When to use ES6 class based React components vs. functional ES6 React components?

New Answer: Much of the below was true, until the introduction of React Hooks.

componentDidUpdate can be replicated with useEffect(fn), where fn is the function to run upon rerendering.

componentDidMount methods can be replicated with useEffect(fn, []), where fn is the function to run upon rerendering, and [] is an array of objects for which the component will rerender, if and only if at least one has changed value since the previous render. As there are none, useEffect() runs once, on first mount.

state can be replicated with useState(), whose return value can be destructured to a reference of the state and a function that can set the state (i.e., const [state, setState] = useState(initState)). An example might explain this more clearly:

const Counter = () => {
  const [count, setCount] = useState(0)

  const increment = () => { 
    setCount(count + 1);
  }

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={increment}>+</button>
    </div>
  )
}

default export Counter

As a small aside, I have heard a number of people discussing not using functional components for the performance reasons, specifically that

"Event handling functions are redefined per render in functional components"

Whilst true, please consider if your components are really rendering at such a speed or volume that this would be worth concern.

If they are, you can prevent redefining functions using useCallback and useMemo hooks. However, bear in mind that this may make your code (microscopically) worse in performance.

But honestly, I have never heard of redefining functions being a bottleneck in React apps. Premature optimisations are the root of all evil - worry about this when it's a problem.

Old Answer: You have the right idea. Go with functional if your component doesn't do much more than take in some props and render. You can think of these as pure functions because they will always render and behave the same, given the same props. Also, they don't care about lifecycle methods or have their own internal state.

Because they're lightweight, writing these simple components as functional components is pretty standard.

If your components need more functionality, like keeping state, use classes instead.

More info: https://facebook.github.io/react/docs/reusable-components.html#es6-classes

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UPDATE March 2019

Building on what was stated in my original answer:

Are there any fundamental differences between React functions and classes at all? Of course, there are — in the mental model.

https://overreacted.io/how-are-function-components-different-from-classes/

UPDATE Feb 2019:

With the introduction of React hooks, it seems as though the React teams wants us to use functional components whenever possible (which better follows JavaScript's functional nature).

Their motivation:

1.) It’s hard to reuse stateful logic between components
2.) Complex components become hard to understand
3.) Classes confuse both people and machines

A functional component with hooks can do almost everything a class component can do, without any of the draw backs mentions above.

I recommend using them as soon as you are able.

Original Answer

Functional components aren't any more lightweight than class based components, "they perform exactly as classes." - https://github.com/facebook/react/issues/5677#issuecomment-241190513

The above link is a little dated, but React 16.7.0's documentation says that functional and class components:

are equivalent from React’s point of view

https://reactjs.org/docs/components-and-props.html#stateless-functions

There is essentially no difference between a functional component and a class component that just implements the render method, other than the syntax.

In the future (quoting the above link):

we [React] might add such optimizations

If you're trying to boost performance by eliminating unnecessary renders, both approaches provide support. memo for functional components and PureComponent for classes.

-https://reactjs.org/docs/react-api.html#reactmemo

-https://reactjs.org/docs/react-api.html#reactpurecomponent

It's really up to you. If you want less boilerplate, go functional. If you love functional programming and don't like classes, go functional. If you want consistency between all components in your codebase, go with classes. If you're tired of refactoring from functional to class based components when you need something like state, go with classes.

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Always try to use stateless functions (functional components) whenever possible. There are scenarios where you'll need to use a regular React class:

The component needs to maintain state

The component is re-rendering too much and you need to control that via shouldComponentUpdate

You need a container component

UPDATE

There's now a React class called PureComponent that you can extend (instead of Component) which implements its own shouldComponentUpdate that takes care of shallow props comparison for you. Read more

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As of React 17 the term Stateless Functional components is misleading and should be avoided (React.SFC deprecated, Dan Abramov on React.SFC), they can have a state, they can have hooks (that act as the lifecycle methods) as well, they more or less overlap with class components

Class based components

state

lifecycle methods

memoization with React.PureComponent

Functional components:

state (useState, useReducer hooks)

lifecycle methods (via the useEffect, useLayoutEffect hooks)

memoization via the memo HOC

Why i prefer Funtional components

React provide the useEffect hook which is a very clear and concise way to combine the componentDidMount, componentDidUpdate and componentWillUnmount lifecycle methods

With hooks you can extract logic that can be easily shared across components and testable

less confusion about the scoping

React motivation on why using hooks (i.e. functional components).

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I have used functional components for heavily used application which is in production. There is only one time I used class components for "Error Boundaries" because there is no alternative "Error Boundaries" in functional components.

I used "class component" literally only one time.

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Forms are easier with functional, because you can reuse form input fields and you can break them apart with React display conditionals.

Classes are one big component that can't be broken down or reused. They are better for function-heavy components, like a component that performs an algorithm in a pop-up module or something.

Best practice is reusability with functional components and then use small functional components to assemble complete sections, ex.- form input fields imported into a file for a React form.

Another best practice is to not nest components in the process of doing this.

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