Document how React treats different attributes

[bvaughn]

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This issue was originally reported by @MMeent via facebook/react/issues/9220

Please refer to the original issue for the related discussion thread.

Text of original issue:

As of #9106 there are quite a lot of tests for HTML's boolean properties on tags. According to these tests HTML properties should get rendered into empty strings or not set, e.g. <div hidden={true} /> should render to <div hidden="" /> and <div hidden={false} /> should render to <div />.

But when I look further, there are other tests which do similar things: <a href={true} /> should render to <a href="true" />. This means that if I take true as my property value, I will not know what the result will be.

The issue being: React currently does not have a clearly-defined and documented way with which they handle their props. Could the docs be updated to specify which type of prop will result in what? e.g. "any number gets parsed to a string (using Number.toString()), an object gets stringified by using arg => Object.entries(arg).map(([key, val]) => ${key}: ${val};).join(), and booleans toggle the property."

In short, these are the bugs/undocumented features/contradictions that the tests expect to happen, plus my expectations:

• prop={true} gets parsed to prop="" or prop="true" for select properties
  expected: either prop="" or prop="prop", not both // as per checked=checked HTML standard
• prop="" gets parsed as prop="" or
  expected: prop=""
• prop="prop" might get parsed as either prop="prop", prop="" or even
  expected: prop="prop"
• prop="foo" => prop="" or prop="foo"
  expected: prop="foo"
• prop={false} => or prop="false"
  expected:
• prop={['foo', 'bar']} => prop="" or prop="foo,bar" (hand-tested, no automatic test available)
  expected: prop="foo bar"
• prop={{foo: 'bar', baz: 5}} => prop="" or prop="foo: bar; baz: 5;" or prop="[object Object]" (hand-tested, no automatic test available)
  expected: prop="foo: bar; baz: 5" // as per style object transform

Hacky showcase: https://jsfiddle.net/ox8a7vfe/3/

Are these features or are these bugs?

If they are bugs, and work is going to be done on the props value parsing, then the following might be a nice addition:

• prop={{foo: true, bar: true, baz: false}} => prop="foo bar" // render all true keys, disregard false keys, like classNames.