react-dom

Major changes

• Initial render now uses document.createElement instead of generating HTML. Previously we would generate a large string of HTML and then set node.innerHTML. At the time, this was decided to be faster than using document.createElement for the majority of cases and browsers that we supported. Browsers have continued to improve and so overwhelmingly this is no longer true. By using createElement we can make other parts of React faster. (@spicyj in #5205)
• data-reactid is no longer on every node. As a result of using document.createElement, we can prime the node cache as we create DOM nodes, allowing us to skip a potential lookup (which used the data-reactid attribute). Root nodes will have a data-reactroot attribute and server generated markup will still contain data-reactid. (@spicyj in #5205)
• No more extra <span>s. ReactDOM will now render plain text nodes interspersed with comment nodes that are used for demarcation. This gives us the same ability to update individual pieces of text, without creating extra nested nodes. If you were targeting these <span>s in your CSS, you will need to adjust accordingly. You can always render them explicitly in your components. (@mwiencek in #5753)
• Rendering null now uses comment nodes. Previously null would render to <noscript> elements. We now use comment nodes. This may cause issues if making use of :nth-child CSS selectors. While we consider this rendering behavior an implementation detail of React, it's worth noting the potential problem. ()@spicyj in #5451)
• Functional components can now return null. We added support for defining stateless components as functions in React 0.14. However, React 0.14 still allowed you to define a class component without extending React.Component or using React.createClass(), so we couldn’t reliably tell if your component is a function or a class, and did not allow returning null from it. This issue is solved in React 15, and you can now return null from any component, whether it is a class or a function. (@jimfb in #5884)
• Improved SVG support. All SVG tags are now fully supported. (Uncommon SVG tags are not present on the React.DOM element helper, but JSX and React.createElement work on all tag names.) All SVG attributes that are implemented by the browsers should be supported too. If you find any attributes that we have missed, please let us know in this issue. (@zpao in #6243)

Breaking changes

• No more extra <span>s.
• React.cloneElement() now resolves defaultProps. We fixed a bug in React.cloneElement() that some components may rely on. If some of the props received by cloneElement() are undefined, it used to return an element with undefined values for those props. We’re changing it to be consistent with createElement(). Now any undefined props passed to cloneElement() are resolved to the corresponding component’s defaultProps. (@truongduy134 in #5997)
• ReactPerf.getLastMeasurements() is opaque. This change won’t affect applications but may break some third-party tools. We are revamping ReactPerf implementation and plan to release it during the 15.x cycle. The internal performance measurement format is subject to change so, for the time being, we consider the return value of ReactPerf.getLastMeasurements() an opaque data structure that should not be relied upon. (@gaearon in #6286)

Removed deprecations

These deprecations were introduced nine months ago in v0.14 with a warning and are removed:

• Deprecated APIs are removed from the React top-level export: findDOMNode, render, renderToString, renderToStaticMarkup, and unmountComponentAtNode. As a reminder, they are now available on ReactDOM and ReactDOMServer. (@jimfb in #5832)
• Deprecated addons are removed: batchedUpdates and cloneWithProps. (@jimfb in #5859, in )