feat(events): Add dataTransfer event property option

[jamsinclair]

What:
Resolves #551

Adds a convenience option to approximate the dataTransfer event property when firing events

Why:

Provides a consistent and convenient way to populate dataTransfer properties, much like populating target properties. This would help developers better simulate testing for drag and drop interactions.

There was previous discussion and feature validation over in testing-library/react-testing-library#339

How:

Update the fireEvent.* functions to populate the dataTransfer property for events only when defined in the eventProperties (second argument).

The property is assigned to the event object in a simple way. It does not construct the DataTransfer object and for good reasons. DataTransfer is privately handled and constructed by the browser and is not supported by jsdom.

MDN also warns:

it is not possible to create a useful DataTransfer object from script, since DataTransfer objects have a processing and security model that is coordinated by the browser during drag-and-drops.
https://developer.mozilla.org/en-US/docs/Web/API/DragEvent

Checklist:

• Documentation added to the
  docs site: docs: add dataTransfer event property info testing-library-docs#459
• I've prepared a PR for types targeting
  DefinitelyTyped N/A
• Tests
• Ready to be merged
  

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[codecov]

Codecov Report

Merging #554 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #554   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines          454       459    +5     
  Branches       111       113    +2     
=========================================
+ Hits           454       459    +5     

Impacted Files	Coverage Δ
src/events.js	100.00% <100.00%> (ø)

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[kentcdodds]

Looks good to me! Just one thing.

[kentcdodds]

We don't only support JSDOM, so let's try the DataTransfer constructor first and if that's unsupported then we can do this workaround. Sound ok?

That being the case, we'll probably need to add /* istanbul ignore if */ or /* istanbul ignore else */ to maintain 100% code coverage.

[kentcdodds]

Super 👍

[kentcdodds]

@all-contributors please add @jamsinclair for code and tests

[allcontributors]

@kentcdodds

I've put up a pull request to add @jamsinclair! 🎉

[kentcdodds]

🎉 This PR is included in version 7.5.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀