Workaround for dealing with exceptions thrown in asynchronous render method

[rppc]

We provide a workaround for this issue by allowing the user to define a callback function to be executed whenever an exception is thrown inside the body of a render method. This function should be called exceptionCallBack, as in the following example, adapted from this issue:

var a = false;

var MyComponent = React.createClass({
    render: function() {
        if (this.props.a) {
            throw 'err';
        }

        return (      
            <h1 onClick={ () => {a = true; run()} }>Hello {this.props.name}</h1>      
        );
    },
    exceptionCallBack: function(e) {
        console.log(e);
    }   
});

function run() {
    ReactDOM.render(
        <MyComponent a={a} name={"Rui Cardoso"} />,
        document.getElementById('example')
    );
}

ReactDOM.render(<MyComponent a={false} name={"Rui Cardoso"} />, document.getElementById('example'));

This produces the expected result of err being logged to the console when the component is clicked.

[jimfb]

Thanks for the PR! It's a good attempt, but error boundaries are a little more complicated than this to get right.

This does a try-catch for every render of every component. In V8, a try-catch is a slowpath operation that causes deoptimization, and thus should be avoided for performance reasons. This also only works if the component that crashed during render has an exceptionCallback defined, but the common case is that the component that wants to handle the error is a parent of the component that crashed (otherwise, why would your component define exceptionCallback instead of just fixing the component in the first place). For these reasons, we're not likely to take this PR, so I'm going to close it out in favor of something like #5602. But the attempt is appreciated!

If you're looking for a way to contribute, I would recommend taking a peak at https://github.com/facebook/react/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+bug%22