There are several differences between HashMap and Hashtable in Java:

1. Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
2. Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
3. One of HashMap’s subclasses is LinkedHashMap, so in the event that you’d want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn’t be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I’d recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

Promise

A Promise handles a single event when an async operation completes or fails.

Note: There are Promise libraries out there that support cancellation, but ES6 Promise doesn’t so far.

Observable

An Observable is like a Stream (in many languages) and allows you to pass zero or more events where the callback is called for each event.

Often Observable is preferred over Promise because it provides the features of Promise and more. With Observable it doesn’t matter if you want to handle 0, 1, or multiple events. You can utilize the same API in each case.

Observable also has the advantage over Promise to be cancellable. If the result of an HTTP request to a server or some other expensive async operation isn’t needed anymore, the Subscription of an Observable allows you to cancel the subscription, while a Promise will eventually call the success or failed callback even when you don’t need the notification or the result it provides anymore.

While a Promise starts immediately, an Observable only starts if you subscribe to it. This is why Observables are called lazy.

Observable provides operators like map, forEach, reduce, … similar to an array

There are also powerful operators like retry(), or replay(), … that are often quite handy. A list of operators shipped with rxjs

Lazy execution allows you to build up a chain of operators before the observable is executed by subscribing, to do a more declarative kind of programming.

The useHistory() hook is now deprecated. If you are using React Router 6, the proper way to navigate programmatically is as follows:

import { useNavigate } from "react-router-dom";

function HomeButton() {
  const navigate = useNavigate();

  function handleClick() {
    navigate("/home");
  }

  return (
    <button type="button" onClick={handleClick}>
      Go home
    </button>
  );
}

React Router v5.1.0 with hooks

There is a new useHistory hook in React Router >5.1.0 if you are using React >16.8.0 and functional components.

import { useHistory } from "react-router-dom";

function HomeButton() {
  const history = useHistory();

  function handleClick() {
    history.push("/home");
  }

  return (
    <button type="button" onClick={handleClick}>
      Go home
    </button>
  );
}

React Router v4

With v4 of React Router, there are three approaches that you can take to programmatic routing within components.

1. Use the withRouter higher-order component.
2. Use composition and render a <Route>
3. Use the context.

React Router is mostly a wrapper around the history library. history handles interaction with the browser’s window.history for you with its browser and hash histories. It also provides a memory history which is useful for environments that don’t have a global history. This is particularly useful in mobile app development (react-native) and unit testing with Node.

A history instance has two methods for navigating: push and replace. If you think of the history as an array of visited locations, push will add a new location to the array and replace will replace the current location in the array with the new one. Typically you will want to use the push method when you are navigating.

In earlier versions of React Router, you had to create your own history instance, but in v4 the <BrowserRouter>, <HashRouter>, and <MemoryRouter> components will create a browser, hash, and memory instances for you. React Router makes the properties and methods of the history instance associated with your router available through the context, under the router object.

1. Use the withRouter higher-order component

The withRouter higher-order component will inject the history object as a prop of the component. This allows you to access the push and replace methods without having to deal with the context.

import { withRouter } from 'react-router-dom'
// this also works with react-router-native

const Button = withRouter(({ history }) => (
  <button
    type='button'
    onClick={() => { history.push('/new-location') }}
  >
    Click Me!
  </button>
))

2. Use composition and render a <Route>

The <Route> component isn’t just for matching locations. You can render a pathless route and it will always match the current location. The <Route> component passes the same props as withRouter, so you will be able to access the history methods through the history prop.

import { Route } from 'react-router-dom'

const Button = () => (
  <Route render={({ history}) => (
    <button
      type='button'
      onClick={() => { history.push('/new-location') }}
    >
      Click Me!
    </button>
  )} />
)

3. Use the context*

But you probably should not

The last option is one that you should only use if you feel comfortable working with React’s context model (React’s Context API is stable as of v16).

const Button = (props, context) => (
  <button
    type='button'
    onClick={() => {
      // context.history.push === history.push
      context.history.push('/new-location')
    }}
  >
    Click Me!
  </button>
)

// you need to specify the context type so that it
// is available within the component
Button.contextTypes = {
  history: React.PropTypes.shape({
    push: React.PropTypes.func.isRequired
  })
}

1 and 2 are the simplest choices to implement, so for most use cases, they are your best bets.