React

• Add a new officially supported context API. (acdlite in #11818)
• Add a new React.createRef() API as an ergonomic alternative to callback refs. (trueadm in #12162)
• Add a new React.forwardRef() API to let components forward their refs to a child. (bvaughn in #12346)
• Fix a false positive warning in IE11 when using React.Fragment. (XaveScor in #11823)
• Replace React.unstable_AsyncComponent with React.unstable_AsyncMode. (acdlite in #12117)
• Improve the error message when calling setState() on an unmounted component. (sophiebits in #12347)

React DOM

• Add a new getDerivedStateFromProps() lifecycle and UNSAFE_ aliases for the legacy lifecycles. (bvaughn in #12028)
• Add a new getSnapshotBeforeUpdate() lifecycle. (bvaughn in #12404)
• Add a new <React.StrictMode> wrapper to help prepare apps for async rendering. (bvaughn in #12083)
• Add support for onLoad and onError events on the <link> tag. (roderickhsiao in #11825)
• Add support for noModule boolean attribute on the <script> tag. (aweary in #11900)
• Fix minor DOM input bugs in IE and Safari. (nhunzaker in #11534)
• Correctly detect Ctrl + Enter in onKeyPress in more browsers. (nstraub in #10514)
• Fix containing elements getting focused on SSR markup mismatch. (koba04 in #11737)
• Fix value and defaultValue to ignore Symbol values. (nhunzaker in #11741)
• Fix refs to class components not getting cleaned up when the attribute is removed. (bvaughn in #12178)
• Throw with a meaningful message if the component runs after jsdom has been destroyed. (gaearon in #11677)
• Don't crash if there is a global variable called opera with a null value. alisherdavronov in #11854)
• Don't check for old versions of Opera. (skiritsis in #11921)
• Deduplicate warning messages about <option selected>. (watadarkstar in #11821)
• Deduplicate warning messages about invalid callback. (yenshih in #11833)
• Deprecate ReactDOM.unstable_createPortal() in favor of ReactDOM.createPortal(). (prometheansacrifice in #11747)
• Don't emit User Timing entries for context types. (abhaynikam in #12250)
• Improve the error message when context consumer child isn't a function. (raunofreiberg in #12267)
• Improve the error message when adding a ref to a functional component. (skiritsis in #11782)

React DOM Server

• Prevent an infinite loop when attempting to render portals with SSR. (gaearon in #11709)
• Warn if a class doesn't extend React.Component. (wyze in #11993)
• Fix an issue with this.state of different components getting mixed up. (sophiebits in #12323)
• Provide a better message when component type is undefined. (HeroProtagonist in #11966)

React Test Renderer

• Fix handling of fragments in toTree(). (maciej-ka in #12107 and gaearon in #12154)
• Shallow renderer should assign state to null for components that don't set it. (jwbay in #11965)
• Shallow renderer should filter legacy context according to contextTypes. (koba04 in #11922)
• Add an unstable API for testing asynchronous rendering. (acdlite in #12478)

React Is (New)

• First release of the new package that libraries can use to detect different React node types. (bvaughn in #12199)
• Add ReactIs.isValidElementType() to help higher-order components validate their inputs. (jamesreggio in #12483)

React Lifecycles Compat (New)

• First release of the new package to help library developers target multiple versions of React. (bvaughn in #12105)

Create Subscription (New)

• First release of the new package to subscribe to external data sources safely for async rendering. (bvaughn in #12325)

React Reconciler (Experimental)

• Expose react-reconciler/persistent for building renderers that use persistent data structures. (gaearon in #12156)
• Pass host context to finalizeInitialChildren(). (jquense in #11970)
• Remove useSyncScheduling from the host config. (acdlite in #11771)

React Call Return (Experimental)

• Fix a crash on updates. (rmhartog in #11955)