/parentheses.js

Lines 286 to 322 in c8f0b6d

	// Walk up the print stack to determine if our node can come first
	// in statement.
	function isFirstInStatement(
	printStack: Array<Object>,
	{ considerArrow = false, considerDefaultExports = false } = {},
	): boolean {
	let i = printStack.length - 1;
	let node = printStack[i];
	i--;
	let parent = printStack[i];
	while (i > 0) {
	if (
	t.isExpressionStatement(parent, { expression: node }) ||
	(considerDefaultExports &&
	t.isExportDefaultDeclaration(parent, { declaration: node })) ||
	(considerArrow && t.isArrowFunctionExpression(parent, { body: node }))
	) {
	return true;
	}
	if (
	(hasPostfixPart(node, parent) && !t.isNewExpression(parent)) ||
	(t.isSequenceExpression(parent) && parent.expressions[0] === node) ||
	t.isConditional(parent, { test: node }) ||
	t.isBinary(parent, { left: node }) ||
	t.isAssignmentExpression(parent, { left: node })
	) {
	node = parent;
	i--;
	parent = printStack[i];
	} else {
	return false;
	}
	}
	return false;
	}

In your case, the printStack is Array (2) [ ArrowExpression, ObjectExpression ], it seems that the while-loop (which will consider arrow functions) is incorrectly skipped because i == 0. I believe the condition should be loosen and only applied to the second if condition where i-- is applied.

PR is welcome. If you don't know how to clone Babel, follow these steps: (you need to have make and yarn available on your machine).

1. Write a comment there to let other possible contributors know that you are working on this bug.
2. Fork the repo
3. Run git clone https://github.com/<YOUR_USERNAME>/babel.git && cd babel
4. Run yarn && make bootstrap
5. Wait ⏳
6. Run make watch (or make build whenever you change a file)
7. Add a test (only input.js; output.js will be automatically generated) to packages/babel-generator/test/fixtures/parentheses
8. Update the code!
9. yarn jest babel-generator to run the tests
   • If some test outputs don't match but the new results are correct, you can delete the bad output.js files and run the tests again
10. If it is working, run make test to run all the tests
11. Run git push and open a PR!

[overlookmotel]

@JLHwung Thanks loads for the swift reply and advice to where to look in the code base.

I will try to put a PR together in next few days. I'll post another comment here when/if I start work on it, so there's no duplicated effort by another contributor.

[overlookmotel]

Failing test: https://github.com/overlookmotel/babel/tree/arrow-function-object-expression-fix

I've started work on a fix.

[overlookmotel]

PR submitted #12082. @JLHwung please have a look when you get a chance.