React: this.state vs this.setState

Scenario 1:

const { foo } = this.state;

Scenario 2:

this.setState(({ foo }) => { ... });

is foo guaranteed to be identical between these two cases? Or will setState ever run async and return a different value based on other setStates?

As the React docs mention:

setState() does not always immediately update the component. It may batch or defer the update until later. This makes reading this.state right after calling setState() a potential pitfall. Instead, use componentDidUpdate or a setState callback (setState(updater, callback)), either of which are guaranteed to fire after the update has been applied.

So we can use the second argument of the setState to pass a callback where we execute our logic which depends on having the updated value of foo. However your initial question was whether the value of foo in const { foo } = this.state; and the value of foo in this.setState(({ foo }) => { ... }); was the same.

In order check this we can compare executing a setState followed by this.state.foo and a setState followed by another setState (the second one will just log the value of foo instead of mutating it). Please refer to the following snippet:

class Example extends React.Component {
  constructor() {
    super();
    this.state = {
      foo: 0,
      boo: 0
    }
  }
  
  handleClickSetState = () => {
    this.setState(({foo}) => ({foo: foo+1}));
    this.setState(({foo}) => console.log("inside setState. foo: ", foo));
  }
  
  handleClickState = () => {
    this.setState(({boo}) => ({boo: boo+1}));
    console.log("outside setState. boo: ", this.state.boo)
  }
  
  render() {
    return <React.Fragment>
      <button onClick={this.handleClickState}>Test state</button>
      <button onClick={this.handleClickSetState}>Test setState</button>
    </React.Fragment>
  }
}

ReactDOM.render(<Example />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
<div id="root"></div>

React: Difference between this.state= and this.setState, You will usually set the initial state like this: this.state = {name1: value}. Then when you need to update it you will use setState : this. React will preserve this state between re-renders. useState returns a pair inside of an array: the current state value and a function that lets you update it, and it would look like this for example :

State might be state when you access the state immediately after using this.setState, as this.setState is async.

If you need to compute on basis on the updated state i.e ( after this.state is invoked ), you can use the 2nd argument for the method which is a callback that gets triggered after the state changes are committed.

this.setState(updateState, () => {
   const { foo } = this.state;

   console.log(foo); // updated foo
});

function updatedState({ foo }) { 
    // you can be sure that `foo`
    // is from the previous state 
    // before setState is called
}

Component State – React, ReactJS: Props vs. State. Why is setState giving me the wrong value? In React, both this.props and� React components has a built-in state object.. The state object is where you store property values that belongs to the component.. When the state object changes, the component re-renders.

setState is async, and pulling values off state to use in setState is a potential source of bugs. setState can take multiple arguments. It can take just a new state, a callback that takes old state + props and returns new state, new state and a function to run after setting new state, or a combination. Example:

this.setState(
  // function taking previous state + props,
  // which should return updated state.
  // this could also just be an object with
  // the new state, if you don't need to rely
  // on previous state.
  (previousState, props) => {
    if (previousState.something) {
      return { something: somethingElse }
    } else {
      return { something: anotherThing }
    }
  }, () => {
    // safe to use new state here,
    // since state has been updated.
    // but it's recommended to use
    // componentDidUpdate instead!
  })

State and Lifecycle – React, We call ReactDOM.render() to change the rendered output: function tick() { const It will use this.setState() to schedule updates to the component local state:. The code above calls a this.setState function and passes in an object with key-value pairs. If the key matches one we already have in state, it updates the value in state to the new value provided. If the key matches one we already have in state, it updates the value in state to the new value provided.

How to become a pro with React setState() in 10 minutes, State Updates May Be Asynchronous. React may batch multiple setState() calls into a single update for performance. Because this.props and� // assuming this.state = { value: 0 } this.setState({value: 1}); console.log(this.state.value); // 0 React will also try to group or batch setState calls into a single call, which leads us to our

React setState usage and gotchas. A React class component has an , Update. State can be updated in response to event handlers, server responses or prop changes. React provides a method called setState for this� React components can, and often do, have state.State can be anything, but think of things like whether a user is logged in or not and displaying the correct username based on which account is active.

Why Not To Modify React State Directly, setState({}) or even this.forceUpdate() , then everything might appear to be just fine. this.state.cart� There are some gotchas with React&rsquo;s setState(). For example, state updates may be asynchronous: React sometimes batches multiple setState() calls for performance reasons. Make sure to set state correctly and to use the latest state. Here are some examples from the official documentation: // Wrong this.setState({ counter: this.state.counter + this.props.increment }); Instead you should