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Last active March 21, 2023 13:59
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connect.js explained
// connect() is a function that injects Redux-related props into your component.
// You can inject data and callbacks that change that data by dispatching actions.
function connect(mapStateToProps, mapDispatchToProps) {
// It lets us inject component as the last step so people can use it as a decorator.
// Generally you don't need to worry about it.
return function (WrappedComponent) {
// It returns a component
return class extends React.Component {
render() {
return (
// that renders your component
{/* with its props */}
{/* and additional props calculated from Redux store */}
{...mapStateToProps(store.getState(), this.props)}
{...mapDispatchToProps(store.dispatch, this.props)}
componentDidMount() {
// it remembers to subscribe to the store so it doesn't miss updates
this.unsubscribe = store.subscribe(this.handleChange.bind(this))
componentWillUnmount() {
// and unsubscribe later
handleChange() {
// and whenever the store state changes, it re-renders.
// This is not the real implementation but a mental model.
// It skips the question of where we get the "store" from (answer: <Provider> puts it in React context)
// and it skips any performance optimizations (real connect() makes sure we don't re-render in vain).
// The purpose of connect() is that you don't have to think about
// subscribing to the store or perf optimizations yourself, and
// instead you can specify how to get props based on Redux store state:
const ConnectedCounter = connect(
// Given Redux state, return props
state => ({
value: state.counter,
// Given Redux dispatch, return callback props
dispatch => ({
onIncrement() {
dispatch({ type: 'INCREMENT' })
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ezmiller commented Mar 4, 2019

Is it accurate to say that connect implements some variant of the observer pattern? If yes or no (or kinda), why?

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@ezmiller i would say - connect() is an observer for sure. Why? Go to the internet

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cyogian commented Feb 20, 2020

I just came here to see how a HOC works in practical. Thanks for this explaination.

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aakash-cr7 commented Mar 15, 2020

@gaearon don't you think the subscription logic should be in the constructor and not componentDidMount? If I am dispatching an action from WrappedComponent's componentDidMount. We won't see the updated state (it wont re render) in WrappedComponent, since componentDidMount of the WrappedComponent will run first before the subscription happens in componentDidMount inside connect()

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Very clear explanation! 👍

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Thank You That was a clear explanation !!

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Thanks, this mental model helping me a lot while learning react with redux :)

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@gaearon when connecting a component and using redux-thunk, dispatching twice would update the state twice. Would that mean that connected components would re-render twice (considering that the changes in state are something that would cause a re-render if used with component's native state)?

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thanks @gaearon

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Thank you 🚀🚀🚀

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Thank you so much. It was so helpful.

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