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Improve error message when calling a class as a function #8948

JonnyBurger opened this Issue Oct 31, 2018 · 1 comment


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2 participants

JonnyBurger commented Oct 31, 2018

Feature Request

Is your feature request related to a problem? Please describe.
In new versions of Chrome and Node, there is a good error message for when you call a class as a function. My proposal would be to add a more user-friendly message in the next major version of Babel.

Consider the following code:

class A {}

In Chrome and Node.js, this results in the following error:

Uncaught TypeError: Class constructor A cannot be invoked without 'new'

In transpiled Babel code, the error is instead:

Cannot call a class as a function

It would be an improvement to the developer-friendliness if the error message is close the one in V8.

Describe the solution you'd like

The above code compiles to:

"use strict";

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var A = function A() {
  _classCallCheck(this, A);


My desired output would be:

'use strict';

function _classCallCheck(instance, Constructor) {
	if (!(instance instanceof Constructor)) {
		throw new TypeError(
			'Class constructor ' + +
				" cannot be invoked without 'new'"

let A = function A() {
	_classCallCheck(this, A);


This would align the error message with Chrome and Node.js.

Describe alternatives you've considered

Teachability, Documentation, Adoption, Migration Strategy
Since users might check the error message using try / catch, I would recommend to only make this change in the next major version of Babel.

I am happy to implement this if the community agrees.

Thanks for considering!


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babel-bot commented Oct 31, 2018

Hey @JonnyBurger! 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
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