iminside / babel-bug

This repo describes Babel.js bug for distinguishing between Arrow Functions, Classes and Regular Functions.

Installation

git clone
npm i

And after installation, for dealing with bug:

npm run bug

for dealing with original JavaScript behaviour:

npm run original

Description

Accordingly with the following gis: https://gist.github.com/wentout/ea3afe9c822a6b6ef32f9e4f3e98b1ba

We might have difference between Functions, Regular Functions or Arrow Functions:

'use strict';

const myArrow = () => {};
const myFn = function () {};

class MyClass {};

And if we might be willing to check what sort of function we are dealing with, we have the ability to check the difference with the following steps:

const isArrowFunction = (fn) => {
	if (typeof fn !== 'function') {
		return false;
	}
	if (fn.prototype !== undefined) {
		return false;
	}
	return true;
};

const isRegularFunction = (fn) => {
	if (typeof fn !== 'function') {
		return false;
	}
	if (typeof fn.prototype !== 'object') {
		return false;
	}
	if (fn.prototype.constructor !== fn) {
		return false;
	}
	return Object.getOwnPropertyDescriptor(fn, 'prototype').writable === true;
};

const isClass = (fn) => {
	if (typeof fn !== 'function') {
		return false;
	}
	if (typeof fn.prototype !== 'object') {
		return false;
	}
	if (fn.prototype.constructor !== fn) {
		return false;
	}
	return Object.getOwnPropertyDescriptor(fn, 'prototype').writable === false;
};

And that is how babel transforms functions:

var myArrow = function myArrow() {};

var myFn = function myFn() {};

var MyClass = function MyClass() {
  _classCallCheck(this, MyClass);
};

Therefore regular checking via JavaScript runtime wouldn't work anymore, because everything becomes functions.

Screenshots

Before babel transforms

After

How to implement correct behaviour?

for Arrow Functions

const workingArrow = () => {};
// right after arrow function definition
Object.defineProperty(workingArrow, 'prototype', {
    value: undefined
});

for Classes

class WorkingClass {}
// right after class definition
Object.defineProperty(WorkingClass, 'prototype', {
    writable: false
});