event.button sent by TestUtils.Simulate.click() is undefined by default

[silvenon]

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

Report a bug.

What is the current behavior?

By default Simulate.click() sends an event with an undefined event.button.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:

https://codesandbox.io/s/00lm97zrjn

What is the expected behavior?

I think event.button should default to 0 because that means "left click". Some libraries like react-router rely on this. Maybe they should default to 0, not React? remix-run/react-router#1534

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?

I'm not sure about earlier versions, but I'm getting this behavior in react-dom@16.2.0.

[gaearon]

You can pass extra data to Simulate APIs so I'd say we leave this out.

I'd also recommend dispatching real browser events in tests instead.