Is React 16's Portal API meant to replace the Context API?

TL;DR => Portals and Context solve different purposes, one is to inject DOM at any level and other is to inject props at any level. Context can mimic Portals but Portals AS OF NOW cannot mimic Context not at least without introducing code smell.

NOTE: Below is based on my understanding of the two concepts, if anyone has further thoughts or corrections on this please feel free to edit the answer.

From what I was able to understand Portals are, as the name suggests, a door way for you to render components that need not be in the component tree hierarchy. This works like perfectly for Modals, Popovers or any component which needs to be stitched at a particular location in the tree.

Context is to communicate with various sibling and child components without having to pass props all the way down from parent to the intended component. Of course, this is an important feature to have but it remained in the experimental phase probably due to the fact that this can be achieved by event-emitters and Redux, MobX for a centralized state management.

I am assuming that your use case for i18n would require a lot of communication across components for which you might want to look at this great article

COMPONENT COMMUNICATION

Portals and Context help achieving this kind of communication but there is a difference. Portals can render or inject DOM at any level and Context can inject props at any level in the subcomponent tree.

You can always achieve what Portals are able to do using Context but I don't think Portals can mimic Context functionality.

EG: Something like this can be done to mimic what Portals do using Context. Where as in Portals AFAIK you can only send DOM Nodes ReactDOM.createPortal(child, container). May be there is a way to achieve the functionality but that would surely lead to code smell.

class Parent extends React.Component {    getChildContext() {        return {            renderModal: this.renderModal        }    }    renderModal = (children) => {        this.setState({            open: !this.state.open,            injectableChildren: children        })    }    render() {        this.state.open            ?            <div>                {this.state.injectableChildren}            </div>            :             null        // JSX    }}class SomeSibling extends React.Component {    static contextTypes = {       renderModal: React.PropTypes.func    }    handleOnClick = (event) => {        this.context.renderModal(renderableChildJSX);    }    renderableChildJSX = () => (        <div>            YAY I AM GETTING RENDERED AT THE ROOT        </div>    )    render() {        return(            <div onClick={this.handleOnClick}>            </div>        )    }}

In my case, I feared from using Context despite it's flexibility due to the fact that React docs always mentioned that it's an experimental feature and warned repeatedly from using this feature full-blown. My guess is that React is looking at stabilising this feature or completely scrape it off the React codebase which is a risk that could go both ways.

CONCLUSION: IMO, Portals in it's current state and the problem it is trying to resolve is entirely different from what Context is built for and it is better to use event-emitters with the only reason that Context might possibly be deprecated.

A Portal API is different from a Context API,

Portals provide a first-class way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.

A portal is useful when you might want to render Modals or Popovers, that need to be outside of the current DOM hierarchy in order for it to have proper z-indexes. Most often you would render these in the top level directly. However with Portal you can render DOM element at any level of hierarchy.

In React 16, portals can be created like

ReactDOM.createPortal(child, container)

Context on the other hand is used to pass the data onto different component without the need to pass if down at each level. You may have components at different level, some of which may be extremely nested and passing the props down all the way at each level might not be a great solution and it comes with significant performance hindrance, since a lot the higher levels may not actually be using these props but will still re-render on such prop changes.

From v16.3.0 React has introduced a new context API and context is no longer experimental.

Context is designed to share data that can be considered “global” for a tree of React components, such as the current authenticated user, theme, or preferred language. Using context, we can avoid passing props through intermediate elements. but it shouldn't be used to just pass props a few levels down

In general you would use context like

export const MyContext = React.createContext();class Provider extends React.Component {   state = {       theme: 'dark'   }   handleChange=() => {}   render() {        return <MyContext.Provider            value={{state: this.state, handleChange: this.handleChange}}           >               {this.props.children}           </MyContext.Provider?>   }}

and for the Component you want to use context value, you would write

import {MyContext} from 'path/to/context'...render() {    return <MyContext.Consumer>         {(theme) => <div>{theme}</div>}     </MyContext.Consumer>}

To piggyback on this topic, is context the best way to manage i18n localisation in react?

Yes i18n localisation is a good usecase for using context since you would need to pass on language/paramerisation selection throughout your app. In cases where you need much more integration with APIs to do localisation, you can think about using Redux.

For more details check this answer on whether to use Context or Redux