Error on duplicate attributes

[jridgewell]

This isn't directly enforced by JSX, but will cause in error in ES5 browsers. Thoughts on including it?

[jamiebuilds]

Why would it error in ES5 environments if it gets turned into an array?

[jridgewell]

Re: babel/babel#2462.

It won't cause an issue with iDOM, until we release the static-hoisting feature. Then it'll cause mayhem with duplicate key attributes.

[jamiebuilds]

I don't see how static hoisting would introduce an issue?

[jridgewell]

See https://github.com/babel-plugins/babel-plugin-incremental-dom/blob/hoisting/src/helpers/extract-open-arguments.js#L44-L47. If we're hoisting, and the key attribute isn't a literal (and we're not eagerly evaluating attributes), we'll push undefined instead of the key variable reference:

statics = ['id', 'id', 'key', 'key'];
// vs
statics = ['id', 'id', 'key', undefined];

If there are multiple key attributes, we'll have several undefineds in the array, and only the final key will be properly set.