When to Use Constructor in React The Complete Guide

When to Use Constructor in React The Complete Guide

There are many functions in the React JS lifecycle and the constructor is the function that gets executed at first. We can do any initialization inside the constructor. However, inside the react constructor, do not use this set State, as it is only for initialization. Each component in react JS is a class that acts as an extension of the react core library and when a class object is created or initialization happens, first of all, it gives a call to react constructor. Within the constructor, a super function call is made to make use of “this” attribute inside the constructor. Inside the constructor, it is not important to pass props for every super call, they are passed only for accessing “this. props”. Generally, we can say that react constructor is a default function for a class to execute an object creation of react component class.

In React, constructors are mainly used for two reasons

Initializing the local state of the component by assigning an object to this. state.

For binding an event handler method to the component or an instance.

Before the component is mounted, the constructor() method is fired. As most things in React have a few rules that you should follow, here are a few that you need to use for constructor:

Call super(props) before using this.props

Never call setState() inside constructor()

Avoid assigning values from this. props to this. state

Bind events all in one place

Avoid using side-effects or subscriptions in the constructor()

The working principle of using constructor in React consists of a few important things which are

Load on Initialization: Every time when the component is initializing, it will load all the initial data. Loading sets the values of the initial state.

No need to Call Manually: As the constructor is getting called on initialization, there is no need to call the constructor function in the reacts component, it gets called automatically.

To perform react constructor-related operations, hire dedicated react developers. Let’s see one real-time example. When designing a page for all available product listings for the customer where product details are fetched from the server, customers have to wait till all the products get fetched from another server. Constructors have a vital role here and for the customer, it helps to set some basic and static products on initialization of components. Thus, customers will see some products instead of any blank page.

The example can be explored as
Let’s perform constructor operation hereby initially setting some values for the constructor and then again changing them with the help of time. Inside SampleConstructor, 

In this component, set an initial value for the msg which is “Loading” on component SampleConstructor initialization (this is object creation of the class).

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Then componentDidMount function will be called after the first rendering of the component and then the value of msg will change accordingly.

Inside ConstructorExample, In this component, a flag is set as initialization inside a constructor. If the value for the flag is true, the SampleConstructor component is called otherwise “different component” is printed. A screen of outputs can be shown as below,

Code:
class SampleConstructor extends React.Component { //generating a constructor and putting super with props so that we can able constructor(props) { // a super function is called inside constructor function so that we can enable this.props super(props); this.state = { msg: 'Loading' } this.cnt = 1; } //do something after rendering html componentDidMount() { this.timerForData = setInterval(function(){ this.setState({ msg: this.state.msg === 'Loading' ? 'Data is still loading' : 'Data Loaded' }) this.cnt = this.cnt + 1; }.bind(this), 1000); } //clear the time interval componentWillUnmount() { //clear timer clearInterval(this.timerForData); } render() { //"return the HTMl message after changing state" return ( <div>{this.state.msg}</div> ) } } class ConstructorExample extends React.Component { constructor(props) { super(props); this.state = { flag: true } } componentDidMount() { this.timerForData = setInterval(function(){ this.setState({ flag: !this.state.flag }) }.bind(this), 10000); } render() { return ( <div> {this.state.flag ? <SampleConstructor /> : (<div>different component</div>)} </div> ) } } ReactDOM.render(<ConstructorExample />, document.getElementById('root'))

HTML Code:
//HTMl file to use above react components <div id="root"></div>

Output:
Output on initialization of components will be the value set from the constructor of SampleConstructor. Loading (React Constructor -1.2) After completing an interval, the state value of the msg will be changed to new. Render will be called on each state change. Data Loaded (React Constructor -1.3) This output comes from the ConstructorExample component where the flag is checked. This flag value is set to true on initialization inside the constructor function where it will keep on changing, Different Component (Output -1.4) To implement the use of constructors in React, it is always good to hire React Native developers from a good development company.

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