How should one use the setState() method in ReactJS?

user_1758831 0
Viewed 5075

I'm new to ReactJS and I'm trying to understand state and setState(). Using setState() I wanted to change a name, but I am not sure where I should call the setState() method in my code:

  1. Inside the constructor OR
  2. Inside the render method OR
  3. Create a custom method and call it at the end of the constructor before render() is called

This is my code:

import React from "react";

class StateBasic extends React.Component{

    constructor(){
        super();
        let personProfile =  this.state = {
                name : "Bob",
                skill : "Art Designer",
                location : "LA"
        }
        console.log(personProfile);        
    }

    render(){
        let changeName =  this.setState({ name : "Frank" });

        return(
            <div className="col-12 col-sm-12 col-md-12 col-lg-12 col-xl-12">   
                <ul>
                    <li> {this.state.name} </li>
                    <li> {this.state.skill} </li>
                    <li> {this.state.location} </li>
                    <li> {changeName} </li>
                </ul>
            </div>
        );
    }
}

// Let's render ReactDOM
export default StateBasic;
setstate reactjs javascript
3 Answers

If you call setState in render() method you will create infinite loop, instead you can use componentDidMount.

class StateBasic extends React.Component {
  constructor() {
    super();
    let personProfile = this.state = {
      name: "Bob",
      skill: "Art Designer",
      location: "LA"
    }
  }

  componentDidMount() {
    setTimeout(() => {
      this.setState({name: "Frank"});
    }, 1000)
  }

  render() {
    return ( 
      <div className="col-12 col-sm-12 col-md-12 col-lg-12 col-xl-12">
        <ul>
          <li> {this.state.name} </li>
          <li> {this.state.skill} </li>
          <li> {this.state.location} </li>
        </ul>
      </div>
    );
  }
}

ReactDOM.render( <
  StateBasic / > ,
  document.getElementById('container')
)
<script src="https://unpkg.com/react@16.0.0/umd/react.development.js"></script>
<div id="container"></div>
<script src="https://unpkg.com/react-dom@16.0.0/umd/react-dom.development.js"></script>

user_46207710

setState usually occurs (but it's not restricted to) when there's some sort of interaction between the user and your application. For example:

  • When the user types in an input
  • When the user clicks a button

class StateExample extends React.Component {
  constructor() {
    super()

    this.state = {
      clickTimes: 0,
      value: '',
    }

    this.handleChange = this.handleChange.bind(this)
    this.handleClick = this.handleClick.bind(this)
  }

  handleChange(event) {
    this.setState({ value: event.target.value })
  }
  
  handleClick() {
    this.setState({ clickTimes: this.state.clickTimes + 1 })
  }

  render() {
    return (
      <div>
        <label>Type here:</label>
        <input type="text" value={this.state.value} onChange={this.handleChange} />
        
        <div style={{ marginTop: 20 }}>
          <button onClick={this.handleClick}>Click-me</button>
          Click times: {this.state.clickTimes}
        </div>
      </div>
    )
  }
}

ReactDOM.render(
  <StateExample />,
  document.getElementById('root')
)
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>

<div id="root"></div>

For more information I recommend reading State and Lifecycle in ReactJS docs.

user_14611310

If you initializing the state then do in the constructor

constructor(){
    super();
    let personProfile =  this.state = {
            name : "Bob",
            skill : "Art Designer",
            location : "LA"
    }
    console.log(personProfile);
    this.state= { name : "Frank" };//initialvalue         
}

If you want to change on some action then make a method (changeName) and use like this in render:

changeName(name){
   this.setState({name})
}

render(){

        let changeName =  this.setState({ name : "Frank" });

        return(
            <div className="col-12 col-sm-12 col-md-12 col-lg-12 col-xl-12">   
                <ul>
                    <li> {this.state.name} </li>
                    <li> {this.state.skill} </li>
                    <li> {this.state.location} </li>
                    <li onClick={this.changeName.bind(this,"hello")} > change Me </li>
                </ul>
            </div>
        );
    }
user_33822040
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