setState callback called with null instead of undefined

[gwmccull]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
In React v16, the callback function that is an optional parameter to the setState function is called and passed a value of null. Previously, in React v15, the function was called with a value of undefined. This change breaks the default parameters feature of JavaScript.

I discovered this behavior while working on a React Native project (hence the example using View & Text).

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/84v837e9/).

import React, {
	Component,
} from 'react';

export default class MyComp extends Component {
	componentDidMount() {
		this.setState({}, (val) => console.log(val))
	}

	render() {
		return (
			<View>
				<Text>{this.props.climbs}</Text>
			</View>
		);
	}
}

What is the expected behavior?
The callback function will be called with undefined

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?
Broken behavior was observed in React 16 alpha 12 and React Native 0.45.1.

I replicated the issue in React 15.4.1 and React Native 0.40.0 and it was working as expected.

[gaearon]

Would you like to submit a failing test for this?

[aweary]

For reference, here's a JSFiddle running 16.0.0-alpha.13 demonstrating that the callback is being called with undefined.

[gwmccull]

It looks like it may only affect React Native. I found a fix to the issue by fixing ReactNativeReconcileTransaction.js

see #10101

[gaearon]

Fixed by #10101.